Preparing Heterodyne observations

• Anatomy of a beam-switched MSB
• Really Important Aside - or is it Inside?
• Elements of an MSB
• Components, Iterators And All That
• The Het Setup component
  • The Frequency Editor
  • ACSIS Special Configurations
• DRRecipe Component
• Site Quality
• Scheduling Constraints
• The Observation component
• The target component
• Moving objects
• The position editor
• The chop iterator
• The offset iterator
• The stare eye
• The repeat iterator and integration time in general
• Adding a note
• Summary: Making your heterodyne MSB.

• Centering the observations
• Shared or Separate offs
• Samples
• Grid Maps
• Jiggle Maps
• Scan Maps

Guided Tour of an Example MSB (Beam-Switched Sample)

A window will pop up containing a list of folders. The little switch on the left of the folder icons is an open/close toggle switch. Push open the folder titled "Samples" and then select the MSB labeled "Beam-switch sample(1X) " and click "Copy" on the toolbar.

Now go back to your Science Program window, and click on "Paste". The MSB will be dropped into your science program. Congratulations! You're now well underway. Now is a good time to save your progress, so use the File->Save option in your Science Program window and carry on.

Really Important Aside - or is it Inside?

Editing the MSB component

Click on the title of the MSB. This will activate the MSB editor panel on the right section of your window. The properties of an MSB are very simple:

• It has a title, at the moment "Beam-switch sample". When preparing your own MSB, this title should be changed to something meaningful describing the observation, usually with target name. Examples might be "B/SW observation of OMC1" or "3 point grid of M83". Please take the time to do this - it really helps the person doing the observing.
• A pull down menu with a repeat counter. Leave that to 1 for the time being.
• The priority of the MSB. Note that this only affects the internal priority of your MSBs - not the priority allocated to your entire project by your TAG. So if you had two different MSBs that rose to the top of the scheduling queue and one had priority "1" and the other "99", the one marked "1" will normally get done first. If all your MSBs are of equal interest to you, just don't bother changing the default. Small print: Bear in mind though that MSB selection is an operational decision - marking your MSBs with priorities 1, 2, 3 etc is no guarantee that they will be done in precisely that particular order. For example, your lower-priority MSBs could be done because they are observable at a time of day where there is not much else in the queue, whereas your high-priority MSBs could be competing against the top-ranked project. Also, the observer/TSS may sometimes select a lower priority MSB because it is the most efficient choice at the time, because of its duration or azimuth.

Elements of an MSB

Components, Iterators And All That

• MSBs are individually schedulable blocks and contain one or more fully specified science observation and any calibrations. For heterodyne observations, calibrations will be described in a show-to-observer note.

• Observations are observations of a single astronomical target and contain components and a sequence.

• Components are configurators for the JCMT systems such as the instrument (het setup component), the telescope (Target component) and the scheduler (Site Quality component).

• Iterators are potentially multiple actions and contain other iterators and one or more eyes.

• Eyes are actions that results in the actual taking of data - a single data file per eye to be precise.

• Notes are a repository of useful information and can go anywhere.

We'll now go through these individually.

The Het Setup component

If you click on the "Het Setup" component you will see the window below. The purpose of this component is to enable the configuration of the required heterodyne instrument (the Front End) and ACSIS (the 'back end') prior to an observation.

Four boxes are displayed in the editor panel.

Front End Configuration

• The first item in the Het Setup component enables the user to choose the required receiver or Front End. The tuning/frequency range over which the selected receiver can be tuned is displayed in the Front End Summary box in the top-right corner.
• The Sp.(ectral) regions box allows the selection of multiple spectral windows. Multiple spectral regions are especially useful if you wish to observe simultaneously two to four spectral lines which lie within the same 1.8GHz passband, e.g. H2D+ at 372.4GHz and N2H+ at 372.67GHz. Commonly used line combinations and configurations are listed in the Special Config(uration)s pull-down menu to the right, and are described here. If the configuration you wish is not in the list, you can create your own using the Frequency Editor described below. The Frequency Editor tool is shown and hidden using the buttons further down the window. Also, please send a message to your Friend of the Project" so we can perhaps add this configuration to the list in the OT.
• The Bandwidths menu item allows the selection of the required bandwidth (in MHz) for each Spectral Region. The resulting spectral resolution (in kHz) is shown automatically updated in the "res" entry in the Frequency Configuration table at the bottom of the Het Setup menu.
• The Mode box allows a choice between double sideband (dsb) and single sideband (ssb) operation. Note that only receiver W allows to choose between ssb and dsb: receiver A is a dsb receiver and HARP a ssb receiver.
• The Sideband menu allows the choice of sideband. Sideband suppression is not absolute in the SSB receivers, so the user should avoid, if possible, the situation where the unwanted sideband corresponds to a frequency where the zenith opacity or receiver noise is high. In most cases, however, you could simply select "Best".

Frequency Setup

Tuning of the receiver requires a rest frequency and a velocity. If the aim is to observe a specific molecular transition the user may use the pull-down lists of molecules and transitions to generate the frequency. (If the particular line/transition you are interested in is not in the list, please send an email to jcmtot@jach.hawaii.edu so that the line can be added for the next release.) The example above shows a special configuration (HARP_H2DN2H_250X2) with the H2D+ 1(1 0 0)-1(1 1 0) transition at 272.4GHz, and the N2H+ 4-3 transition at 272.7GHz, both using the 250MHz bandwidth mode, observed simultaneously. Alternately, the tuning frequency may be specified explicitly in GHz. The frequency will appear in RED until the Accept button is clicked. This is essential to ensure acceptance of the input. This method will also result in the phrase 'No line' appearing in the molecule and transition fields, even if the frequency does in fact correspond to a known transition. (Look-up occurs from molecule/transition to frequency only - no look-up is done in the opposite direction).

The velocity of the target may be entered either here or in the Target Information component; this second option is activated by ticking the box marked Default tuning velocity to target velocity. If entering the velocity here you must also specify the Velocity Definition and Velocity Frame from the two pull-down menus, and click the "Accept" button (to the right of the frequency field). (Note that the velocity will appear in RED until you click on the Accept button. Failure to click on the Accept button will mean the velocity you entered will be lost.)

• If your source has a very large radial velocity, you should:
  • Select the receiver within whose tuning range the (redshifted) line will fall. For example, at a redshift of 2.0, the neutral carbon [CI] J=2-1 line, whose rest frequency is 809 GHz, will fall into the tuning range of RxA.
  • Specify the radial velocity of the source, as above, either in the Het. Setup velocity field, or in the Target Info. velocity field. (If you choose the latter, make sure the Target Info. component is on the same level as the Het. Setup component, and not embedded in the observation sequence.) If you enter the velocity in the Het. Setup component, be sure to click the Accept button before proceeding.
  • The OT now corrects its lookup table of transitions for the entered velocity. Select a molecule and transition from the pull-down menu, or enter the rest frequency of the spectral line (and click the Accept button). Beware, however, that if you enter a value manually into the frequency field, the OT will not check to see if the redshifted line is in the tuning range of the receiver you selected.

Frequency Editor

Clicking on the Frequency Editor button will show you something like this:

The frequency editor tool is particular useful when trying to configure the backend for multiple lines, multiple resolution windows, or for receivers operating in double sideband mode, where two differing sky frequencies end up with the same IF frequency. In each of these cases finding the correct values for the setup of each window is tricky and the visual presentation from the frequency editor is indispensable, if only to verify the setup. The tool now handles multi-spectral window selection and can be used interactively in choosing observing frequencies and bandwidths for each window or in combination with special ACSIS configurations, or simply to check the atmospheric opacity at the observed (or sideband) frequency. Before clicking the 'Show Frequency Editor' button, please make sure to select the desired number of spectral windows under 'Sp. Regions' (See above). Once inside the frequency editor, the frequencies and bandwidths can be selected or adjusted.

Working roughly from top to bottom, left to right, in the example above showing only one Spectral Subsystem (USB) with two spectral windows, we see:

• the observing and image side-bands, marked as LSB and USB
• their locations within the respective LO2 ranges and their widths
• the identification of the selected emission line associated with a window, marked Line
• the IF (Intermediate Frequency), sometimes in Hz sometimes GHz
• the bandwidth, in MHz
• the Resolution that the selected bandwidth produces, in kHz
• the molecular transitions in this vicinity, marked by vertical bars, on a line labelled Emission Lines
• the frequency scale immediately below Emission Lines is the frequency in the rest frame of the source i.e. the emission lines show at their nominal laboratory frequency regardless of the doppler shift.
• the line selected is shown in red
• the receiver temperature (TRx) is plotted as the red line (although there is no scale currently). It's wise to check this since even a minor change in radial velocity can shift the observed frequency into a region of very large atmospheric opacity.
• the FE Frequency scale, shown below the plot of TRx is the observing frequency in the rest frame of the receiver i.e. the tuning frequency at the telescope. You will notice this scale will shift if you enter a non-zero velocity, and will also shift if you change the velocity definition.
• the Local Oscillator frequency, shown as LO1, located within the full tuning range of the receiver.
  • Note the upper frequency scale, lower frequency scale and LO1 are only approximate because the correction from the receiver rest frame to the chosen rest frame requires the Doppler correction for the Earth's motion, which varies with time.

Among the dynamic functions, we find that

• the positions of the windows can be changed by clicking on the window sliders and dragging . . . Note how the LO1 and LO2 (ie the IF) both change
• the bandwidth can be changed by clicking on the pull-down menu
• clicking anywhere along the 'Emission lines' level will identify the nearby lines.
• keeping the mouse button depressed while scrolling down through the menu makes the line appear in green
• The LO1 frequency can be changed by right-clicking on the slider and dragging. This changes both frequency scales and the line rest frequency. Moving the LO slider can be a convenient means of choosing a band centre frequency such that particular spectral lines are included or excluded from your passband.

The above example shows a setup with three 250 MHz windows and one, lower resolution, 1000 MHz window for receiver A (for HARP only two windows are possible). Of the 250 MHz windows, one is centered on a line in the LSB, two on lines in the USB. The line-id for the top window is 'greyed' because it corresponds to the observing frequency specified in the main Het. Setup component and it 'anchors' all windows. Moving the slider of the top window will also slide all other windows. By contrast the remaining three windows can be moved independently from one another within the IF band.

To generate such a multi-line, multi-Subsystem configuration:

• In the Front-End configuration window, select a value of Sp.(ectral) regions greater than 1. The Frequency Configuration block at the bottom of the screen will show the appropriate number of 'Region's, although initially they are populated with default values.
• Select the first line (for Region 0) as above.
• Press the Frequency Editor button to generate a Frequency Editor window with the appropriate number of Subsystems. The pre-selected line is in red.
• Click at the level of 'Emission Lines' in the general vicinity (frequency space) of the next desired line
• keep the button pressed and scroll down until the line is met, and shown in green
• click on the 'Line' box appropriate for that subsystem to center the spectral window and identify it with the selected line.
• Repeat (from Click) for further lines if necessary
• Press the Hide Frequency Editor button to transfer these data to the Heterodyne Setup window

Note: Please make sure that the window is anchored on a line if it is intended to be centered on that line. While the observation will proceed with a 'no line' label, the line identification is important for archival purpose.

DRRecipe Component

With the rapid data acquisition rates now afforded by HARP and ACSIS, reducing data by hand has become a very slow and tedious process. Automatic data reduction pipelines have become a powerful tool for carrying out these repetitive tasks. ORACDR has been developed for reducing and combining JCMT (and UKIRT) data, primarily for producing "preview" images for assessing data quality at the telescope, but can also be used for the final reduction. ORACDR uses recipes to carry out all the usual reduction steps that were once carried out by hand. Note that these recipes are still under development and thus are likely to change and evolve.

The DRRecipe component is used to specify the recipe that is to be used by ORACDR for reducing the data. The example shown in the figure above (see Elements of an MSB) shows the DRRecipe component below the HetSetup component, although it need not be positioned exactly here. (Note that the DRRecipe component needs to be paired with a HetSetup component, however.) Open the DRRecipe component by clicking on it. The bottom portion of the window displays a list of the available recipes, with a very short description of each. (See below for a fuller description.) Click on the desired recipe, then click the "Set" button in the top portion of the window corresponding to the desired observing mode.

Recipe names primarily reflect the method used for fitting a baseline, since how this step is carried out differs depending on the line profile(s) expected. Effectively the recipe chosen tells the fitting routine how to deal with baseline features. To illustrate this imagine a narrow spike on a broad profile: this can be a narrow line on a broad baseline wiggle, a noise spike on a broad line, or a core plus outflow profile. This process is tricky in this sense: the baseline fitting can easily remove a feature of interest from the preview image. Observers should ALWAYS go back and inspect the raw data carefully to assess the accuracy of the preview images and pipeline reduction.

Note that for all observations the outer ~5% of channels on either side typically are too noisy to be useful and are excluded from the reduction described below. Typically, smoothing and clump-finding algorithms are used to exclude emission free regions from the total intensity and intensity weighted velocity collapses. These moments maps are thus derived based on the line-widths of each spectrum. The exact methods differ from recipe to recipe and are still under investigation and development. Specifically the is aim to provide the observer with useful preview images that are representative of their data.

The following standard reduction recipes are currently being developed for use with the ORACDR pipeline, with an emphasis on scan-map data. An outline is provided below, but expect changed and redesigns as we gain experience. The main purpose of the routines is to

• a) fit a baseline to the spectra and
• b) select emission from the background

• reduce_science_gradient
  • Intended for nearby galaxies and sources which have a significant velocity gradient (rotation) across the field-of-view. Assumes linewidths in excess of 25 channels (~10 km/s @ BW=1000Mhz and ~1 km/s @ BW=250MHz). Baseline regions are determined for each spectrum individually.
• reduce_science_broadline
  • Intended for extragalactic and other sources with very wide lines. Linear baselines are fitted to the outer 1/6 of the spectral range of the coadded cube (after truncating the outmost ~5% of the channels on each end). The integrated intensity and intensity weighted velocity map are derived from the inner 2/3 of the spectral range.
• reduce_science_narrowline
  • Intended for Galactic sources with not much of a velocity gradient across the field of view. Baseline regions are determined from a global average spectrum. This single set of regions is used to fit the baseline for all spectra in the cubes.
• reduce_science_lineforest
  • Intended for spectral-line surveys, or where multiple (narrow)spectral lines are expected within the spectral range.
• reduce_science_continuum
  • Intended for continuum sources, or for absorption line spectra in front of a continuum source. No baseline subtraction is done.

Site Quality

Your TAG normally allocates your program a certain range of tau (opacity), so you probably don't need to change anything here unless your program is using multiple receivers and the TAG-allocated tau range is not sufficiently specific for this MSB. You should specify an appropriate range of tau to match the demands of the receiver to be used. But beware: the resulting range of tau in which the MSB can be observed is then the intersect of the TAG allocation and your input: make sure this is not an empty set ! For example, if the TAG allocation for your project is weather band 4 (tau(225GHz) between 0.12 and 0.2) and you request a tau range between 0.1 and 0.12, then your project will be picked up for observation ONLY if the tau is EXACTLY 0.12.

The default value of the sub-millimetre seeing is "Don't Care".

It is useful to set the "τ for noise calculation" value, however, to the average or expected value of tau during which your observations are expected to be obtained. (Note that the OT does not know what τ range was allocated for your project.) The OT uses the value in this field to calculate the estimated RMS of the observation specified in the phot/sam etc. "eye"-cons.

Scheduling Constraints

The Scheduling Constraints component allows one to restrict or broaden the time over which an MSB will be selected. To insert this component, click on the Component button on the left side near the bottom of the OT window, and select the "Sched. Constraints".

The main reason you might want to insert this component is if your science targets have a very low or very high declination (i.e. δ < ~-35^o or δ > ~+80^o). By default, MSBs are only selected when the sources are above about 30^o elevation, so low- and high- declination sources may never be selected unless you specify a lower minimum elevation. Similarly, sources at ~19^o declination transit almost directly overhead, and you may wish to restrict such sources to be selected when they're not at extremely high elevations.

Another reason to insert this component is if your observations are time-critical, i.e. they must not be done before a certain date or after a certain date. This can be specified by changing the Earliest and/or Latest Schedule Date.

You may also need to use this component if you have a project which requires periodic observations, for example if you were monitoring a variable source. In that case, you can specify the repeat time in the "Reschedule every..." box. The MSB will be reactivated at the appropriate time.

The Observation component

The first element inside the Observation component can be the "Target information" component. Note, however, that in the templates the target component is outside the Observation component. This allows one to construct an MSB to perform multiple observations on a single target: such MSBs would therefore contain multiple Observation components. More importantly for heterodyne observations it allows the Het. Component to access velocity information that may need to be obtained from the Target Component. Further details on this issue of component nesting may be found in the section on advanced topics.

Following the Target component in the example is a sequence iterator which contains a repeat iterator, then (depending upon the observation type) perhaps a chop iterator or an offset iterator - a position-switched scan centred on the Target, as in this example, needs neither - and then, finally, a single (action) "eye". The sequence iterator doesn't do anything per se, but it is important to note that it represents the sequence of events at the telescope. The repeat iterator allows the user to control the number of repeats carried out for a certain observation. The "eye"-con represents the observation itself.

The "Flag as calibration" button should be toggled ON if, indeed, this observation is intended as a calibration for others. This fact is then made known to the TSS who may decide not to execute the observation (er . . calibration) if a suitable calibration has recently been done already.

The target component

In this component you will enter the coordinates and velocity of the source, and the coordinates of the reference position (for position-switched observations). The "Target Type" is a pull-down menu that allows you to select RA/Dec (most common), Orbital Elements (for comets, etc.), or a Named Planet. The window changes for each selection. For RA/Dec targets, enter the R.A. and Dec. in the appropriate boxes, using colons as separators. One of several different coordinate systems can be selected with another pull-down menu, such as J2000, B1950, etc.

It is recommended that one specify the radial velocity of the source in the target component, either as a radio, optical, or relativistic velocity, or redshift (z). Note that if a velocity is specified in the Het. Setup component, this overrides velocities specified in the target component.

To specify a reference position, click on "REFERENCE". The default is to enter the offset (in arcseconds) from the source position, but by unchecking the "Offset" button, you can enter an absolute position for the reference. NOTE: the reference position is required for position switch observations as the location for the sky reference. If present in beam-switched observations, the three-load calibration will be carried out at that location.

Instead of entering the source coordinates manually, you can download them from the Web. Assuming you have an Internet connection you can enter the name of your target, select the on-line catalogue to search (i.e. SIMBAD or NED) from the pull-down menu, and hit "Resolve Name". After a brief pause the RA and DEC co-ordinate fields should become populated with the SIMBAD/NED co-ordinates for the source. The SIMBAD/NED name for the source will also be indicated next to the "Resolve Name" button.

WARNING: Positions of astronomical objects are often wavelength dependent. For example, the SIMBAD coordinates for IRAS16293-2224 are many arcminutes different from the JCMT coordinates. The catalogues you may be using will have defined the astrometric positions therein on the basis of optical- or infrared- or radio- observations. You should check that the positions now showing are valid for your submillimetre studies.

Moving Objects

Top: Orbital elements as found, in this example, on the NASA JPL NEO orbit diagrams page
Bottom: Elements specified in the Target Component Elements tab. Note the different format for Epoch and Time of Perihelion Passage, in particular.

The OT can use the Horizons database to resolve a given minor planet's name into elements. An example is given below.

Name resolution for moving objects: When "Orbital elements" is the selected Target Type, the "Resolve name" button queries the Horizons database for elements. Note that the name format is somewhat picky and you may have to use trial and error to get something that works for a particular target. Also note that the OT uses a resource at JPL which is intended for human reading, not computer processing. As such the output format is subject to arbitrary changes, so there is no guarantee that this search feature will work or that the results are accurate; for example it is possible that elements may end up in the wrong field or omitted completely. Always double check the returned values.

Allowing for Proper Motion

Proper motion entered as milliarcseconds per year in the target component. These values are taken into account when slewing the telescope.

But wait - that's not all.

The position editor

A whole new window will pop up - this is the position editor. Looks a bit boring at the moment though, with just a small green crosshair in the middle. Let's make it a bit more interesting - go to Catalog menu and into the Image Servers item and chose a Digital Sky Survey near near you:

It's now full of stars. The display application, by the way, is is based on JSky, for those familiar with it.

If you're having problems with the position editor, the most likely explanation is that your version(s) of JRE, JAI and/or java may not be sufficiently up-to-date; check the download page for more information. If you are behind a firewall, you may need to check your proxy server settings.

The DSS may not be the most useful survey for planning sub-millimetre observations, but you can read in any FITS image with an appropriate WCS header using Open under the File menu. You can also fetch a FITS image over the Web. The screenshot below shows the SCUBA commissioning scan map of W48 (image courtesy Tim Jenness). If you have an Internet connection, go to the File menu, select "Open URL...:" and type in the following URL: http://www.jach.hawaii.edu/software/jcmtot/w48.fits or download the image by clicking on the link and and use File->Open to read it in. Notice that the RA/DEC position fields of the target component must be set reasonably near to the world co-ordinate of the FITS file otherwise the image may not load properly.

You will then see the DSS image replaced with the sub-millimetre image.

A short technical note if you are planning on generating your own image for import: if you want to import a SCUBA map in NDF format, convert it to FITS by using the Starlink convert utility with the following arguments:

ndf2fits encoding=FITS-IRAF bitpix=32 comp=D

If you have difficulty despite doing this, let us know.

As was mentioned before, the green cross-hair is the position of your science co-ordinates. Click on the button on the left side of the position editor entitled "Sci Area". The circle that is drawn is the HPBW of the chosen heterodyne receiver.

Now for the useful thing mentioned earlier: While leaving the position editor window open, go back to your science program window and click on your chop iterator that is inside your science observation. Now look at the position editor. The chop beams and the area in which they will rotate during integration are drawn. Well, that's no good - we're chopping onto bright stuff!

This is where the position editor comes into its own. We are going to use it to specify where exactly we would like to chop. In the chop iterator, use the drop down menu to change the chopping co-ordinate frame from AZEL to TRACKING (i.e. RA/DEC). You will notice that in the position editor the uncertainty circles have disappeared, since we will always chop in the same position in the sky:

Now back in the position editor, click on the Drag button on the upper left side of the window. Then click on the centre of one of the chop beams in the display and drag it away from the emission:

You will note that the values in the chop iterator have automatically changed to reflect the new chop throw and angle values. Neat, eh? Remember that by default, any calibrations for your observations will be performed with the same chop as the science observation.

You can also click on the target component and use Drag to change your science co-ordinates if you wish.

The chop iterator

The chop iterator has a list of chop configurations (in this example only one 60 arcsecond chop). Each chop needs to be specified by a throw, an angle, and co-ordinate frame of the angle. In this example the frame is AZEL (Azimuth Elevation) but other common options are TRACKING (RA & Dec) or FPLANE (Focal Plane). A 60 arcsecond chop with an angle of 90 degrees AZEL (i.e. 60 arcsec in the azimuth direction) is typical for beamswitched observations of compact sources. If the source is extended, one might want to increase the chop throw, or if the source was elongated in one direction on the sky, chop in a rotated direction relative to the RA/DEC frame. The maximum recommended chop throw is 180 arcsec. There is a software limit of 240 arcsec. Chopping with large throws is less efficient than using small throws and also there will be a slight distortion of the beam due to observing off-axis. Clicking on Add one can add chop configurations. Each configuration will result in a separate observation (and data file).

The offset iterator

It's often very useful to be able to specify a position with the Target component, but to take an observation at a position offset from that specified. That can be done with the "offset" iterator. In addition, with Grid maps, a whole series of offsets can be specified in either a regular or irregular spacing.

If an offset iterator is not already specified in the MSB, one can insert it below the Sequence iterator , usually just above the stare "eye"-con . Click on the iterator below which the offset iterator is to be inserted (the 'chop' iterator in the image above), then click on the iterator menu on the left-hand panel of the OT window and select the offset iterator. Be sure to "drag" the stare eye-con to the right, so that it is indented with respect to the offset iterator (see figure above).

The offset is specified in arcseconds in the 'p' and 'q' boxes. Note the orientation of these axes to the left of the 'Title' box. In an unrotated frame, 'p' is in the positive r.a. direction (i.e. east) while 'q' is in the declination direction (i.e. north). If a rotated frame is required, enter the position angle (measured east of north) in the 'PA' box. If you then click on the 'Display Derotated Offsets' button, the specified offsets in the unrotated frame will be briefly displayed in the table above.

To add more offsets, click the 'New' button, and enter the offset in the 'p' and 'q' boxes as above. Repeat for as many positions as required.

A regular grid can also be specified using the 'Grid Pattern' section of the window. See the section on Grid Maps for a description. Note that all offsets at the JCMT are defined in the Gnomonic (TAN) projection.

The stare eye

The Continuum Mode box can be checked if you need an accurate measurement of the continuum level. Typically this only makes sense when doing a BMSW observation, because the switching time for PSSW observations are too long for a reliable determination of the continuum. For BMSW observations, the effect of checking 'Continuum mode' is to run the chopper much faster. This will result in higher overheads which can significantly increase the time it takes for the observation to complete. Hence, this mode should only be used when really needed. As a concrete example: the maximum time between chops typically allows for the completion of a full 9x9 jiggle with 0.1sec step, i.e. 8.1 secs. The chopper will then spend ~3 secs in the off-position, resulting in an 11-sec cycle plus some overhead. In continuum mode the chopper will move to the reference every 0.1secs and spend equal times in the 'on' and 'ref' position. A 9x9 jiggle will thus take 16.2 secs plus significantly more overheads due to transit times of the chopper. Limited testing so far suggests that the typical penalty for using continuum mode is an increase of the observing time by a factor of 1.7.

The repeat iterator and integration time in general

The repeat iterator acts on things that are inside the repeat iterator (i.e. indented under it) in turn. So if you set it to 2, your science observation consist of two spectral line samples; i.e you will end up with precisely two spectral line observations. If you require a long integration of a source, it makes sense to break the observation down into shorter integrations of around ten minutes or so, so that data quality can be assessed as the observation progresses. In practice, you set up the integration time required per spectral line observation using the stare eye and then use the repeat iterator to define a sufficient number of repeats to give the total required integration time.

Now you can see how your time usage builds up. Working our way inside out (and bottom to top):

• The stare eye is set to an on-source integration time (and shows you the estimated RMS noise) which will end up in a single file.

• The Repeat iterator multiplies the number of times the sample is carried out. If you increase it from the default of 1, don't forget to take that into account when calculating how deep you will go.

• The observation component shows you the estimated time of the sequence it contains.

• The MSB component will give you the estimated time of the observations inside it. For the heterodyne OT, you only need to define MSBs for your science observations. Obviously, before the observation, the TSS will tune the receiver (when necessary), focus the telescope, and do a pointing observation. Specific calibration instructions, for example, the observation of a spectral line standard and/or a planet should be described in the note attached to the MSB. Be sure to remember to click the show-to-observer tick box for the note. In conclusion, the estimated time for an MSB only includes observations of the science target - one should remember there will be instrument set up and calibration overheads on top of this when planning your observing program.

• The Science Program will give you the estimated time of all MSBs inside it including the number of repeats. So if you have a 1hr MSB (as in this example) and you set the MSB counter to 2, your MSB estimated time will be 1hr and your Science Program time will show 2hrs. Remember that different MSBs may be observed on different nights, whereas observations within a single MSB will be observed within a aingle night.

Adding information in "Note" elements

• A note for general remarks concerning the program
• A calibration note for specific instructions for the calibration observations. In the future we hope to be able to replace this note with better methods for including calibration observations.

The note component consists of a text window where you can include textual information, with the option of showing the note to the observer and TSS at the time the observation is made. If you click on the note icon, the right hand panel allows you to enter the name of the note and the main text body of the note. The "Show to observer" check-box should be checked if you want the note to be visible for the observer/TSS at the time of observation. It is strongly recommended that each of your MSBs contains at least one "Show to observer" note. All of the information about the observation not covered by other elements of the MSB should be included in "show to observer" notes. This would be much of the material which would have previously gone into the "Overall Strategy" and "Observing Cookbook" sections of heterodyne observing templates. It is important to note that calibration details are not yet explicitly included in heterodyne MSBs, so the observer should spell out the required calibration observations in a "Calibration" note.

Setting the "Show to observer" tick box brings up a "Completion Parameter" and "binning" field. These should contain information which help the observer and TSS to decide when to consider your observation to be complete. By default, your program will be executed with exactly the number of integration and number of repeats as specified. However, where possible, an attempt will be made to optimize the process using the specified Completion Parameters. The detailed logistics of the scheduling may prevent this, but when possible this may save time for your project or guarantee the results. For instance you may want to allow for more time than the MSBs nominally take to allow for the addition of extra repeats in order to ensure the requested rms. Setting a S/N target may save unnecassary repeats, etc. The "Completion Parameter" should include a value for the total expired time, and/or a required signals-to-noise, and/or RMS noise value. Best is to provide all three and whichever one is reached first determines the completion. The assumed bin size, either in km/s or MHz, should be entered in the "Binning" field. The above screenshot shows an example of a possible "Show to observer" for an extragalactic spectral line observation. Be aware, however, that ultimately it is up to YOU, the PI, to determine when the data have sufficient S/N, by downloading and analyzing your data soon after they are acquired and updating your MSBs as required.

Additional notes can be inserted at any point in the MSB from the icon menu and you may have more than one "show to observer" notes in an MSB. For example, you could choose to insert a "show to observer" note above the "chop" element to clarify the use of an unusual chopping scheme.

Finally, there is no need to repeat information in notes that is already included in other components of the MSB. This might include receiver tunings, chop throws, offset positions and science targets. All of these should be set up in the appropriate MSB components. If they are reproduced in notes, there is a risk that changes made to main body of the MSB will not be reflected in the notes, leading to conflicts between the notes and the MSB components resulting in confusion at the telescope.

Summary: Making your heterodyne MSB

• Choose the corresponding MSB template from the heterodyne library and copy/paste it to your science program. Each of these is described briefly below.
• Fill out the het setup component with the frontend/backend details for your observation.
• Set up the site quality component, normally this means simply leaving the "allocated" button checked.
• Enter the target information for your target. This step can be as simple as entering the RA/DEC and epoch. Don't forget there are the useful tools of the Name resolver and the position editor to help you out here.
• Set up the chop iterator. Again this can simply mean entering the chop throw angle and reference frame, or this iterator can be set up via the position editor.
• Set up the stare eye. Remember to enter the total on-source integration time per sample.
• Set up the repeat counter to give the number of repeats of the time set up in the stare eye to get your total required integration time. If you want a total integration of 30 minutes on-source, you should set up an integration time of 5 minutes in the stare eye and set the repeat counter to 6. When observing, you can then monitor the observation by inspecting the data file produced every ~13 minutes, and co-add the spectra as you go along. In general, if you want to build up long integrations, you will want the duration of the basic observation to be of the order 10-15 minutes ("on+off+overheads").
• Add at least one "show to observer" note. This should contain additional information on the overall aims of your science program, details of required calibrations and any other special notices you want the observer and TSS to act upon when the observation is made.
• Click on the outermost level of the MSB. In the right hand panel you should give the MSB a meaningful name. Be aware that observers and operator typically will see only the start of that line in the browsing tool, hence put the most significant information at the start. E.g.
  'IRC+10216 spectral line survey, CO molecule, transition 230.538'
  'IRC+10216 spectral line survey, CO molecule, transition 220.399'
  will look similar without expanding the name field in the browser. Instead
  'IRC+10216 230.538, spectral line survey, CO molecule'
  'IRC+10216 220.399, spectral line survey, CO molecule'
  would be a better compromise. Below the name field, there is pull down menu labeled "Observe". This counter allows you to control how many times you want the whole MSB observed. If you want it done just once, leave this as (1x). You can set the relative priority of the MSB within your science program using the "Priority" pull down menu. Set this between 1 and 99, with 1 signifying the highest priority.

That's pretty much all you need to know to start preparing your MSB. The remainder of this primer covers the Heterodyne MSB library.

The Heterodyne MSB library

• Single-position Samples: this is the most fundamental of the observing modes, in being a single spectrum observed toward at a single position. The sample can either use a position switch (Sample-PSSW) or a beam switch (Sample-BMSW). PSSW observations typically spend of order 30 secs on the source before going to the reference position for 30 secs, but they can accommodate large switches exceeding several degrees. BMSW observations by contrast offer a better sky cancellation by switching much faster between the source and reference (up to several Hertz in 'continuum mode'), but are restricted to a switch of 180" at the most. Also, the telescope will nod: half the time will be spent chopping to one side of the source, half to the other side, requiring both sky positions to be free of emission.

  The offset iterator can be used to observe at a position offset from the one specified in the Target Component. If you specify multiple offsets these modes will be equivalent to a Grid-PSSW or a Jiggle-Chop, although the detailed execution will be different in case of the latter (see below).

• Grids: these observations are Samples carried out at multiple offsets, either in a regular grid or as a series of explicit offsets. The observations from each grid point end up in a single cube, sharing calibrations and possibly offs. The observations at each position can be either position-switched or beam-switched. Position switched patterns are executed as OFF-ON(s)-ON(s)-OFF-OFF-... Grid maps are used where relatively small, sparse maps are required with longish integration times per point.

  (Note: it is possible to do a grid of jiggle-maps by putting a "Jiggle-Eye" inside an "Offset" iterator. However, in this case the jiggles will be executed as fully independent observations at each grid point, resulting in independent cubes.)

• Jiggles: this mode makes a small map by "jiggling" the secondary mirror rapidly over all the points in the map and is similar to Sample-BMSW/Sample-PSSW observations on a regular, pre-defined grid, such as a 4x4 pattern. There is an important distinction: rather than fully completing each position before going to the next, Jiggles will combine sequences of short integrations at all positions and possibly cover the whole jiggle pattern many times to build up the integration time at each point. Thus, it will typically observe multiple source positions before going to the reference position for a relatively longer time, although this is configurable through the Separate offs tick-box.

  Jiggles are new to heterodyne observing at the JCMT, and specifically designed for HARP although useful for single receptor receivers as well.

• Scan-PSSW: This mode is useful for making large maps rapidly. The telescope continuously takes data while scanning across the source in e.g. a back-and-forth pattern. It will typically go to the reference between rows and an interpolation of the references on either side of the row will be used for each of the samples in the row. This mode is much more efficient to map large regions than Grid maps.

A detailed description of the various observing modes, rms noise, and durations is available as a document Heterodyne Obsmodes. Even though the information is not required in order to set up a science program, experienced as well as novice JCMT observers are encouraged to read the document as a general background. The discussion below concentrates on the mode-specific components in the JCMT OT.

Centering the observations

For HARP the situation is more complicated. Firstly, it has 16 receptors, each of which in principle can be aligned with the target position for receptor-centered observations. But, secondly, and more to the point, the 4x4 receptor array does not have a center receptor. For regular grids or jiggles this results in maps with even dimensions, such as 16x16. The center of the map will thus fall between the center four pixels. Since the output pixels are the locations observed by the receptors, this also means that center-position of such a map will not have been observed precisely by any receptor. For fully sampled maps the consequences should not be significant, but for user-defined sparse grids they can be disastrous since possibly no receptor may come even close to the target position.

Consequently, for HARP there are three possible alignments:

• receptor-centered: the target position is placed on one of the center-four receptors. Since these receptors are offset (15",15") from the array center, this will result in strongly asymmetric maps with the target position displaced 21" towards one of the four corners. Which corner will depend on the elevation of the observations, since it depends on which of 4 possible angles, differing by 90 degrees, the k-mirror will select. Combining receptor-centered observations from different elevations may thus result in a wind-mill pattern with spectra from only 9 of the 16 receptors overlapping in all observations (see figure below). For compact targets smaller than the area mapped by a single receptor this may still be the observing mode of choice, since the whole source will be mapped by a single receptor. Which of the center-four receptors will be the "tracking receptor" is set in a telescope configuration file and is in principle not user configurable. All patterns with names like NxN (3x3, 4x4 etc.) are receptor-centered.



 

• array-centered: the target position is placed in the center of the array between the center-four receptors. The resulting map will be symmetric around the target position and independent of the K-mirror orientation. However, none of the receptors will align exactly with the target position, which may not be an issue for an extended source, but will probably be a serious problem for a sparse grid of a source with a point-like core. In case of a fully sampled observation using one of the special HARP jiggle patterns this mode is referred to as map-centered since the target position will be between the center-four pixels in the output map which will be either 7.5" or 6" apart depending on the pattern selected. The map-centered harp jiggle patterns are named harp4_mc and harp5_mc in the JCMTOT.
• pixel-centered: the target position is placed on one of the center-four pixels of a jiggle map, slightly offset by half a pixel in each direction from the center of the map. This mode is only available when using one of the special HARP jiggle patterns and is the counter-part to the map-centered observations above. As is the case for the receptor-centered map, discussed above, the map will be asymmetric around the target but only by one map pixel. A wind-mill pattern resulting from the K-mirror flipping by 90 degress will only affect a 7.5" or 6" rim along the edges of the map. The pixel-centered harp jiggle patterns are named harp4 and harp5 in the JCMTOT.

  The figure below illustrates the pixel-centered and map-centered centering for the HARP jiggles: the cross shows the location of the target position in the output map. Note that the HARP4 and HARP5 pattern, shown here offset to the top-left corner, can be offset to any other corner depending on the elevation of the observation. For receptor-centered grids the cross would be at the same location as the middle of the dark-blue poststamp in the center of the HARP5 images. Another discussion of the HARP jiggle patterns can be found here.

  

   

  Shared or Separate offs

  The concept of shared or separate offs almost exclusively applies to jiggle and scan maps. Offs will also be shared for position-switched grids with very short integration times (≤ 15s), but that choice is not user configurable. Shared offs are also required for scan-maps. However, Jiggles can be configured either with shared or separate offs.

  Shared offs means that the observations at a number of positions in the map, i.e. ons, share the sky reference, the off. This concept originates from traditional scan-map observations where the telescope scans across an extended source before going to a sky reference. By necessity all positions observed along the scan will share the same sky reference or off. Since 50% of the noise observed in a spectrum (i.e. on-off) results from the off this would lead to severe 'striping' of successive rows unless the integration time in the off position is sufficiently increased to drop its noise significantly below the noise of the on measurements. Calculations show that the integration time in the off needs to be multiplied with sqrt(np), where np is the number of on positions in the row. Of course, there is nothing special about scan-maps: the same applies for any observations where a number of points share the sky reference, as is the case for HARP jiggle-maps where the full 4x4 or 5x5 jiggle pattern is observed before going to the off.

  Observations benefit from shared offs in two ways and the more so the more points between the offs (the larger the scan-map). To illustrate, consider 1s observations of 16 positions (which would be a very small scan map). Separate offs would require 32s on the sky equally split between ons and offs. Using shared offs the observation needs only 16+4=20s. But also for each point individually the noise is reduced because of the longer observation of the off. Expressed in a time required to reach a particular rms the combined effect makes shared off observations 4 times faster than separate observations in the limit of many points. More precisely, the factor is ~4/(1+2/sqrt(np)) or ~2.7 for the relatively small HARP jiggle patterns and ignoring fixed overheads (which reduce the factor to closer to ~2). Nevertheless, one night instead of two nights is a huge gain, which is why shared offs are the default for JCHOP observations. The figure below shows the benefits in terms of the duration of the HARP4 jiggle observation, including realistic overheads, needed to reach a certain rms (assumed Tsys = 250 K and frequency channels of 1 MHz). Note that shared/separate offs are only relevant within a single observation, which typically take 10-20 minutes. The figure shows that e.g. a 15min observation using shared offs (blue line) will have the same rms noise as a 33min observation using separate offs (red line). Note that coadding successive observations will decrease the noise with the usual sqrt(nr spectra) factor, irrespective of the shared mode chosen.

  

   

  Of course, the above gain comes at a price: all 16 or 25 jiggle-positions from each receptor are correlated because they have the same sky reference (off). As long as the pixels are kept separate this is not an issue because noise remains dominated by the on measurement and the points can be regarded to be only weakly correlated. However, any operation in which pixels are combined will 'suffer' from this correlation in the sense that it will combine pixels that are not fully independent and the noise will hence not drop with the customary sqrt(np) factor. Note that this both applies to operations that use a spatial convolution (smoothing) as well as total flux measurements that sum or average over pixels such as aperture photometry. In the extreme, when averaging data over the whole footprint of a harp4 observation, the noise will be 1/4th (16 independent receptors) instead of 1/16th (196 pixels) of the noise per pixel. Finally: note that JCHOP continuum mode observations always will have separate offs, because the secondary will chop to the off position faster than it moves around the jiggle pattern.

  In conclusion: if imaging or velocity/frequency data are the primary aim of the observations, shared offs are hugely beneficial and recommended; if accurate flux measurements over extended regions are the primary aim or the final maps will be spatially smoothed significantly, separate offs are recommended instead.

  Samples

  A sample denotes an observation consisting of one pointing per science target. It can be beam switched (BMSW) or position switched (PSSW). If you open up the PSSW sample MSB in the heterodyne library, you will see that it contains the same elements as the BMSW MSB, with the exception of the "chop" component. Opening the "Target Component" component, however, reveals two rows in the bottom right hand window, a SCIENCE and a REFERENCE row. Clicking on one of them allows the user to specify the co-ordinates of either the SCIENCE, or "on" position, or the REFERENCE or "off" position. The REFERENCE position can either be specified as a co-ordinate offset from the SCIENCE position, by checking the "offset" tick-box, or as an absolute co-ordinate, by leaving the "offset" tick-box unchecked.

  Important: If you put or leave a REFERENCE position in the Target Component of a chopped observation (Sample-BMSW, Jiggle-Chop) that position will be used when doing a three-load calibration (hot-cold-sky).

  For all sample modes, the total "on" integration time per sample is indicated in the "Stare" eye. Note that integration time no longer is the "on+off" time, only the "on" time.

  Grids

  Observations of a single science target at multiple positions are generically referred to as Grid observations. Note that the user can define any list of positions and that there is no requirement that the pointings form a regularly shaped grid. Opening the various grid MSBs in the heterodyne library shows they are similar to the Sample-PSSW or Sample-BMSW MSBs except for the addition of an Offset Iterator. This iterator enables the user to specify the required pointings for the observation.

  Suppose the user requires three pointings along the long axis of a galaxy, at a half beam width spacing of 10 arcseconds. Let us assume the long axis of the galaxy has a position angle of 30 degrees on the sky. First enter the position angle in the "P.A." field. The rotated frame has axes labeled p and q, so in order to obtain the required pointings along our galaxy we require (p,q) offsets of (0,-10), (0,0) and (0,10). One adds a new pointing by clicking on the "New" button. One then modifies the offsets via the p and q boxes directly above the "New" button. The highlighted pointing can be removed using the "Remove" button. The "Rm. all" button removes all of the user-added pointings. The arrow buttons to the right of the box displaying the pointings allow the pointings to be re-ordered. The highlight pointing can be advanced one position up or down the pointing list, or to the top or bottom of the list by clicking on these arrow buttons. For our example, the completed Offset Iterator will appear as shown below.

  

  If you want to see what the offsets are in the sky frame (which will be the co-ordinate frame specified in the "target information" element), click and hold the "Display Derotated Offsets" button.

  The Offset Iterator has the ability to automatically generate regularly spaced grids without the need to enter each pointing individually. The "Grid Pattern" section of the "Offset iterator" allows the user to specify the offset of the top-left corner of the grid (i.e. most positive 'p' and 'q'), the grid point spacing, and the dimensions of the grid, in grid points. To create a 3x3 grid, with a ten arcsecond spacing, one would fill in the fields of the grid pattern section as illustrated below, and then click on either "Create/Centre on Base" button, or the "Create" i.e. on corner button. The pointings generated then appear in the pointing list box above.

  Notice that "Create/Centre on Base" chooses the the geometrical centre of the grid for the (0,0) position, and ignores the contents of the "initial offset" fields. The "Create" (on corner) button, on the other hand, will place the top left pixel at the offset specified in the "initial offset" fields. For example, if you want a 2x2 grid with 10 arcsecond spacing at offsets (0,0),(-10",0),(-10",-10"),(0,-10"), then set the initial offsets to p=0,q=0, spacing p=10",q=10", and dimensions 2x2, and click the "Create" button.

  

   

  Jiggles

  Jiggle observations, like Grids, map a region around a source. This mode is primarily intended for array instruments with widely spaced receptors for which 'jiggling' is needed in order to achieve a Nyquist sampling. Nevertheless it is available for single-pixels receivers as well. As opposed to Grids, the chopping secondary is used to move over the points in the pattern. This allows for a fast switching of the position and makes it possible to observe e.g. the whole jiggle pattern before going to the reference position. (In "continuum mode", however, where the chop frequency is much higher, half the time is spent at the reference position.) Jiggle-chops are beam-switched observations, and hence the distance to the sky reference can be no more than 180". Jiggle-PSSW use a jiggle to maps the source, but a position-switch to move to the reference.

  Opening the various chop MSBs in the heterodyne library shows they are similar to the Sample-BMSW example above except that there is a "jiggle eye" instead of the "stare eye". It offers various jiggle patterns: 3x3 through 11x11 plus HARP-specific jiggles (such as HARP4 and HARP5).

  Since any spacing can be specified using the NxN patterns, in order to guarantee that the target falls on one of the pixels in the output map (as opposed to between e.g. widely spaced pixels), these patterns are receptor-centered. Four special patterns are also available for making fully sampled maps with the HARP array receiver. HARP4 & HARP4_mc are 4x4 patterns with 7.5" pixels designed to fill in the footprint, but slightly under Nyquist sampled, and HARP5 & HARP5_mc are 5x5 pattern with 6" pixels, i.e. slightly better than Nyquist sampled.

  • HARP4 and HARP5 are pixel-centered: these will give you fully-sampled maps with the target position on one of the center 4 pixels of the output map. The resulting map will be slightly asymmetric and maps observed at different time may have a different orientation. However, the affected area will only be a single-pixel wide rim along two or more edges of the map.
  • HARP4_mc and HARP5_mc are map-centered: these will give you fully-sampled maps with the target position in the exact center of the output map, between the 4 central pixels. The resulting map will be symmetric around this position.

  For a more extensive discussion see Centering the observations and the figures there.

  For the NxN patterns the "Jiggle Spacing" needs to be defined by the user: typically 0.5 or 1 beam-spacing in arcsecs. The PA and System should be left at "0.0" and "Tracking" for the typical user. Finally, the integration time for each jiggle position can be specified as "secs/jig posn". (Note that this is the total "on-source" time per position.) As for the "stare" eye, the estimated RMS noise per channel is shown in the "Noise" box.

  

   

  "Continuum mode" can be selected as is explained for the "stare eye". Due to the resulting increase of the time needed to complete the observation, by a factor of approximately 1.7, only check this if you really need the measurement of the continuum level.

  Important: If you put or leave a REFERENCE position in the Target Component of a chopped observation (Sample-BMSW, Jiggle-Chop) that position will be used when doing a three-load calibration (hot-cold-sky).

  Scans

  The format of the position switched scan MSB is similar to the sample MSB, except that the "stare" eye is replaced by the "scan" eye. The fields in the "Area" panel allow you control the size and sampling of the scan map. The resulting Nr. of samples/pixels and Nr. of scans/pixels in each dimension is shown. The total required on-source integration time per sample point can be specified in the "Sample time" box. The system will automatically break up the observations into multiple passes over the map.

  Setting up scan maps is significantly different when using a non-array receiver (i.e. RxA or RxW), or an array receiver (i.e. HARP). Read the example below for setting up MSBs for an RxA/RxW Scan Map, and for a HARP Scan Map.

  Making a Scan Map With a Non-Array Receiver (RxA/RxW)

  When using a single-pixel receiver such as RxA or RxW, the "sample spacing" will be the pixel-size along scan direction of the telescope. The "scan spacing" will be the pixel-size in the cross-scan direction i.e. between the scanned rows. Be aware that there will be a pixel centered on the base position only if the number of pixels in each direction is odd. The size of the map must be a multiple of the pixel-size in each direction, and is defined as the distance between the centers of the outmost pixels!

  Suppose, for example, that you want to use RxA to make a 238"x28" map. A sample spacing of 7" and scan spacing of 7" will result in a pixel map of 35x5. Note that it is customary to oversample the scan-direction to counter-act smearing, although this often requires a regridding of the final map to recover the intended rms/pixel.

  

  The sample time box allows the user to set the amount of time data is taken for for each map "point" as the receiver is scanned across the sky. The RMS noise per channel of the final co-added map is also indicated, as is the estimated time to complete each row and the complete map.

  It is possible to set the scanning direction, but for most purposes this should be left to the default values.

  Making a Scan map With an Array Receiver (i.e. HARP).

  HARP is a 4x4 square array of receptors, which are separated by 30". To make a scan map, the telescope scans continuously along (by default) the longest axis of the map (defined by the width and height of the map). The array is rotated by 14.04º to the direction of the scan. This results in a fully sampled map, with 7.3" pixels. Thus the HARP sample spacing is set by the OT to 7.2761". At the end of a scan, the array is shifted up by the scan spacing and the telescope makes another scan across the area to be mapped. A pull-down menu allows one to set the scan spacing, so that one can step by the full array (i.e. no overlap of pixels during subsequent scans), by 3/4 array, 1/2 array, 1/4 array, 1/8 array or by a single pixel (i.e. a shift of 7.28").

  

  Effectively the largest single map one can make is one square degree. Larger maps can be made, but they should be broken up into smaller pieces, one piece per MSB, with an offset iterator for each piece. Note that mosaicing such large maps will likely tax most computing systems beyond their capacities.

  In the lower left panel of the SCAN window, one can specify the direction and orientation of the scan, using two pull-down menues, PA and System. For all but very special applications, the System should be left in its default, TRACKING coordinates (e.g. R.A./Dec.). If PA is left in its default state (automatic), then the telescope will scan along the longest axis of the scan box defined by width, height, and PA specified in the Area panel. One can, however, set the scan direction to any arbitrary value, by selecting user def from the PA pull-down menu, and typing in the required scan direction in degrees, where PA=0 is north and PA=90 is east.

  If the scan map is to be repeated, a useful technique is to "basket weave" the scans, i.e. make a scan map by scanning along one axis of the map, and then repeat by scanning along the other axis. This minimizes striping in the resultant map. One way to do this is to edit a science observation for a scan map, set the parameters as required, then copy the entire science observation and paste the copy below the original. In the original scan "eye"-con, set the SCAN PA (lower left panel) to Along Width. Do the same in the copy scan "eye"-con, but set the SCAN PA to Along Height.

  Sample Time: The sample time is the total integration time per pixel in the completed map. The minimum time depends on the setup of the backend. It is about 0.3 seconds with 8192 channels (e.g. single 250 MHz band), but can be as low as 0.1 second with 2048 channels. There is no physical limit for the maximum sample time. However, it should be noted that the telescope does not check the reference position until at least one row is completed. Thus one should make sure that the total time per row is less than about 60 seconds (or 120 seconds under stable weather conditions), and the total time for the MSB is no more than 1.5 to 2 hours at most. For large maps the integration time per pixel (sample time) should be set to a small value, e.g. 0.3 seconds per pixel, and then repeat the MSB to build up integration time.

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