Hints on preparing SCUBA programs with the OT
Hints on preparing SCUBA programs with the OT: Frequently Asked Questions |
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This page is intended to serve as a SCUBA-specific FAQ for preparing programs
with the Observing Tool software. However, your first reference tool should
be the JCMT OT primer webpage. Note also that each project has been designated a
"Friend of Project" - a support scientist ready to help you prepare
your program and answer any queries you have. They (and their email address)
can be found on the Time
Allocation page for your project.
Contents
General issues
- Do I need to define a calibration target?
No. When your calibration observation is being performed the SCUBA software will ask the observer to pick the best available source from the list of SCUBA calibrators. You can make a suggestion for a calibrator in the notes if you have a preference, but don't define it as a target. This is because the Query Tool, used at the summit to search the MSB database, checks to see whether all the sources in an MSB are observable before offering it as an option to the observer. This is also the reason why you shouldn't put two sources that are far separated in the sky into the same MSB.
However, it's worth checking out our Guide to calibration while observing to see what your likely calibrators are, since some of them have special observing requirements that you should mirror whenever possible in your science observation. Two of our secondary calibrators are also unuseable at 450 microns; if data at this wavelength are important to you, you may wish to add a note that these sources should not be used.
- How often do I need to point?
Pointing should be done before each new target (including calibrators) and if staying on one source, approximately every hour or less. Hence pointing should be done about once every (3 × 40) photometry integrations, about once every (2 × 10) jiggle-map integrations and once every full scan-map repeat (i.e. once per six individual maps). We also strongly recommend pointing at the end of each photometry MSB, just to reassure yourself that you haven't moved significantly off target; this observation is included in the library photometry MSB.
- Do I need to define a pointing target?
No. On the OT pointing eye page, there is a small square at the bottom of the page marked "Automatic target". If this is checked (as it is by default) then the observer will be prompted for a target just before the observation starts, and the Query Tool even searches the pointing catalogue for usefully close options. Hence an appropriate target can be selected on the spot. If you have a preference or suggestion, it's best to put that in a note. However, if your actual science target is already in the pointing catalogue (which you can see here) then you can uncheck this box and your science target will be used also for pointing - check our coordinates agree with yours!
- Do I need to include noise measurements, skydips etc?
These type of observations can all be inserted by hand into the SCUBA observing queue by the observer, and the frequency at which they are required often depends on conditions and on the observing sequence on a given night. Hence we suggest you don't put these into the MSBs explicitly, but instead describe what you want in your notes (e.g. "Knowing the opacity very accurately is very important for this project so please do skydips every hour").
Photometry
- What chop throw should I use?
(Firstly, see the note above regarding calibration choices, since this may have a bearing on this decision.)
For photometry, this depends on your knowledge of the source. If you know it to be extended in one direction, either chop in an orthogonal direction or make your chop throw large enough (but below about 180") to avoid emission. If your source is expected to be a point source, or you just don't know its extent and shape, leave the settings at the default template values. Since you are not map making it will be difficult to know once you have data whether you chopped onto some emission, although for bright sources looking at the time series of each bolometer may be informative. (See the data reduction pages for further advice on analysing data). Be sure to set your calibrator chop throw to be the same as your science chop throw.
- What's the "wide photometry" option on the Photometry/Sample page?
This is an option which is like a hybrid of jiggle-mapping and photometry. Only experienced users of the telescope should use it, and you should contact us before selecting it if it wasn't explicitly mentioned in your JCMT proposal. Most users can simply ignore this option.
- Do I need to change the chop parameters for a 2-bolometer chopping MSB?
No. By selecting the 2-bol photometry template from the SCUBA library in the OT, the SCUBA component will be set up in such a way as to automatically override whatever is in the chop component anyway. (Take a look in the SCUBA component for the two photometry options in the library, and you'll see that the bolometer list includes the second bolometer on the longwave array in the 2-bol mode - the shortwave array doesn't need to be explicitly mentioned since the bolometers match up on the two arrays.)
Jiggle maps
- What chop throw should I use?
(Firstly, see the note above regarding calibration choices, since this may have a bearing on this decision.)
For jiggle-mapping, this depends on your knowledge of the source. If you know it to be extended in one direction, either chop in an orthogonal direction or make your chop throw large enough (but below about 180") to avoid emission. If your source is expected to be a point source, or you just don't know its extent and shape, leave the settings at the default template values. Once you have a map, you should be able to see negative holes half the amplitude of your source at the chop throw distance to either side - if one or both holes is missing when the noise is low enough to see it, you may not have chopped off source sufficiently. Be sure to set your calibrator chop throw to be the same as your science chop throw.
Scan maps
(Note that the discussion below and the scan-map template in the library are for the Emerson II scan-map technique, which is our default method. If you want to use an alternative method and have questions, please contact your Friend of Project before starting to prepare your MSBs.)
- What chop throws should I use?
The recommended chop throws are 30, 44 and 68" . These replaced the previous set of 20, 30 and 65" when their coverage in Fourier Space was found to be preferable. The direction of your chop throws should be at 0 and 90° to the RA/Dec plane (or TRACKING in the OT). This is the only combination proven to work with SURF (and therefore ORACDR) - if you're writing your own data reduction software though you can use whatever you think will work, providing the two directions are at 90° to each other. Note that, unlike other SCUBA modes, scan-map making does not require that you chop off of the emission, although it is best if your map has at least some regions of blank sky.
- What scan direction should I use?
Generally you should leave the scan direction parameters as you find them in the template. SCUBA will then choose the scan direction in the most efficient way as and when your MSB is going to be observed. However, if you for some reason want to restrict the way SCUBA scans your map area, you must use angles which are a multiple of ((n×60) + 15.5)° in the FPLANE - i.e. with respect to the Nasmyth focal plane that SCUBA sees. This ensures that the array gets suitably sampled in each scan pass.
- When should I use the fast scan template?
The fast scan template should be used whenever you are certain that you care only about the 850 micron data and not the 450 micron data. The differences in the fast scan template are that the scan spacing and the sample spacing are both larger - these are smaller in the 450/850 template to ensure that the 450 micron map is fully sampled. Of course, since the higher sampling rate reduces the noise in the 850 micron map anyway, using the alternative template doesn't save you total time to get down to a given flux level; however the individual maps can be made in less time, which may be preferable if your source doesn't rise very high each night, for example.
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