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Up: CGS4Handbook

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How to write EXECs

Almost all CGS4 observations are taken by running an EXEC. EXECs are ascii files which contain high level commands to control the telescope and instrument and take data. You can write an exec to do an entire nights observing, including calibration observations, loading source coordinates for the telescope operator, taking data while nodding the telescope along the slit or to blank sky and instructing the data reduction system to use specified reduction sequences. However since observing usually has natural breaks, e.g. to check finding charts, or fine tune the telescope pointing on near-stars, it is most efficient to write several execs and then modify them as the night progresses. Most observers write an exec for flats, arcs, darks (which can be run while the operator slews the telescope) and one or more for standard stars and sources. This method keeps the Exec to a readily editable length.

Execs can be nested - for example there is a standard pre-defined exec called "quads" which does two pairs nodding the telescope between object and sky. If you want to do 10 pairs your exec can say

DO 5 QUADS.

There are commented examples of EXECS at the end of this document.

1. How to write an EXEC :

Enter the name of the EXEC you would like edit. The name does not need any extension. This can be either the name of a pre-existing EXEC, or any name you choose for a new EXEC. If you wish to edit a pre-existing EXEC and save it with a new name then enter an output name in the second row of the menu. If this name is left blank then the output version of the EXEC will be saved with the same name as the original. If you are new to CGS4 observing it helps to start with one of the system Execs detailed below. After entering the names use the PF1 key to exit the menu. Make sure the names do not have any leading spaces in them.

This puts you into the EDT text editor. Now edit a file containing your EXEC commands.

If you are unfamiliar with the Vax editor the following may help. You can move around with the arrow keys. Use the return key to move to the next line or insert a line. The PF4 key can be used to delete a whole line. EXEC commands are not case sensitive.

When you have finished editing your exec, exit from the editor by doing control-Z.

2. The SYSTEM EXECS

Two SYSTEM example execs which contain examples of EXECs to do calibration observations and source observations have been written. For normal observing you can make your own versions of these using your chosen CONFIGS, number of pairs to observe, etc. These EXECs are called EXAMPLE_CALIBRATE and EXAMPLE_SOURCE and they contain the following commands:

! EXEC example_calibrate
CONFIG EXAMPLE_K
FLAT
ARC
SET OBJECT

! Example_K, for observing point sources in the K-window, does 4 pairs.
CONFIG EXAMPLE_K
SET OBJECT
BREAK  (to peak up in the slit)
STARTGROUP
DO 2 QUAD_SLIDE
ENDGROUP

The EXEC QUAD_SLIDE reads the position angle of the slit on the sky and nods the source a specified number of arc seconds up (in detector rows) the slit, and it contains the following commands

!QUAD_SLIDE nods along slit 26 pixels
object
slide slit 30.5
sky
sky
slide slit 0
object

If you want to nod more or less than 26 pixels make your own version of QUAD_SLIDE by editing it from the EDIT_EXEC menu. Note that a positive value in "slide slit" will move UP the slit in row numbers.

If you are using the Echelle, the pixel scale is a strong function of grating angle and in this case it is best to measure the nod distance for your configuration and then edit QUAD_SLIDE accordingly.

The "slide slit" command will check whether or not the autoguider is turned on. If you are autoguiding then to execute the nod it will turn off the autoguider, move the crosshead and the telescope the specified amount, turn the autoguider back on and wait a second or two for the image to settle. If you are not autoguiding then only the telescope is moved appropriately.

The EXEC QUADS can be used to observe extended source by nodding the telescope to blank sky. The telescope operator can define any distance and angle of nod for you. If you want to nod to blank sky be sure to tell the TO where you want to nod to. The EXEC contains the following commands to nod the telescope.

! QUADS nods to sky, autoguides on source position only.
object
tel offset
sky
sky
tel main
object

The EXEC QUADS_NOGUIDE is the same as QUADS except it assumes that you do not want the autoguider turned on at all.

3. Useful EXEC commands

The most commonly used exec commands are:

CONFIG name - recall the config called name

FLAT - drive the motors to the positions specified by the most recently recalled config, and take a flat observation using the sampling, lamp and exposure times defined in the flat information in the config.

ARC - drive the motors to the positions specified by the most recently recalled config, and take an arc observation using the same sampling as for your object and the lamp and exposure times defined in the arc information in the config. If the Echelle is in use and the config contains a different CVF position for the arc then this will be set as well. If you are taking low resolution data with a central wavelength of 2.8um or longer then the blocking filter will automatically be set for observation of the 1.4-2.5um lamp lines in second order.

SET OBJECT - drive the motors to the positions specified by the most recently recalled config, and set the corresponding exposure times/coadds/sampling for the object etc. Does not start data taking.

OBJECT - take an object observation (after driving the motors to the config positions if necessary).

SKY - take an observation as above for OBJECT, but flag it as SKY for the data reduction.

BREAK - interrupt the exec here and wait until told to resume. When a break is executed the terminal will beep.

STARTGROUP - flag following observations as belonging to a group of object-sky pairs. This is necessary so that the data reduction recognises that it needs to subtract observations in pairs and then add the pairs.

DRCONFIG drconfigname - tell the reduction system to use a previously defined reduction configuration.

BIAS - drive the motors to the positions specified by the most recently recalled config, put the blanks in and take a bias observation using the default bias settings. These are exposure time 0.12 seconds, 30 exposures per integration and 3 integrations per observation. Bias observations can only be taken in STARE mode. The CGS4 bias level is very stable - you do not normally need to take bias observations other than the one that its taken at the begining and end of the night when the array_tests EXEC is run.

DARK - drive the motors to the positions specified by the most recently recalled config, put the blanks in and take a dark observation using the same exposure time, exposures per integration and integrations per observation as defined in the CONFIG for object observations. If you are observing by nodding CGS4 for background subtraction you do not normally need to take darks, because the dark current is also subtracted when "object-sky" pairs are formed.

Full details of all the EXEC commands are given in the EXEC documentation included in the handbook, kept at the summit, HP and JAC.

4. Examples of EXECs.

There are three examples of EXECs documented here. Each command in the exec is preceded by a comment line, marked by !, explaining what it does. Two of them are the system EXECs, Example_calibrate and example_source and the third is a modified version of example_source to show how you might change it if you wanted to observe a very faint source by nodding to blank sky instead of along the slit.

4a) Sample exec for taking calibration observations.

! EXEC example_calibrate
! Recall the configuration defining the desired wavelength and 
! exposure times etc, in this case for low resolution K spectra.
CONFIG EXAMPLE_K
! take a flat using this configuration
FLAT
! take an arc using this configuration
ARC
! set up ready to take object observations.
SET OBJECT

The last command gets CGS4 all set up ready to take data or for the TO to peak up. It also makes sure that the lamps get turned off.

No dark or bias observations are taken because they are not necessary for observing object-sky pairs or for the flat/arc because the exposure times for these are short and the dark current is <~ 1 e/s.

4b) Sample EXEC for taking observations of a relatively bright star.

! System example of an Exec for observing point sources
! in the K-window, does 4 pairs.
!
! Recall the configuration defining the desired wavelength and 
! exposure times etc, in the case for low resolution K spectra.
CONFIG EXAMPLE_K
! set up ready to take object observations
SET OBJECT
! stop the exec so that you can peak up
BREAK  (to peak up in the slit)
! tell the data reduction to start a new group of pairs.
STARTGROUP
! Use the EXEC that moves between two positions on the slit. Each 
! QUAD_SLIDE does two pairs, object/sky/sky/object. Do 4 pairs.
DO 2 QUAD_SLIDE
! tell the reduction that this set of pairs is finished.
ENDGROUP

4c) Sample EXEC for taking observations of a faint extended source.

! Recall the configuration defining the desired wavelength and 
! exposure times etc, in this case for low resolution K spectra.
CONFIG EXAMPLE_K
! set up ready to take object observations
SET OBJECT
! stop the exec so that you can peak up
BREAK  (to peak up in the slit)
! tell the data reduction to start a new group of pairs.
STARTGROUP
! Use the EXEC that moves between object and blank sky. Each 
! QUADS does two pairs, object/sky/sky/object. 
DO 40 QUADS
! tell the reduction that this set of pairs is finished.
ENDGROUP