Appendix A. OBSERVER Commands

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A summary of the most useful OBSERVER commands is given in Table A.1. The command MAXIMISE displays all the commands available at the current time (this changes depending on what state the CCD is in); MINIMISE displays only the most useful commands in each state. HELP can be used to obtain online help on any command.

Table A.1 OBSERVER commands

Command Brief description Example 
ABORT Stop the current exposure and do not read out the chip   
BIAS Take a zero second exposure with the shutter closed  BIAS
COLOUR Select the look-up table for the XMEM  COLOUR GRAY
COMMENT Add a comment to the data header  COMMENT Hi there
CONTINUE Re-start an exposure suspended with HOLD  
DARK Take an exposure without opening the shutter  DARK 1800
DUMMY Take a non-archived exposure  DUMMY 10
FITSINFO Check the values of the INSTRUMENT and OBSERVER keywords   
GLANCE Take an exposure which is not written to disk  GLANCE 1
HELP Provides help on the usage of any command  HELP REPEAT
HOLD Pause the current exposure; restart with CONTINUE  
INSTRUMENT Set the instrument name in the FITS headers  INSTRUMENT 13
KEEP Transfer a DUMMY or GLANCE exposure to the archival disk. KEEP GLANCE
OBJECT Set the object name  OBJECT NGC7078
OBSERVER Set the observer's name in the FITS headers  OBSERVER Smith
REPEAT Repeat the next exposure n times  REPEAT 5
RUN Take an archived exposure  RUN 1800
SPEED Select read-out speed  SPEED SLOW
STOP Stop the exposure and read out the chip   
TIME Set the exposure time  TIME 1800
WINDOW Select the portion of the chip to read out  WINDOW EEV2_RGO


Setting up the CCD

Several set-up parameters should be chosen before you begin observing.
1. The CCD read-out speed is set by typing
SPEED speed
where speed is one of XTRASLOW, SLOW, NORMAL, FAST and NONASTRO. (Note that XTRASLOW is not available for the EEV2 and not generally used for the MITLL CCDs. The characteristics of the CCDs at the different speeds are shown in Section 2.7.
2. The region of the chip which is read out is chosen using
WINDOW windowname
windowname is a text file containing information on the rows and columns which are being read out, the overscan region, and the on-chip binning (if any). OBSERVER will look in the directory which is defined by the logical name WINDOW_DIR, which is currently given by




Table 2.10 shows popular windows for each CCD when observing with the RGO. Note that time-series observations require special windows (see Appendix B).

If you want to define your own windows, you will need to specify the directory where the window file is explicitly. Your support astronomer or night assistant will be able to help you to modify a window file, which requires access to a VAX.

3. The lookup-table used by the XMEM to display the data as it is read out is changed using
COLOUR colour
where colour is one of GRAY, CONTOUR, FALSE, GREY, or GRJT. GRAY is usually the most useful: it displays the data in greyscale with saturated pixels shown in red, so you can easily tell if you have saturated your data.

Taking Exposures

There are three ways to take exposures: RUN, DUMMY and GLANCE, which differ in where the data is written.
RUN takes an exposure which is written onto the archival disk of the VAX. The data is written onto the disk DISK$INST, into a directory which depends on the CCD controller which is being used and the date, with sequential run numbers. Thus a RUN command will result in a file like You are not allowed to delete data from this disk; if you decide you do not want to keep an exposure, you can set the next run number to be the same as an existing file and a higher version number will be written. This data is visible from the Sun workstation as although note that only the highest version can be seen from the Sun.

DUMMY takes an exposure which will not be archived: it is written onto DISK$DATA and the files look like

which can be seen from the Sun as DUMMY exposures should be used for test exposures which you do not want to keep. If you decide you do want to keep a DUMMY exposure, you can issue a KEEP which transfers the data to the archival disk.

GLANCE exposures are read into the XMEM and displayed but never written to disk; this can be useful for checking exposure times. Again, a GLANCE can be kept using the KEEP command.

There are also some specialised data-taking commands:

BIAS takes a zero second exposure with the shutter closed; the object name is automatically set to BIAS_nnnn, where nnnn is the run number.
DARK n takes an n second exposure with the shutter closed.
SKY, ARC, FLAT, DFLAT (for dome flat) and SFLAT (for sky flat) are all equivalent to RUN except that they set a flag in the header to identify the type of frame, and set a default object name which can be over-ridden with OBJECT.

More Acquisition Commands

Other commands which are useful for taking exposures:
TIME sets the default exposure time to be n seconds. This default can be overridden by explicitly giving a time to the RUN (or DUMMY) command, as RUN 10; thus if you are alternating object and arc exposures, you can set TIME 1800; then using RUN will start an 1800 s exposure, but RUN 1 will start a 1 s exposure. The exposure time can be changed after an exposure has started by giving a TIME command again - though make sure you don't set the time to a value less than the current exposed time!
OBJECT sets the default object name. Note that the OBJECT can be changed after an exposure has started (or during read-out), since it is not written to the header until the data is written to disk. You therefore need to be careful not to type ahead.
COMMENT can be used to record comments to the header. As this is not written until the data is read out, this can be useful for adding information about the exposure, for example
REPEAT repeats the next exposure indefinitely; with a value n, it will repeat the next (or current, if given while exposing) exposure n times in total. The current value of the REPEAT counter can be seen in the central column of screen. The counter can be disabled using SINGLE. Note: do not confuse REPEAT with MULTIPLE, which allows multiple exposures without reading out the chip, and is not normally used for spectroscopy.
SINGLE sets the REPEAT counter to 1, ending REPEAT mode.
NEXTRUN allows you to set the run number of the next exposure; this can be useful if you want your data to be numbered sequentially from night to night (by default the run number starts again at 1 each night), or if you want to supersede a previous exposure.

Stopping Exposures

There are several ways to stop an exposure prematurely:
The exposure time can be reset during an exposure using TIME n, provided you have not yet reached n seconds.
STOP stops the exposure immediately and reads out the CCD.
HOLD closes the shutter and pauses the timer; CONTINUE re-starts it.
ABORT aborts the exposure and does not attempt to read the CCD out. You can do an ABORT even when the CCD has begun reading out.


You leave OBSERVER by typing which will prompt you to see if you want to log out of your login session also. This should be done at the end of every night, since a new directory has to be created for the following night's data.

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Ray Stathakis
Last Updated 22/3/2002