Appendix 2 - Creating OFFSET_RUN files

NB: The following discussion still holds - some more features however have recently been added. For a description of these added bits, see here.

The OFFSET_RUN file format was originally designed for controlling IRIS (the AAT's infrared camera). It therefore allows control of exposure parameters which are not relevant to CCD imaging -  specifically, the control of `CYCLES' and `PERIOD'.  The parameters you can control are  

  1. DARK exposure or RUN exposure
  2. OBJECT name
  3. Telescope RA offset (in arcsec from starting position)
  4. Telescope DEC offset (in arcsec from starting position)
  5. Offset in TRUE arcsec? (ie. in arcsec on the sky)
  6. Exposure TIME
  7. Number of CYCLES
  8. PERIOD (only for IRIS readout mode 4)
  9. Number of REPEATs

The OFFSET_RUN file is executed from the ICL terminal with the command

ICL> OFFSET_RUN filename

The default directory is DISK$RAW:[OBSERVER], and .DAT is the default file extension. DISK$RAW is specific to the VAX 4000, so you may find it more convenient to create the file on another disk.

This command initiates a sequence of runs. At each run the next line of the file is acted upon. This may involve offsetting the telescope, taking a dark exposure, or changing one of the other parameters. Runs continue until the end of the file. Note that you cannot alter the optical configuration or mode of taking data: these must be set before you begin.

A sample file follows. Each of the paremeters above is spcified by a single word - spaces are ignored, but DON'T USE TABS. Also, if you create it on some other directory ENSURE THAT IT HAS WORLD READ AND EXECUTE PROTECTION; failure to do so results in an error reporting "failure to open file".

! Sample offset run file
! Comment lines starting with `!' are ignored
!                  (")    (")       (s)  
DARK      dark       0      0 TRUE    0     1      1      1
RUN       north      0    100 TRUE   40     1      1      1
RUN       west    -100      0 TRUE   40     1      1      2


This file will take a BIAS frame at the `base' position, then offset the telescope 100 arcsec north of base to take a 40 second exposure. Finally, two 40s runs are taken 100 arcsec west of base.

Things to note here are:

  1. DARK, RUN, FLAT and SKY are not allowed as object names; also avoid names beginning with a digit.
  2. TRUE gives offsets on the sky irrespective of declination; i.e. the telescope moves in RA by sec(dec) times the entered figure.
  3. The CYCLE and PERIOD columns are not used in CCD imaging but must be included nonetheless. The PERIOD column is not used in IRIS readout method 1 must be included nonetheless.

This telescope control mode ensures much more reliable mosaicing and offsetting, and is easier on observers and night assistants, but there is an extra overhead between runs of about 10 seconds compared to conventional observing.

When running under ICL the messages announcing recording transfer of data to the VAX appear on the ICL terminal.

There are some tricks to watch in the event of a system crash when running under ICL. You can abort a telescope offset run by Ctrl C on the ICL terminal. But you should clean up by typing TIDY on the same terminal twice before you try to run again. The telescope will be left where it last offset to, and you should resume the offsetting without driving back to the start position. But if you either log off from ICL (and on again) or slew the telescope, then the offset data sent to the telescope will be reset and you should then resume the next mosaic pattern with the telescope at the start position.

If you do have to restart the entire software, the procedure is:

  1. ICL> EXIT
    40a> LOGOUT

  2. Press the black cold start button on the XMEM.
  3. Ctrl C on the OBSERVER (ie. the right hand) terminal. If nothing happens, wait a second or two and do so again. Eventually you will descend to the server:

    Enter username> type anything polite here

    Local> logout
    Local -020- Logged out port 6

    It is necessary to get completely out of the server before starting up OBSERVER under ICL. Now startup as usual.

Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters

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This Page maintained by : Chris Tinney (
This Page last updated:  29 Feb 1996, by Chris Tinney