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defineobs is a GUI application for creating or modifying observation definition files.

USAGEdefineobs [observset]
INPUTobservset.obsdef is an existing observation definition file
OUTPUTobservset.obsdef a new observation definition file

New alternative versions of defineobs:

  • -- ships with source/binary (as of v2.20), a command-line version of defineobs, written in Python, and can be run with the standard Python built into any current machine running Linux or Mac OS X. (NOTES)
  • -- available separately here, a GUI version for machines running Mac OS X 10.6.x (64-bit) and later. (NOTES)


defineobs may be invoked either with no filename or with the name of an existing observation definition file for modification. A GUI window is opened:

defineobs gui

The "Mask", "Dewar Offset File", "Grating Order", and "Grating Angle" boxes take typed input. "N&S;" is an on-off button. The other boxes display menu selection when the left mouse button is clicked. The entries should be filled-in or modified from the top down: the choices for "Camera" depend on the setting for "Instrument", and the grating parameters depend on the settings for "Camera" and "Mode". If saving a file with the same name as an existing observation definition file, the existing file filename.obsdef is moved to filename.obsdef% before writing the new file.

We have recently updated defineobs to allow the user to edit an additional parameter, labeled as the "Disperser Misalignment," which will be written as "D_ALIGNROT" in the obsdef file. D_ALIGNROT is the change of the ALIGNROT parameter in degrees, which is defined in the opticdef.dat for each grism. This parameter can be used to correct for rotations of a grism with respect to the CCD. It is mostly needed in LDSS3 data where the instrument can shift slightly, but it can also apply to IMACS data (see Step 3 in the Cookbook).

As of Cosmos 2.20, we have reverted back to the previous layout used in Cosmos 2.17. In Cosmos 2.18 and 2.19, the "Observation Date" field was removed, and the "Camera" drop-down menu contained more options for each dewar used at both cameras. If you are still using either of these older versions, please refer to the tables below for help with the "Camera" field.

f/2 (Short camera)
Date Choose
date < August 2005 f/2 Mosaic1 (SITE)
August 2005 < date < March 2008 f/2 Mosaic1 (SITE2)
date > March 2008 f/2 Mosaic2 (E2V)

f/4 (Long camera)
Date Choose
date < September 2011 f/4 Mosaic1
date > September 2011 f/4 Mosaic3

Notes for is an interactive command-line version of defineobs that asks for the same fields to create or modify an observation definition file. [-o observset]
observset.obsdef is an existing observation definition file
-h print the help file
observset.obsdef a new observation definition file

Here is a sample session with

* Press <Ctrl+C> at any time to quit *
Enter year of observation (e.g. 2012): 2014
Enter month of observation (e.g. '5' for May): 5
Choose Instrument:
(2) LDSS2
(3) LDSS3
Enter number: 1
Enter Mask: MaskName
Enter Dewar Offset File: DewoffName
Choose Camera:
(1) LONG f/4
(2) SHORT f/2
Enter number: 2
Choose Mode:
(1) Direct
(2) Spectroscopic
Enter number: 2
Choose Grism:
(1) 150 l
(2) 200 l
(3) 300 l
(4) 300R l
(5) 600 l
Enter number: 3
Choose Nod & Shuffle Mode:
(1) No
(2) Yes
Enter number: 1
Enter Grating Order (1): 
Enter Disperser Misalignment (0.0):
Enter Observation Definition File: ObsdefName
Writing the following to ObsdefName.obsdef:

DATE            5/2014
MASK            MaskName
DEWOFF          DewoffName
MODE            SPEC
DEWAR           E2V
GRATING         IMACS_grism_300
GR_ORDER        1
GR_ANGLE        0
D_ALIGNROT      0.0

Wrote ObsdefName.obsdef

Things to note:

  • For the "Grating Order" and "Disperser Misalignment" fields, the default value is in parentheses. I just pressed "Enter" for these to accept the default values.
  • When loading an obsdef file, the current values will show up in parentheses. Just hit "Enter" to accept these values, and input different values for the ones you want to change.

Notes for is an additional option for Mac users, and is installed and launched similar to other Mac OS X apps. No dependencies need to be installed and it should work right "out of the box" on Mac OS X 10.6 (64-bit only) and later.


  1. Download the app here
  2. Unzip the compressed folder, if it wasn't automatically done
  3. Copy the uncompressed folder to your Applications folder


  • You can launch by going to your Applications folder in Finder and double-clicking on defineobs.
  • Alternatively, you can drag defineobs from Finder to your Dock to make it more readily available.
  • The interface is similar to the original Perl version of defineobs, except that all user options are under the "File" menu from the top of your screen. defineobs mac gui

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