Calibration (GCAL) Observe Elements

Note: a set of basic calibrations for your observations will be acquired by the Gemini staff for all queue-scheduled data. See the performance and use section of the relevant instrument pages for details. These and any additional calibrations must be defined by the PI in the OT.

Of the four observe elements that take data, three are associated with calibration (the fourth takes science data). Each observe command generates one dataset. A dataset is one file and may contain multiple frames within the multi-extension FITS structure depending on the specific instrument (e.g. the number of coadds, if supported and not coadded within the instrument, whether it has multiple detectors etc.). Each observe is shown separately in the sequence list and timeline. The three calibration observes are:

• Bias - takes a very short exposure dark (usually only relevant for destructive-read stare exposures with IR detectors). The parameters available in the detailed element editor are the number of coadds per observe and the number of observe commands issued for each observe element. (At present the bias exposure time defaults to a nominal value of 1 sec for display in the sequence).
• Dark - takes dark data (shutter closed and/or detector blanked off) with the exposure time, coadds and observes specified in its detailed element editor. (The default is 1 observe containing 1 exposure of 10s).
• Flat / Arc - takes flat-field or arc lamp data using the current instrument configuration illuminated by the GCAL facility calibration unit. See below for more details about the flat/arc detailed editor. Explicit sky flats are not normally taken (except by Gemini staff to calibrate the difference between GCAL and telescope illumination or at mid-IR wavelengths where GCAL does not operate).

An observation may contain multiple calibration (or for that matter, science) observes. For example, if the highest precision of wavelength calibration is required (or, heaven forbid, there is significant instrument flexure as a function of telescope elevation) you might choose to take arc frame and flat-field data immediately before or after the science data. The sequence below shows an observation in which arc frames are taken before the telescope offset sequence collects science data (for flat read arc in the timeline):

You can of course specify the calibration data in a completely separate observation and include both it and the science observation in a group.

Currently both arc and flat observes are erroneously labelled as flat in the sequence list and timeline.

The detailed editor for taking flat/arc data is accessed in the usual manner, by selecting the flat/arc observe element in your science program [replace with image of current editor]:

This editor is used to define the GCAL configuration: lamp (choice of flat-field and arc lamps) neutral density and colour-balance filters optical or IR diffuser See the GCAL pages for a complete description of the capabilities. The exposure time, coadds (if supported by the instrument) and number of observe (commands) must also be specified. The save button accepts the latest changes and stores the program to the local database, the undo/redo button (and, transiently, the edit pencil) toggles pending and saved changes and the close button closes the science program editor (saving any changes to the local database).

Last update May 3, 2001; Phil Puxley