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Once a user has made a selection of the astronomical observations to retrieve from the archive, the appropriate calibration data may be found with the CALSELECT command. This command will find observations of a specified `Observation Type' (see section ) within a specified number of days before and after (each of) the astronomical observations already selected (in the default table).
The observation type is an integer code number primarily used to separate the various calibration exposures from the astronomical observations. A short list is given in table ; all the observation types used in the catalogue can be listed by typing:
SHOW CODE_GROUPS=OBS_TYPE , or: SHOW CODE=OBS_T
The default setting for the CALSELECT command is to search in a window of 7 days before to 7 days after the date of each astronomical observation, and on observation type 10, which means that all calibration exposures encountered will be collected. The command, given without further qualification uses the default setting:
However, in many cases it can be better to search for specific calibrations with a more appropriate window setting, taking into account some knowledge of observing practise. For instance, observers tend to take flat-field exposures at the beginning or the end of their runs, but arcs much nearer the the actual science spectra. Instead of the single, unqualified, CALSELECT search, a sequence such as the following may be more appropriate:
CALSELECT OBS_TYPE=(10 to 19), DAY_LIMITS=(1,1), window=4 (standards)
CALSELECT OBS_TYPE=(20 TO 29), DAY_LIMITS=(1,1), window=3 (bias, darks)
CALSELECT OBS_TYPE=(30 TO 39), DAY_LIMITS=(5,5), window=6 (flats)
CALSELECT OBS_TYPE=(40 to 49), DAY_LIMITS=(1,1), window=2 (arcs)
CALSELECT OBS_TYPE>=50, DAY=(3,3), window=5 (others)
This is a whole mouthful, and clearly an area for experimentation; it may be worth the effort to define such specific calibration-selection commands as special symbols (next section).
The CALSELECT command can dig out quite a number of calibrations, in particular if the search boundaries are taken generously. Searches within one day of observation is normally more than sufficient and quite likely it will even then be necessary to weed out unwanted entries, for instance because they relate to other, unwanted observations taken in the same night, or, because observations from other observers than the one who took the data, are mixed in as well. This will usually require the removal out of specific entries. A straightforward way of doing that, without the need for complex select constructions, is by using the (VAX-based) EDIT facility. First, create a list file:
LIST/FILE , or LIST/FILE item, item, item, ... , then do:
EDIT/EDT , or EDIT/TPU
which puts you in edit mode, and enables you to delete any line from the table. On output of the editor the new query table will automatically be generated.
What if CALSELECT fails to uncover any calibrations? One possibility is that you happen to work on a part of the catalogue which is not yet provided with astronomical codes (see section ), and the only way to find calibrations is then to inspect other observations taken in the same night (or nights, by the same observer). Most calibrations can be identified by their name ( FF, ARC, etc. ), so it is generally not too difficult to find your data by selecting on such names and inside an appropriate time interval:
In the case of ARCs for spectroscopic observations it is helpful that many of these are taken at the same sky position of the actual astronomical object, so a search on position will dig them out. In particular with WHT data it is possible to find most calibrations in this way (check on exposure times; arcs and flats usually have less than 100s).