Once the object coordinates have been typed into the system computer and
added to the object list, an object may be acquired by typing
GOCAT <object-name> on the TCS interface (this is usually done
by the TO). Although the WHT should point to within 3 arcsec, you
might want to take a quick exposure to check that the object is indeed
within the field; this can be done by typing:
GLANCE <channel> <time> at the ICL interface,
where the exposure time is in seconds, and <channel> is either PFIP or AUX. A GLANCE exposure will not be saved to the system disk, but may be saved to the SPARCstation via the DMS interface (without headers).
Once the object has been acquired, longer exposures may be obtained by
RUN <channel> <time>,
where again the time is in seconds. Multiple exposures can be done
MULTRUN <channel> n <time>,
where n is the number of exposures.
A run may be prematurely ended via the following commands:
ABORT <channel>- data will be lost
FINISH <channel>- exposure is ended ``gracefully", and
data saved to disk with the new, lower exposure time
Finally, an exposure may be paused, and then continued, by:
In order that cosmic ray events may be identified and removed in subsequent processing, it is recommended that long exposures be split into several shorter exposures. Each shorter exposure should be long enough to be sky-noise limited, in which case the only penalty to be paid is that of the extra readout time. As well, to improve flat-fielding, the telescope should be shifted (``dithered") between each exposure. The dithered frames can be combined later to produce a dark sky flatfield which can be divided into each program frame (once they have been dome flatfielded). This dark sky flat can also be used to remove fringing which is often seen in long R and I exposures. Users who are very concerned about registering their frames should think carefully about the radial variation of the field distortion; for instance, at 5 arcmin from the optical axis at Prime Focus, the radial distortion is 0.2 (see Section 1.1).