Up: A Night on the Bare Mountain
Next: Observing with the Cross-disperser
Previous Page: Focus
Next Page: Afternoon Activities
Having finished the set-up, you are ready to observe. This Section goes first over some general concepts and then takes you step by step through your observation routine.
If disk only is selected as the data destination then the number of available CCD runs will be limited by the disk space. CCDSPACE will tell you how much disk space is still available. It is important that users should copy important images to tape as soon as possible and then PURGE so that disk space is not limited. Better still, select tape as the dump device so that every exposure is automatically copied to tape. After the D-tape has been copied to a C-tape by support staff (or the observer if he has time!), the disk can be purged and overwritten if necessary. The EAGLE disk has space for 500 CCD runs. At least 60 CCD images can be stored on a 2400 foot tape (1600 bpi). The tape writing gives a slight overhead to the duration of an observation - although the next one may be started during tape writing. However, the Lexidata must not be used whilst tape writing proceeds. Only if you have many short exposures may you choose to dump the data to disk and then FITSOUT the runs to tape at the end of the night. But remember that the data is not safe until it has been written to tape. The EAGLE disks failed twice in 1988 with loss of all the data on the disk in both cases.
An estimate of the bias level is used by the software when it does correlations or similar operations. This bias level is an electrical zero offset within the CCD camera and can be measured from the ``underscan'' portion of an exposed image (see Appendix B). The bias level may be measured using PHOTOM/STATS after taking the first exposure, and the software default may be corrected if necessary (using a command BIAS b where b is the value to be entered).
If preflash is needed for your chip (find out from your SA) the level should be set for all observations. At present the default is zero, use PREFLASHDEFAULT=n to reset the default.
The normal high-level command that is used for an observation is of the form RUN t ``NAME'', where t is the exposure time in seconds and NAME is your object. Several (n) equal exposures (of t seconds) can also be obtained using the command MULTRUN n t ``NAME''. Such exposures are recorded on a sequential series of disk files and would be written to tape, if selected. An alternative method for a trial exposure is to use GLANCE t, where the image is put in a scratch-file which is overwritten by the next GLANCE. This image can be kept permanently if desired, by using the command KEEP ``NAME'' before another GLANCE is performed.
A RUN consists of various lower-level operations as follows: telemetry check, clear chip, timed exposure, readout to disk, display, (copy to tape). The above operations take about 1, 4, t, 30, 5 (and 60) seconds respectively. The PHOTOM and SPECTRUM utilities make available several basic image/data analysis options for use by the observer. There are also other more general facilities such as image arithmetic operations, IPCS external memory usage, etc. Chapter 5 summarises the ADAM software commands.
No temperature reading or other monitoring of the CCD camera is provided directly. The remote telemetry is automatic and only becomes visible to the user if a fault develops (see also the ADAM command TELEM).
After initializing the system and doing the SETUP, the following frames should be obtained: