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Data acquisition

The detector and data acquisition are controlled via ICL from the same VAXstation that controls the instrument. The most common commands are listed in Table 7.

Data are formatted and saved to disk on the Vax 4000 system computer by the Data Acquisition System, which is described in detail in ``The Data Acquisition System'', by G.T. Rixon and S.W. Unger. In ICL, normal integrations are started using the command RUN. On completion a RUN will read the data into the DMS, and then automatically copy it to a file in .NDF format on the VAX in a new directory for each night of observing, e.g. DISK$WHTDATA:[OBSDATA.01-JAN-1992]. These files can be examined from any of the data reduction VAXes. Of particular importance when observing with LDSS, they can be read by LEXT running on LPVS3.

The DMS stores only the most recent readout, whereas a new VAX file is created for each run. The file names are generated using the automatically updated run number. For test and set-up frames which you don't want to save, it is possible to use the command GLANCE, which does an integration, but writes the data only to the DMS; no VAX file is created and the run number remains unchanged.

The data should be archived to tape (usually on a Digital Audio Tape), using the procedures FITSINIT and WRITE_FITS. These procedures can be run from any Vax in the cluster which is connected to the relevant peripheral device, but the normal procedure is to use the OBSERVER account on the data reduction Vax LPVE. It is the responsibility of the observer and the support astronomer to produce the FITS format archive tape, or D-tape; the observers copy, or C-tape, can be produced either in FITS format or using the VMS BACKUP utility by the observers, or as a direct copy of the D-tape by operations staff.

Data in any of the DMS buffers can be also be transferred to the Sparcstation connected to the DMS, but this is not done automatically. The Sparcstation is useful for some set-up procedures where the IRAF package can be used to inspect the data, so it is worth setting it up to get data from the DMS. To do this you must use the mouse on the Sparcstation to select the DMS buffer being used by LDSS. The buffer number being used should be written on the whiteboard in the WHT control room (note that it may not be called LDSS in the DMS window on the Sparcstation). Then, once an integration has been read into the DMS buffer, the mouse on the Sparcstation can be clicked on the relevant button to copy the DMS frame into an IRAF file. There are several options for naming the IRAF file, including one which takes the currently active run number. In this option it is important to realize that if you get the data from the DMS after you have started the next run, the IRAF file will have the name corresponding to the run in progress, not the run that has just completed which corresponds to the data in the DMS.