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Etalon scanning

A TAURUS scan normally consists of a large number (typically 60) of separate 2D images taken sequentially, each with a different setting for the etalon gap. Before each image is taken, the etalon gap is altered by changing the offset applied to the Z channel of the CS100 servo-controller. The set of 2D images are built into a 3D datacube in the Detector Memory System.

Acquiring a spectral line datacube therefore requires the process of data acquisition to be synchronised with the stepping of the etalon gap. There are two different ways of doing this, depending on whether the detector being used is a CCD or IPCS.

Data acquisition with a CCD is relatively straightforward, and is carried out under control of the system computer. The procedure is as follows:

Syscomp tells DMS to set up and clear 3D data buffer
LOOP for all etalon steps
Syscomp tells TAURUS 4MS to step etalon
Syscomp tells DMS which plane of datacube to expect next
Syscomp tells CCD to take image
CCD image read out into one plane of 3D data buffer
Keep datacube

Given the overheads involved in clearing and reading out the CCD, it can easily take 1-2 hours to scan through a complete datacube. During this time it is possible that the atmospheric transparency will vary, possibly producing spurious structure in the spectra. This problem can be minimised by, instead of scanning the datacube sequentially starting at plane number 1, scanning the planes in the datacube in a random order.

When the IPCS is used as the detector, the effects of changes in atmospheric transparency are minimised by using the 'rapid-scanning' technique. During a scan, the integration time at each step is kept to a minimum so as to complete a scan through the wavelength range as quickly as possible. The required signal-to-noise ratio of the data is then achieved by co-adding the results of a large number of successive scans in the DMS.

A typical rapid-scan might involve stepping the etalon every 100 milliseconds, in order to scan through the entire cube in less than 10 seconds. Thus synchronisation between the etalon stepping and data acquisition becomes time-critical, and can no longer be carried out over the utility network under control of the system computer. This synchronisation is therefore carried out via a dedicated link between the TAURUS and IPCS 4MS systems. Whilst the data is being taken the TAURUS 4MS is slaved to the IPCS 4MS, stepping the etalon when requested to do so. The procedure is as follows:

Syscomp tells DMS to set up and clear 3D data buffer
Syscomp tells TAURUS to slave itself to IPCS
Syscomp tells DMS to start IPCS data acquisition
LOOP for number of scans
LOOP for all etalon steps
IPCS tells TAURUS 4MS to step etalon
TAURUS 4MS steps etalon, notifies IPCS when succesful
DMS takes exposure with IPCS, coadded into DMS
Keep datacube

Thu Apr 7 00:29:52 BST 1994