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The fixed format of FOS makes it a very stable instrument. However, it is a good idea occasionally to check the focus and alignment. Moreover the best focus setting will need to be reset if either a below-slit filter, or more particularly the field lens, are used with FOS in any mechanism below the slit.
To check the focus, set the ISIS slit width to 40 microns, turn on an arc lamp, put in the comparison-lamp mirror and take a short exposure. Select a strong but unsaturated line and use the DMS command 10 Y-FIND to measure its FWHM. If the line is in focus, the FWHM should be about 1.6 pixels, slightly better in the blue. If the FWHM exceeds 1.8 pixels, the focus must be adjusted by control of the motor-micrometers (see below). A more accurate check of the focus can be made by comparing exposures taken with either the left or right Hartmann shutter closed, these exposures can be analysed with the Hartmann routines described in Section 8.3.3
For data-reduction routines to work well, the slit must be aligned with the CCD rows to within 0.2 pixel over the full slit length. This can be checked using the data frame obtained for focus, by measuring the line centres with 10 Y-FIND at opposite ends of the slit.
The focus and tilt of the FOS CCD are determined by the positions of 3
push rods on which the ring bearing the CCD is supported.
These push rods are moved along their axes by 3 motor-micrometers,
which are controlled from the engineering terminal. The commands
HELP - lists all available commands
STATS - lists status of Hartmanns, motor-micrometers, focus and tilt
xxx FOCUS - adjusts focus to position xxx (0 - 511). Each motor-micrometer moves by the same amount
ALLSTOP - stops all mechanisms immediately
xxx 1MOVE - moves motor-micrometer 1 to position xxx (0 - 511)
xxx 2MOVE - moves motor-micrometer 2 to position xxx (0 - 511)
xxx 3MOVE - moves motor-micrometer 3 to position xxx (0 - 511)
xxx HANGLE - sets horizontal tilt to xxx (-255 - 255)
xxx VANGLE - sets vertical tilt to xxx (-255 - 255)
a b HARTS - operates Hartmann shutters (a, b = OPEN or CLOSED)
The most-recent setting of the motor-micrometers, focus and tilt is recorded in the FOS instrument logbook. Check whether this is for FOS in the configuration that is required.