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Hartmann test using the DMS

Set the slit width and dekker with:--

SLIT 150
DEKKER 6

The comparison lamp filters and the exposure time should be set to give a reasonably strong comparison spectrum but the intensities in the lines to be used, as measured with the CALCULATE option on the DMS STATISTICS menu, should not exceed 32767. Close the left Hartmann shutter by typing:--

BHART 1 or RHART 1

at the ICL interface and take an exposure by typing:--

GLANCE RED ARC <time> or GLANCE BLUE ARC <time>

When it has read out, at the DMS control window type:--

FOCUS

Select three strong lines evenly spaced from the one end of the spectrum to the other by placing the cursor on each in turn and pressing the space bar on the DMS keyboard. Each line profile will be shown on the DMS display, and the line can be rejected if it appears too weak, or blended with or close to another line. The best results will come from strong isolated lines. When three lines have been selected type:--

FOCUS-LEFT

at the DMS control window. The DMS will then list the centroid and an estimate of the full width half maximum in CCD pixel units at three positions along each line, and store these values. The arrangement of the listed positions on the alphanumeric screen maps to the DMS display, so the first position listed is in the top left hand corner of the image as displayed on the DMS display.

Now open the left Hartmann shutter and close the right by typing:--

BHART 2 or RHART 2

at the ICL interface and take a similar exposure. At the DMS control window type:--

FOCUS-RIGHT

This routine will list the positions and the full width half maxima of the lines together with the shifts since the first exposure. These Hartmann shifts indicate how far from the focal plane the photocathode is at each point. The amount of collimator movement required can be calculated from the mean Hartmann shift; a Hartmann shift of 0.5 pixels implies that the collimator needs to be moved about 1000 microns. The direction of the shift will depend upon which on-chip amplifier is being used, and will have to be determined by trial and error.

After each move of the collimator the Hartmann test should be repeated. After a move of the Blue collimator it probably will be necessary to run FOCUS again as unclamping and clamping this collimator can cause line shifts of several pixels.

The final FWHM of the lines with a narrow slit should be in the region 1.5 to 2 pixels, although with the 150 micron slit recommended they will be somewhat broader.

At the end of the focus procedure both Hartmann shutters should be opened by typing:--

BHART 0 or RHART 0

The focus is somewhat temperature dependent, but unless there is a sudden change of ambient temperature the spectrograph, once in focus, should remain so for several days.