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The focus should be determined with a grating setting and data format appropriate
for the observations of the coming night.
Set the slit width and dekker with:--
A comparison lamp, usually the Copper-Argon lamp is used for the focus. The
neutral density filters should be set to give a count rate of no more than
one photon/pixel/second at x8 resolution along the spectrum and x2 resolution
alongthe slit. The ICL procedure IPCSFORMAT should be used so set up
an appropriate data window. Close the left Hartmann shutter by typing:--
at the ICL interface and take an exposure of at least 200 seconds
EXPOSENP IPCS 200
at the ICL interface.
When it has finished, at the DMS control window type:--
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:--
at the DMS control window. The DMS will then list the centroid and an estimate of the full width half maximum in DMS 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:--
at the ICL interface and take a similar exposure. At the
DMS control window type:--
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 1 pixel implies that the collimator needs to be moved about 1000 microns. A positive Hartmann shift implies that the collimator must be moved to lower values.
As the IPCS data builds up in the DMS you can type FOCUS-RIGHT part way through the exposure to see how it is going, then repeat it at the end.
After each move of the collimator the Hartmann test should be repeated. 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 should be around 3.5 pixels, it is rather better in the red and rather worse in the blue. If, with the spectrograph in focus and a narrow slit in place the FWHM is much greater than this then the Image Tube focus is incorrect and should be adjusted. This must only be done by RGO technical staff.
At the end of the focus procedure both Hartmann shutters should be opened
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.