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This page details the special preparations that the support astronomer needs to do for MOS setups, in the afternoon of the start of the run. Grism installation is described in the general spectroscopic staff instructions.

Mounting the slit masks (i.e. MOS plates)

Nowadays, MOS runs typically involve many masks per run, and only 3 or 4 masks are used per night. As the support astronomer is only around the first night, it is practical to teach the observers how to mount the masks by themselves.

For this we prepare 3 or 4 place-holder slots in the aperture wheel, called MOS_mask_1, MOS_mask_2, MOS_mask_3 and MOS_mask_4. These placeholder names can be selected using the EditAlfosc program.

Make sure to empty the slots that are to be used for MOS-masks, and to show the observer how to put in the masks. Note that the observer should not have to use the EditAlfosc program.

Even though during a run different fields will correspond to a single place-holder name, the unambiguous identification of the masks will always be present in the RA and DEC FITS headers.

How to align the masks with EditAlfosc

Staff only! Edit the wheel step number: the aperture-wheel step number has to be offset -1400 steps from the nominal position, e.g. for aperture wheel position #1 step value 40000 becomes 38600.
Note: in Dec 2013 i had to use the offset -1000 to get the Copenhaguen-made masks aligned with the detector, and all the slits within the FOV.

Mount the calibration lens

Finally, mount the calibration lens in one of the FASU wheels. The FASU calibration lens flattens the lamp-light distribution over the full CCD, but care should be taken as to not introduce spectral shifts. It needs to be checked in the first afternoon of the run if the use of the lens is advisable or not; this may depend on the setup (grism). It may be that without the lens the outer slits are not properly illuminated.

The calibration lens is kept in one of the larger optics drawers on the wall in the electronics room.

The calibration lens has an entry in the EditAlfosc program.

Back to top Last modified: 02-Dec-2013