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Aperture masks

The aperture masks for the programme should have been manufactured well before the start of the run, and should be installed during the afternoon before the first night of observing. The focus and matrix masks (see Section 1.2) are required for some setup operations. Loading the aperture wheel is a task for qualified personnel only. However, it is useful to know the general procedure and the observer may need to perform some of the non-critical items in this sequence

• Take the telescope to the zenith

• Unlock the doors using LDOOR 0

• Unclamp the aperture wheel using LDSS_UNCLAMP APERTURE

• Go out to LDSS-2 with new masks and open the doors

• Manually release the spring clamp and turn the aperture wheel by hand to the desired unloading/loading position, letting the clamp relocate to hold the wheel in the new position

• Remove the old mask by turning each of the three spring-loaded clips and lifting out the mask. Take care not to touch the underside of the mask, since this could damage the slit edges.

• Place the new mask in the holder so that it seats down flat; the three locating points on the cell are such that it will fit properly only in the correct orientation and the right way up.

• Make sure that the mask cell is held tightly against the locating points while you lift and turn the three clips to hold it in place. Check that the cell has been clamped firmly up against the locating points by trying to slip a piece of paper between the cell and each of the three pins (this is the most difficult part).

• Once satisfied that the mask is correctly installed, unclamp the aperture wheel and move it ready to change the next mask, making sure to note which mask is in which position

• Close the doors and return to control room taking old masks with you

• Lock the doors using LDOOR 1

• Reinitialize the wheel from an arbitrary position using SEND LDSS OBEY APERTURE_WHEEL GINIT

• Use LAPERTURE_SETUP to redefine the contents of the wheel.