change: change filters in the INT WFC

Purpose:

change allows the user to change one filter of the set of six mounted in the WFC's filter-wheel. The nominated filter is moved in line with the filter-access door and the user is then invited to open the door and change the filter. When the system detects that the door has been closed again, it identifies the filter by reading a code-strip on the filter-holder and looking up that holder in a table of filters mounted ready to go into the wheel. At the end of the command, the newly-inserted filter is deployed in the telescope beam.

Usage:

   change filter-name
where filter-name is the name of the filter to be taken out of the wheel, as shown on the WFC GUI before the start of the command. The change action can be invoked from the WFC GUI by clicking on a filter name with the shift key held down.

Example:

Replace the Harris V filter with something else:

   change Vh

Notes and caveats:

Before changing filters, make sure that you have told the computer which filters are mounted in which holders: see wfcmdb. If this mapping is not up to date then the filter names will be wrong. Loading the table after changing the filters doesn't fix the problem unless you restart the observing system as well.

This command is intended to run from the character-cell terminal on the access gantry, near to the instrument when the telescope is at access park. Textual prompts are given to guide the user through the change without reference to the WFC GUI.

The filter-access hatch is on the side of the instrument body, where the back-anodized casing sticks out below the thermal shielding. To open the hatch, turn the black knurled knob and pull. Don't touch the adjacent red knob: the red knob releases the mechanical clamps on a large and heavy part of the instrument which might then drop on your foot. The computer senses the position of the hatch's locking bars so you should turn the knob back to lock the hatch when you close it.

Please open the hatch only once, as directed, during each change. Once the computer detects that the hatch is locked after having seen it open, it will move the filter wheel to identify and deploy the new filter. If you open the hatch when the computer isn't expecting it, the wheel movement, and the change command, will be aborted for safety.

To change a complete set of filters you need to run the command six times.


This page was last revised on 1997-10-30, for s5.1.