The WFC has a six-position filter wheel, the filters of which cover both the science array and the autoguider. There are more than six filters available and filters can be exchanged individually between the wheel and storage; there is no provision to exchange the entire wheel for another preloaded with alternative filters.
The available filters are listed in the filter database.
The filters themselves are clamped into handling mounts that slot into the filter wheel. Each mount is a machined metal unit in which the filter butts against a narrow flange. The filter is held in by a plastic packing ring and the ring is screwed to the metal mount in six places. The outside of the mount has machined guides which match the slot in the filter wheel, and has a clip to secure it into the wheel. On the outer rim of the mount, next to the clip, is a metal strip. This presents a binary code to a microswitch in the instrument, by which the system identifies which filter mounts are in the wheel.
To deploy one of the six mounted filters into the beam, click on the symbol for the filter in the WFC mimic (having first activated that mimic using its enable button), or, equivalently, use the filter command from the command line. The observing system will calculate the difference in optical thickness between the old and new filter and will adjust the telescope focus accordingly. (See focusing the WFC for subtleties of this feature.)
Filters should be taken in and out of the wheel under control of the ICS. With the observing-system on-line in the control room, log into the system computer (as intobs) from the terminal on the trolley on the access gantry.
Filter changing is easiest with the rotator at 180 degrees. Send it there with the rotate command.
Use the change command for the filter you wish to change; if you are changing more than one filter, invoke the change command once for each.
In the change command, you name the filter you're taking out, not the one you want to put in.
You get at the filter through a hatch that is on the left-hand side of the instrument as you face the observing floor. The hatch has a black knob: turn the knob to release the hatch and pull, but not until the computer tells you to do so. (If you open the hatch when the wheel is moving, the safety features stop the wheel and abort the change command.) Opening the hatch reveals the edge of the filter mount and the bronze catch holding the mount into the wheel. Squeeze the catch and pull out the filter mount. There is no need or excuse to touch the glass of the filter. Slot in the mount with the new filter and push it in until the catch clicks into place. Now close the hatch and turn the black knob to lock it. When the system senses the hatch is locked, it will identify the new filter and leave it deployed in the beam.
If you make a mistake with the hatch and change gets aborted, run the command again for the same filter.
If the terminal on the gantry is unavailable, you can run change from the prompt in the control room. Clicking on a filter in the mimic with the shift key held down also runs change for that filter.
Filters should be kept in the special case provided when not in the wheel.
Currently, there are exactly as many filter mounts as there are filters. This is good; swapping filter between mounts invites errors in the filter-identification scheme. Spare filter mounts for new filters will be delivered next year.
The filter should first be registered in the general ING database of optics. Registration yields a catalogue number for the filter.
Next, load the filter into its mount and read the mount's code-strip. The code-strip presents five bits of information, read at 8mm centres, with the least significant bit nearest the clip: a raised area indicates binary 1. You need to know this code, converted to a decimal number, when you tell the system about the filter.
The instrument can read which filter mounts are in the wheel, but you must tell it which filter is in the new mount. You need to do this before loading the filter into the wheel. At the observing-system prompt in the control room, run the program wfcmdb. This is a rollodex-like viewer for the filter table.
Remember the 5-bit code from the code-strip, converted to base 10. Scan through the table, using the `>' and `<' buttons, looking for this number in the filter-holder field. If you find a matching entry, you can over-write the information with the details of the new filter. Otherwise, press `new item' to get a blank entry and fill that in.
The invocation field is the name which you're going to use for the filter in the filter command. It needs to be unique; e.g. differentiate Harris R and Kitt-peak R (if we had such a filter) by Rh and Rk.
The waveband, photometric system and unique-id fields are used when logging and archiving observations; the unique id is the number from the optics database.
The focus offset is in millimetres relative to the Harris filters. In practice, this means that you need to determine the offset experimentally, on the sky (see `focusing the WFC'). The offsets are positive and are larger for thicker filters.
When you have entered the filter details, press `commit changes'. You should then scan through the list using the `<' and `>' buttons to check that the invocations are still unique.
When you have finished, press `save and exit'. The observing system will now use the information you've entered each time the ICS starts up. You can load the filter into the wheel using the change command.