3. Shutter and Filter Wheel

The filter wheel/shutter has a 4 inch shutter and holds 8 4X4 inch filters, or smaller sizes with adapters. We have 4-inch ``Harris'' UBVRI filters on loan from KPNO; their descriptions and tracings can be found at ftp://ftp.noao.edu/kpno/filters/4Inch_List.html. The filters can be found in the plate-loading room on the observing floor.

The filter position can be controlled using the CCD computer.

3.1 Changing filters
(The .html version of this manual contains helpful pictures which help to explain the following steps.)

The filter wheel must be removed from the shutter/wheel mount on the telescope to install filters. The wheel will hold 8, 4X4 inch filters as well as 8, 2X2 filters when using the adapters for the small filters.

Note: Please handle these filters carefully and use extreme caution when removing and inserting the wheel in to its holder. These are very expensive filters and we do not have any spares.

3.2 Removal and insertion of wheel

The filter wheel is large and can be heavy when all the filters are installed. You should move the telescope down so the wheel can be removed or inserted easily from the floor. Do not try to remove or install the wheel, while the telescope is at zenith.   Here's  a picture of the filter wheel on the telescope and  here's  another.

  • Unscrew the knob that secures the access door. Carefully pull the door up by the knob along the support rods and lay it down. It does not remove, but must lay well over the dewar to permit access to the wheel.
  • Carefully pull the wheel up and out of the holder. You will notice that the center of the wheel facing you at this point, has a round center section with metal arcs milled into it. This is the position encoder system. The encoder disc must face the dewar when installing the wheel. Be careful not to damage this.
  • Lay the wheel encoder surface down on a bench covered with Kay Drys. This will protect the encoder surface.
  • In the Schmidt filter box is a container with screws and an allen wrench which you need to remove the filter holding frames. These frames are used to hold the 4X4 filters as well as the adapters for the 2X2 filters. There are special rotated 2X2 adapters for the Schmidt. These will allow the filters to be rotated so they line up with the rows and cols. The rotation of the holder does not matter, it will always be correct, however you must have the correct side facing the dewar. If the filter wheel is on the bench, with the encoder resting on the Kay Drys, the adapters go in with holding clip down, what would appear upside down. You will have to turn the wheel over to insert the 2X2 filters.  Here's a picture of the 2x2 adapters and  another If there is a question about which side must face the dewar, hold the adapter up and make sure the filter lines up with the rotation of the dewar while holding the holder square with the wheel.


    Note: We have had light leaks around the edges of 2X2 filters from the coatings of the filters not going to the edges. If you see this problem you will have to cover the edge of the filters with black tape. If you can see through the filter, hold it up to the light and you can see if there is a problem.

  • Insert your filters and hold them in place with the frame. There are numbers engraved on the wheel which determine the filter position, please keep track of these numbers and what you have mounted so this can be entered into the computer. Please note that the frames will only go one way. and the screw holding the frames must not extend above the frame work of the wheel. You can use strips of cork on the frames to protect the filters.
    NOTE: DO NOT USE ANY FORCE ON THE SCREWS HOLDING THE FRAMES, OR YOU MIGHT BREAK THE FILTERS. THE SCREWS ONLY NEED TO SET FIRMLY.
  • After all the filters are in, blow off the filters just before installing the wheel.
  • Carefully lift the wheel. Open the access door if not left open. insert the wheel with the center encoder facing to the dewar. You do not need to pay attention to the position of the filter numbers on the wheel as the set up will take care of position. Rotate the wheel by hand until the detent lockes it into place.
  • Carefully lift the door and slide it down the rods. Secure it with the knob.
  • Press the white button ``STD reset" on the 4X4 filter interface power supply box. This will reset the shutter and filter wheel to it starting zero point. Any time the filter wheel is removed and replaced, press the reset button.

    If at any time you feel the shutter is not working right, press the ``STD reset" button. This will zero the shutter.  Here's  a picture of the  filter contol box and the reset button.

    3.3 Controlling the Filter Wheel

    We are working on integrating filter control into the CCDtool software (see Sect. 6). In the meantime, you can control the filter wheel using the program schcom on budgie (the Sparc Ultra 1).

  • From an xterm window on budgie, type kermit
  • At the kermit prompt, type set line /dev/ttyb
  • In another xterm window, you can send filter wheel commands. The filter wheel commands are summarized in Table 3.1.

    Note that it is important to query the filter wheel position every time the power is turned on to turn off the LED which senses the filter position --- otherwise the LED's light leaks onto the CCD.

    Table 3.1. The filter wheel commands.
    Request You Type:
    What's the filter wheel's current position? schcom 'pfil?'
    Change to filter 5 schcom '5 pfil'
      Note: the single quotes (') around schcom's argument are necessary. 

    2. Operating the Telescope & Guider    4. Computers at the Burrell Schmidt