PNS MANUAL: operation / filter change

The PN.S filters are stored either in the PN.S cupboard in the WHT building, or in the filter cupboard in the Coude room of the INT building. There are currently four filters (named A, AB, B and C). They can be identified by the codes engraved in their sides: A has code 2002, AB 502.66/4.32, B 1401, C 3001.

Exchanging the filter

Before exchanging a filter, the filter holder needs to removed from the spectrograph. This must be done through the SQUARE access hatch in the side of the instrument. The round hatch is too small.
  1. Open the square hatch
  2. It is a good idea to place the filter tuning tool on top of the filter as shown in the picture, to protect the filter. Tune the filter to zero degrees which will make it easier to extract later. See figure

  3. Remove the six bolts that hold the filter holder to the instrument.
  4. Carefully lift the filter holder out of the instrument. Under no circumstances touch the filter with bare hands. Use the white gloves provided for this purpose in the PN.S cupboard.
  5. Set the filter holder down on a table. Remove the filter tuning tool. Turn the filter holder over.
  6. To remove the filter, the inner metal cylindrical sleeve holding it in place needs to be removed. This requires the three small headless screws holding the sleeve in place to be removed. Use the magnetized screwdriver (red top) to extract the screws. See figure

  7. Once the screws are out, slide the sleeve out of the holder. This can be tricky: it is a tight fit and the sleeve can easily jam. Be patient and be careful not to touch the filter. Use gloves!

  8. Now the filter is loose in the holder. DO NOT TRY TO LIFT IT OUT! Instead use the felt-covered tool in the shape of a cylinder on a square base. Set the tool down on the table, cylinder up. Place the filter holder with filter gently over the cylinder, and slide it down off the filter. The filter is now lying on top of the cylinder and can be picked up and put away.See figures

  9. Now reverse the process to insert a new filter:
  10. Place the to-be-inserted filter on the cylinder, shiny side down (if you can tell the difference!). Lift the holder up around it and off the felt-covered tool.
  11. Carefully insert the sleeve into the holder. Again, be patient as the sleeve jams easily. Take care that the screw holes align.
  12. Replace the three headless locking screws, using the magnetized screwdriver. Turn the filter holder over, and replace the tuning tool in order to protect the filter.
  13. Now re-insert the filter holder into the instrument. Align it so that the long mark on its circumference points straight out of the square hatch.
  14. Replace the six screws holding the filter holder in place. Tune the filter to the desired angle. Then remove the filter tuning tool!
  15. Replace the square hatch, and note the filter you've just inserted and its angle.
  17. Make sure to re-focus spectrograph. Tests with Filters A and B concluded that this is realisable as a constant focus shift. Keep careful note of capstan positions, and the direction in which they must be turned. Include the information on the focus offsets archive.


    Exchanging the filter

    The filter holder was made for Filter B and is currently only suitable for filters of the same thickness. Filter A is just too thick. Filter C has not been tried (as of 16/9/2001). See also John Hart's email on interchangability problems.

    Access to the filter is easiest via the square hatch on the flat side of the PN.S. Under no circumstances touch the filter with bare hands. Use the white gloves provided for this purpose in the PN.S cupboard.

    Currently we only have one filter holder, which resides in the filter tilt mechanism. A picture of the filter unit is available as a doc file.

    There are two wire rings; one retains the filter and the other retains the sleeve containing the filter. They both snap into grooves in their respective bores.

    Both rings are secured by means of a tiny grub screw passing through the wall of the respective housing, and protruding into the narrow gap in ring. This spreads the ring diametrally to lock it in place. We included a drive key for the screws in the box containing the telescope adapter ring.

    To remove the filter (from an email by John Hart 13/7/2001):

    1. Remove the screws mounting the filter unit in the camera body, and withdraw the filter unit unit from the camera body.
    2. Place the unit "face down" on a clean flat surface.
    3. Retract the grub screw that locks the outer wire ring, and remove that ring by prising one end out of the groove. Be careful!
    4. Remove the spring ring.
    5. Slide the inner cell (filter and sleeve assembly) out of the outer housing. This gives access to the grub screw that locks the inner wire ring.
    6. Retract the grub screw that locks the inner wire ring, and remove that ring.
    7. Slide the filter and associated pack rings out of the sleeve. You will need surgical style gloves to avoid getting finger prints on the filter.

    To install the filter, reverse the foregoing procedure. Note that the shiny side of the filter goes face down so that when the filter unit is (inverted and) installed the shiny side is facing the collimator.


    The procedure is tedious, but it can easily be done during the day.

    Note: don't attempt to wipe the filters, even if they have condensation. Let the filters warm up in the control room so that the condensation has gone, then enclose them in their wooden box and place them in the MC (mezzainine cupboard).

    About this Document

    We need your input to keep these notes up-to-date. Please add to this document, or create new ones which can be linked from it. Send your input by email. Last updated 21-04-2004

    Documents of this background colour are part of a set linked from the manual