Calcite

Mounting calcite plates in the aperture wheel

For spectropolarimetry you need to mount another calicte plate, found in the Calcite plates drawer in the filter carousel, and this one has a polarimetry slitlet mounted on its top. Per default the Pol_1.8" wide slitlet is used (actual width is 1.5"), unless the observer wants a narrower slit.

Status: From April 2003 to November 2008 the two calcite plates were mounted at 90 degrees orientation in their holders. This way the slit was horizontal and spectro-polarimetry done with grisms in the vertical (default) orientation. In November 2008 this was changed to a 0 degree orientation of the calcites, needing the slit to be vertical and the grisms horizontal! The Pol_1.8" slitlet was checked for alignment with respect to the calcite on the 9th Feb 2010 (AAD). Mounted in position 0 in the aperture wheel its best alignment with respect to the CCD was at encoder position 317400. (AAD)

Staff Instructions:

• Handle the calcite plates with care. They are very soft material and should never be cleaned with any liquids nor rubbed in any way. Remove dust by a light blow of air only! Don't touch with fingers. When they are not mounted, the calcite plates are stored in their holders wrapped in optical paper inside a plastic envelope in the Calcite plates drawer in the filter carousel.
  
  
• If you have to take the calcite plates out of their holders, then check their orientation as follows. Use the piece of paper in the calcite drawer of the filter carousel called: Calcite alignment tool, which contains lines rotated by angles 0, 45 and 90 degrees. Place the calcite plate (in its holder) such that the largest edge of the holder is parallel to the lower line of the frame of the plot. If it is mounted correctly, all lines will appear as double except for the horizontal one (because it is displaced horizontally). If this is not the case, then loosen the inner tube by unscrewing the screw holding the tubes together (need a small hex key), and correct the alignment. NB! It is crucial that the two calcite plates are both oriented in exactly the same way (i.e. not even 180 degrees offset), otherwise the slit aquisition for spectropolarimetry will not work.
  
  
• The polarimetry slitlet should be mounted on top of the calcite plate (on the side where light enters from the telescope), and is fixed to the calcite plate holder by tape. Align it by looking through the calcite at the slit, which now will appear as two slits horizontally offset to each others. Look directly at the calcite such that you see your eye reflected in the center. The slit is aligned with respect to the calcite plate when the two slit images are on a line. See below for aligning the slit with respect to the CCD.
  
  
• Mount the calcite with the slit in the aperture wheel. You have to unscrew all four screws and take out the the whole slitholder. (Take care that screws don't fall into the instrument!)
  
  
• Update the wheel content using the Setup Editor. Now align the slit with respect to the CCD:
  
  
  1. Put the polarimetry slitlet in the beam and set all other positions to open.
  2. Set the internal ALFOSC focus to 1070 (default is 1940).
  3. Turn on the ThAr lamp in the ALFOSC calib unit.
  4. Make a small window by typing alfosc.polwin in the sequencer.
  5. Take an image using for instance exp 1.
  6. Use imexam to find the x-position of the top and bottom part of the two slitlets split by the calcite. Measure the x-positions with the key 'j'.
  7. If the slit is tilted, change the encoder position of the wheel in the program EditAlfosc and iterate until the slit is aligned. About -1400 units is needed to rotate the slitlet clockwise by an amount corresponding to dx = 3 pixels from upper slit top to lower slit bottom. Note that the slit will be displaced horizontally on the image when you change encoder values. This is normal.
  8. When the slit is vertical, type resetxy and take a full image to determine the slit position (i.e. the xy values for the center of the upper slit) in order to update the file ~obs/alfosc/slitpos.def. Instruct the observer to use the upper image of the target for slit acquisition.
  9. Make an appropriate window for spectroscopy for the user by setting xbeg, ybeg, xsize, ysize in a script such as for instance:
     

     #!/bin/tcsh
     alfosc.resetxy
     alfosc.ybeg 800
     alfosc.ysize 300
     alfosc.xbeg 700
     alfosc.xsize 1348
     

Retarder plate

Mount 1/2 wave or 1/4 wave plate in FAPOL

• Make sure the FAPOL carriage is out of the beam.
• Power off the FASU controller.
• Remove the round connector from FAPOL.
• Disconnect the DB25 connecter on the FASU controller side.
• Unscrew the 4 screws that mounts FAPOL in FASU. The unit is heavy and will fall down a few mm if not held.
• Slide the FAPOL unit carefully out of its hole and bring it down to the control room.
• Sit at a table and carefully unscrew the 4 screws with rubber washers that hold the retarder plate in place. The 1/2 wave plate can in principle go in at any orientation (standard stars will measure the zero point of the angle of linear polarisation), but please align such that the red pen mark aligns with the white zero on the holder. The arrow points to which side is up. Both the 1/2 and the 1/4 wave plate should be mounted such that the circular mark on the plate holder points to the circular mark on the carriage (which is at the outer end of the carriage).
• For the 1/4 wave plate fix one screw per side of the plate. For the 1/2 wave plate two screws on two opposite sides is the best way.
• Bring the unit back, slide it in and fix it, connect the cables again and power on the FASU controller. Don't forget to open the FASU shutter.

Physically mounting the FAPOL unit un FASU

• There is a free way of 13 mm from the top side of FASU B wheel to the FAPOL unit. FASU filter holders stick out about 2mm above the wheel. To be conservative, don't put in thick filters in FASU B which will protrude more than 9mm out of the filter holders.
• Power off the FASU controller.
• Remove the black plate that is facing towards the staircase.
• Mount FAPOL in the hole and fasten it with the screws.
• Remove the blind round connector to the right of the hole.
• Connect the round connector from FAPOL to the connector to the right of the hole.
• Connect the DB25 connector to the FASU controller.
• Power on the FASU controller (Don't forget to open the shutter).

Connecting FAPOL on a normal instrument mount

• Connect the network cable to the network switch mounted on the adapter. You can use whatever port you like.
• Connect the 220V cable to a 220V outlet.

Changing the FITS hdr keyword for the retarder plate

• shutdownobssys alfosc
• Change the user from obs to alfosc by typing su alfosc in the terminal (if you don't know the password check it from the retarder plate box).
• Edit the file /home/software/alfosc/src/FAPOL/fapoctrl-socket-db.wsh
• On line 37 you will see the line
  set retarder_mounted "lambda/2"
• Edit the "lambda/2" to reflect the mounted retarder plate.
• startobssys alfosc

Misc

Start of the control programme

• /usr/local/bin/fapo-control

16 retader plate positions

• For testing purposes there are 16 retader plate positions available (the step size is 22.5 degrees). Non-UIF positions are not updated in the UIF, i.e. the retader-plate might have different position than the UIF indicates, however the FITS-headers are updated in a correct way. The use of the "hidden" positions is the normal FAPO-script way e.g. "!fapo-112.5" etc.

Comments to Anlaug Amanda Djupvik