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About focusing FAPOL

Spectropolarimetry

  1. Set the internal ALFOSC focus to 1070 for spectro-polarimetry. At this focus both components of the 1.8" wide slitlet are in focus and the width of the slit actually measures 1.5".
  2. Make sure FAPOL carriage is in
  3. Put in Cal-90 in the aperture wheel
  4. Preferrably use the standard I-band filter in ALFOSC (i#12). Its focus offset is only -5 with respect to no filter. This value you add back later.
  5. Inital focus
    • Linear polarimetry (lambda/2):
      Use the best focus for direct imaging and foc-del +475 ? (check!)
    • Circular polarimetry (lambda/4):
      Use the best focus for direct imaging and foc-del +275
  6. Set TV-focus to about 520 and start auto guiding
  7. Make a suitable readout window ( alfosc.polwin) .
  8. Take one by one exposure while changing the telescope focus value step by step. (In good seeing, steps as small as 10 can be used.) Use imexam in the sequencer and measure the fwhm of the two components. The best focus is found when the two values are as equal as possible.
  9. If needed, put the focus pyramid in the grism wheel and and take 15 seconds integrations. The best focus for spectropolarimetry is when the two components give about the opposite focus offsets (e.g. say +50 for the left and -50 for the right component).

Note, there is strong dispersion in the calcite, the location of the blue light from the extraordinary component being at a longer separation from the ordinary component than the red light.

Imaging polarimetry only

If you are just occasionally taking polarimetric imaging you can use the found offsets from normal imaging mode, assuming you have already focused the telescope.
  1. Use default internal ALFOSC focus (1940)
  2. Make sure the FAPOL carriage is in
  3. Put in Cal-90 in the aperture wheel and select the I-band (i#12) filter (its focus offset is only -5 with respect to no filter).
  4. Inital focus
    • Linear polarimetry (lambda/2):
      Use the best focus for direct imaging and foc-del +725
    • Circular polarimetry (lambda/4) :
      Use the best focus for direct imaging and foc-del +525
  5. Set TV-focus to about 600 and start auto guiding
  6. Make a small readout window (alfosc.polwin)
  7. Take one by one exposure while changing the telescope focus value step by step. (In good seeing, steps as small as 10 can be used.) Use imexam in the sequencer and measure the fwhm of the two components. The best focus is found when the two values are as equal as possible.
  8. If needed, put focus pyramid in the grism wheel and and take 15 seconds integration. The best focus for polarimetry is when the two components give about the opposite focus offsets (e.g. say +50 for the left and -50 for the right component )