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How to measure filter transmission curves with ALFOSC/FASU

  • Mount the smallest pinhole that you can find in the carrousel/optics cabinet.
  • Have the to-be-measured filter mounted in ALFOSC or FASU.
  • Mount suitable grisms (horizontal/vertical doesnt matter), that cover the wavelength region you are interested in. Beware of second-order effects.

  • Put all wheels open.
  • Put the pinhole in the beam.
  • Put the grism in the beam.

  • Switch on a suitable lamp:
    • either the internal halogen (red)
    • or the baffle flat (blue).

What frames to obtain:

  • Make a spectrum without the filter in the beam, and check that the highest counts are above 10000 ADU and below 40000 ADU.
    Note that the exposure times should not be below 2 seconds.
  • Then make a spectrum with the filter in the beam, with the same exposure time as the previous spectrum, and check the peak counts. If the counts are below 10000 ADU, then make new spectra with and without the filter, with equal expo times.
  • Switch off the halogen lamp.
  • Take the filter out of the beam, and make separate He, Ne, and/or ThAr spectra. See this table to find suitable arc lamps.

Data reduction:

  • Use the halogen spectrum taken without a filter to define the extraction aperture and background region.
  • Extract both halogen spectra *with* background subtraction, using the above defined aperture definition.
  • Extract the arc-line spectra using the above defined aperture definition.
  • Do wavelength calibration.
  • Make ratio of reduced spectra of halogen-with-filter/halogen-without-filter. This will give the filter transmission normalised to 1.
  • Multiply transmission curve by 100, and make nice plots.

How to measure grism transmission curves with ALFOSC

Analog to the above. Put in a suitable cross-disperser (gr#10, gr#11, gr#12) in the filter wheel. Put the to-be-measured grism in the grism wheel, at orthogonal dispersion direction wrt to the crossdisperser.

Use the above instructions, regarding the to-be-measured grism as a filter.

Back to top Last modified: 25-Jan-2008