4.3 Aligning the Dewars

At the start of a series of FTS observing runs it is sometimes necessary to check that the FTS dewars are properly aligned with the optical axis of the rest of the FTS and the telescope, especially if a small cold stop is to be used during observations. This procedure is normally carried out by the CFHT support staff, but is included here for completeness.

  1. Stop down the iris at the entrance aperture of the FTS to a small diameter (5 arcsec).
  2. Insert the large aperture cold stop (normally 12 arcsec).
  3. Select a pair of filters with a bandpass in the thermal infrared, preferably at about 5 .
  4. Connect a voltmeter to the test output of one of the preamps.
  5. Since the iris at the entrance aperture of the instrument is not cooled and is smaller than the aperture cold stop it acts as a thermal source. Alignment of a dewars is carried out by adjusting the large hand screws at the bottom of the dewar to maximize the signal received by the detector, which occurs when the detector is aligned along an axis passing through the center of the entrance iris.
  6. Repeat the procedure for the other dewar.

Please send comments and suggestions to: veillet@cfht.hawaii.edu