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
- Stop down the iris at the entrance aperture of the FTS to a small
diameter (5 arcsec).
- Insert the large aperture cold stop (normally 12 arcsec).
- Select a pair of filters with a bandpass in the thermal infrared,
preferably at about 5 .
- Connect a voltmeter to the test output of one of the preamps.
- 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.
- Repeat the procedure for the other dewar.