Geometry at Cassegrain

Rotator Centre

The relevant Task Common parameters are TVXCTR, TVYCTR, GDXCTR and GDYCTR, which represent the position of the rotator centre in the TV probe coordinates, where the TV cross-wires define the probe position, and in guide probe units, where the tracking centre defines the probe position, respectively.

These four quantities can be measured as follows:

1. Set the nominal aperture offset parameters AOXNOM and AOYNOM to zero.
2. Centre a star on the probe (either the TV or the autoguider).
3. Rotate the turntable to the cardinal points in turn (0, 90, 180, 270) and at each position angle move the probe so that the object is centred on the probe position (as defined above).
4. At each position note the (x,y) probe position as shown on the MIMIC display.

The rotator centre position in probe coordinates is now the centre of the circle on which the measured points lie, i.e. the mean of x and y measurements if the cardinal points have been used.

Nominal Aperture Offset

The parameters of concern here are AOXNOM and AOYNOM, which are used to allow for the fact that the reference position is off-axis on the rotator. Their values must be redetermined if the A&G box is somehow shifted. Define the relevant reference point as follows:

1. For the IDS and FOS, use the intersection of the slit and the dekker. All dekker apertures, in the IPCS setup dekker at least, are concentric apart from the comb dekker.
2. For other instruments and non common-user instruments, which do not have any easy reference positions, use the TV crosswires (with the TV probe centred) with the flip mirror in the field position, or use the centre of the detector.

Measure AOXNOM and AOYNOM as follows:

1. Set up the system so that the values of AOXNOM and AOYNOM are roughly correct, i.e. so that a star image stays in the field of the probe when the turntable is rotated. In fact, at Cassegrain it is good enough to set these parameters to zero.
2. Point the telescope to a star near zenith (and track, of course).
3. Rotate the turntable to the cardinal points and measure or mark the position of the star on the TV probe with Grinnell cursors (see chapter ).
4. Determine the centre of rotation from the measured positions.
5. The corrections to the values of AOXNOM and AOYNOM already in the system are the distances between the reference position and the apparent centre of rotation. To determine this, put the star on the calculated position of the centre of rotation and use SNAFU/A to determine the offset to the reference position. Using the BEAMSWITCH NOM and A buttons, it should now be possible to offset and switch back and forth between the two positions (circle centre and reference point). The required values of AOXNOM and AOYNOM will then be displayed on the information display.
6. Enter the correct values for the parameters and test their validity by centering an object on the reference position and seeing whether its position alters as the turntable is rotated.