Geometry at Prime

 

The Prime Focus Camera Unit is now mounted and dismounted with the probe `park' position defined by a mechanical template, which ensures a very reproducible geometry from one run to another. It is therefore very unlikely that the geometry parameters at Prime will have to be modified: In other words, be sure to have a very good reason for changing any parameters. If you find something odd with the geometry try first to attribute it to some other cause. In particular, check that the telescope was initialized with the correct configuration file for the CCD chip mounted: either with GO INT, PFRCA, COMPILE or GO INT, PFGEC, COMPILE (see section ).

The relevant geometry parameters at Prime, as shown in figure , are:

  
Figure: Prime Focus geometry parameters

for the centre of the CCD chip. The GD PARK position is at the top right-hand corner of autoguider field. Further, for the position of the rotator axis, we have:

The current values of these parameters can be inquired at the TCS terminal with the INPSCF task (see section ). Note that figure refers to a rotator position of 180. The sign convention is such that distances measured in x below GD PARK (as shown in the diagram) are positive, and distances measured in y towards the left from GD PARK are positive. This means that AOYNOM and TDATA(12) are normally negative. The x measures are close to zero, and therefore unpredictable.

Because at prime focus both the detector and the autoguider probe are positioned off-axis, the default beamswitch apertures A and B are defined such as to enable viewing of the autoguider field with the Finder TV camera and the CCD detector itself:

• The aperture A (offset) coordinates represent the offsets of the rotator axis from the GD PARK position (i.e. the top right-hand corner of the autoguider field) with the same sign convention as before, and equals the amount the telescope will move North and West in order to view the autoguider field on the Finder TV. The offset values are (must be) the same as TDATA(11), TDATA(12).
• The aperture B coordinates are the offsets of the GD PARK position from the centre of the chip, and equal the amount the telescope will move North and West in order to transfer a star at the chip centre to the parked autoguider probe. The offset values are (must be) the same as {AOXNOM + TDATA(11)} , -{AOYNOM + TDATA(12)} (the sign of the x coordinate is due to the sign convention).

The current values of the aperture offsets can be queried via the User Interface by typing:

AD(VISE)/T(ELESCOPE)

In order to test and re-determine the geometry parameters, proceed as follows:

1. • Perform a GLANCE to display a brightish star at about the centre of the chip.
   • Do a STARFIT to measure on which pixel the star is centred.
   • Park the autoguider probe by typing GDXY 550 900 at the ADAM terminal.
   • Type AC/TEL at the User Interface terminal.
   • Press the BEAMSWITCH B button and acquire the star with the autoguider.
   • Press the ACQ button on the Autoguider Panel, and when the small circle on the monitor has disappeared, stop the autoguider and press the BEAMSWITCH NOM button.
   • Do a STARFIT to find on which pixel the star is centred, and subtract from the measured coordinates those of the chip centre.
   • Change the sign of the x coordinate (because the detector x coordinate runs against the autoguider x coordinate.
   • Convert the offset values thus obtained into arcseconds by multiplication with 0.74 (RCA) or 0.54 (GEC).

   We now have determined corrections to the APERTURE B offset coordinates.

2. In order to correct the rotator centre coordinates, continue:
   • Rotate the rotator through 180. The telescope will move in an attempt to keep the star centred on the chip, using the current (approximately correct) values of AOXNOM and AOYNOM.
   • Perform another GLANCE and do a STARFIT to find which pixel the star is now on.

   The mean of the two sets of measured pixel coordinates gives the coordinates of the apparent rotator centre. The differences between these and the pixel coordinates of the chip centre, converted to arcseconds as in the preceeding paragraph, and converted to radians by dividing by 206264.8, result in the corrections to the current values of AOXNOM and AOYNOM. Update these parameters in the sense that, if the apparent rotation centre is above the chip centre, ADD the difference to AOXNOM, and, if the apparent rotation centre is to the right of the chip centre, ADD the difference to AOYNOM.

3. Calculate the new APERTURE A coordinates by subtracting the new AOXNOM from the new APERTURE B x coordinate, and by adding the new AOYNOM and the new APERTURE B y coordinate and changing the sign.
4. Calculate the new rotator axis coordinates from the following expressions:

   GDXCTR = 39.3 (550 + TDATA(11)/0.97)
   GDYCTR = 39.3 (900 + TDATA(12)/0.97)

5. Enter the new geometry parameters into the Telescope Controle Software with the INPSCF task. Subsequently, inquire the new aperture offset values by typing at the User Interface terminal:

   AD/T

   and check that:

   AP/A equals TDATA(11) , TDATA(12)
   AP/B equals TDATA(13) , TDATA(14)

6. Finally, communicate the new geometry to the ADAM data acquisition software by typing at the ADAM terminal:

   TV\?OFFSETS

   and follow the prompts (Beware: defaults are not accepted; you must enter values, even if they are unchanged):

   • On ``offsets (arcsecs) from rotator centre to parked probes'': Take the Aperture A coordinatess and enter them as -Ap.Ay Ap.Ax.
   • On ``offsets (arcsecs) from rotator centre to CCD centre'': Enter -AOYNOM +AOXNOM.
   • Chip size in arcseconds: Enter 237 379 for the RCA chip or 208 312 for the GEC chip.
   • Finally, the angle offset in milliradians between TV axes and RA, Dec is currently 10.

You should now be able to find guide stars in any orientation of the rotator. If you can't, report it to the Astronomy Group.

All default values are loaded automatically at startup. If you are sure the new geometry is correct, inform the Software Group, who will enter the new values as defaults in the proper `.STD' configuration file. Figure shows the current geometry of the INT Prime Focus Unit.