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To check and correct the INT prime focus geometry

This should no longer be necessary, since the PFCU is now mounted and dismounted with the probe park position defined by a mechanical template. Before this was done, the geometry changed from run to run. It is included here in case it is needed again, but beware of red herrings such as the rotator position angle giving wrong readings etc. In other words, be sure to have a very good reason for changing any parameters.

In what follows, all directions refer to the scale drawing in the User Guide with the rotator in the 180 position.

1. On the Telescope Control System console, type:
   INPSCF <c/r>
   @ AOXNOM (produces current value - in radians - of -dist of chip centre below rotator axis)
   @ AOYNOM (produces current value - in radians - of -dist of chip centre from rotator axis)
   @ TDATA(11) (produces current value - in arcsec - of -dist of rotator axis below GD PARK, that is, top right corner of autoguider field)
   @ TDATA(12) (produces current value - in arcsec - of -dist of rotator axis from GD PARK)
   @ TDATA(13) (produces current value - in arcsec - of -dist of chip centre below GD PARK)
   @ TDATA(14) (produces current value - in arcsec - of -dist of chip centre from GD PARK)
   @ GDXCTR (produces current value - in m - of the rotator axis -coord)
   @ GDYCTR (produces current value - in m - of the rotator axis -coord)
   END
   

   Note that the sign convention is that distances measured in below GD PARK (as shown in the diagram in the User Guide) are positive, and distances measured in towards the left from GD PARK are positive. This means that AOYNOM and TDATA(12) are normally negative. The measures are close to zero, and therefore unpredictable.

2. On the User Interface console, type AD/T to read telescope parameters. Amongst the parameters is: Apertures: Ax,y(") [-value, -value]
   Bx,y(") [-value, -value]
   The first, Aperture A, is the coordinates of the rotator axis from GD PARK with the same sign convention as before, and is the amount the telescope will move North and West in order to view the autoguider field on the Finder TV. It is the same as TDATA(11),TDATA(12).

   The second, Aperture B, is the coordinates of GD PARK from the centre of the chip, and is the amount the telescope will move North and West in order to transfer the star from the chip centre to the parked autoguider probe.
   It is the same as {AOXNOM + TDATA(11)} , --{AOYNOM + TDATA(12)}.
   (The coord is not of the expected sign as RA increases towards the east.)

3. • Perform a GLANCE to display a brightish star at about the centre of the chip.
   • Do a STARFIT to find which pixel the star is centred on.
   • Park the autoguider probe by typing GDXY 550 900 on the ADAM console.
   • Type AC/TEL on the User Interface console.
   • Press the button BEAMSWITCH B and pick up the star with the autoguider.
   • Press ACQUIRE on the autoguider buttons, and when the small circle has disappeared, stop the autoguider and press the button BEAMSWITCH NOM.
   • Do a STARFIT to find which pixel the star is centred on and subtract from its coordinates the chip centre coordinates.
   • Now change the sign of the coordinate as the pixel coordinate is in the opposite sense of the autoguider coordinate.
   • Convert these to arcseconds by multiplying by 0.74(RCA) or 0.54(GEC).

   These are the corrections to update the APERTURE B coordinates.

4. • Rotate the rotator through 180. The telescope will move in an attempt to keep the star centred on the chip. This it does by the control program knowing 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 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 last paragraph, and then converted to radians by dividing by 206264.8, give the corrections to the current AOXNOM and AOYNOM to update them 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.

5. Calculate the new APERTURE A coordinates by subtracting the new AOXNOM from the new APERTURE B coordinate, and by adding the new AOYNOM and the new APERTURE B coordinate and changing the sign.

6. Calculate the new rotator axis coords from the following:
   GDXCTR = 39.3 (550 + TDATA(11)/0.97)
   GDYCTR = 39.3 (900 + TDATA(12)/0.97)

7. On the TCS system console, type
   INPSCF <c/r>
   AOXNOM < new value in rads>
   AOYNOM < new value in rads>
   TDATA(11) < new Ap.A coord in arcsec>
   TDATA(12) < new Ap.A coord in arcsec>
   TDATA(13) < new Ap.B coord in arcsec>
   TDATA(14) < new Ap.B coord in arcsec>
   GDXCTR < new value in m>
   GDYCTR < new value in m>
   END
   

8. On the User Interface console, check (by typing AD/T) that
   AP/A = TDATA(11) , TDATA(12)
   AP/B = TDATA(13) , TDATA(14)
   

9. On the ADAM console, type TV_OFFSETS, and follow prompts.

   You will first be asked for the ``offsets (arcsecs) from rotator centre to parked probes''. Take the Ap.A coords and enter them as -Ap.A Ap.A.
   Next, you will be asked for ``offsets (arcsecs) from rotator centre to CCD centre''. Enter -AOYNOM +AOXNOM.
   Then you will be asked for the chip size in arcseconds. Enter 237 379 for the RCA or 208 312 for the GEC.
   Lastly you will be asked for the angle offset in milliradians between TV axes and RA,Dec. It is currently 10.
   Beware. Defaults are not accepted. You must enter values, even if they are unchanged.

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.