April, 2001

This short note gives details about how to gather pointing data (do a pointing run), reduce the data using "TPOINT" to obtain a pointing model, and then introduce this model into the telescope control system.

To do a pointing run, use the scope window and hit the button to start a pointing run. This will modify the scope window to add 3 buttons (Skip, Save, Next). The Start button brings up these buttons and initializes a raster pattern to run around the sky and gather pointing stars. Any catalog can be used, but the FK5 stars have always been used thus far.

Use the next button to search the catalog for stars near the next raster position. Select one of these stars and use the Go button to move the telescope to it. Then use the paddle and offsets to center the star. Then use the Save button to save a record to the pointing file. If the star cannot be found, use Skip, otherwise Next will refuse to move to a new position until the star is saved.

It is entirely possible to do a pointing run without using the raster search guided by the Next button. To do this, select stars from the catalog (any catalog) and use the Go button to move to them, center them as above, and use the Save button to record the data.

Pointing data goes to hacksaw:/home/mmtop/catalogs/pointing.dat Before beginning a run, ensure that this file is empty, usually by renaming it to something you like. (new data always gets appended to the file.) Even more importantly, when you finish the run rename the file to something to preserve it. Also, mail the file to tom with an explanation.

Here is what one record in a typical pointing file looks like. Each record is tagged with a prefix such as RAA: and new records (with new tags) may be added at any time.

CAT: FK5-0808        21:31:33.538 -05:34:16.25 +00.141 -000.82  2.91 G0  J2000.0
RAA:   52.6554527  184.5903414
AA:   52.6747292  184.9251872
TAA:   52.6536137  184.9073619
LST: 21:42:45.86
CRD: 21:30:45.06 -05:33:44.27
SVEL: -0.0009 0.0068
SERR: 4 9

This record gets converted to one line (see below) and the header prepended as follows.

! Here is pointing data from 9/1/2000

! This is a wholly new 123 star set.

! MMT pointing data in tpoint format 4
! Next follows the header record. (Tpoint manual p. 23)
!       (TPOINT run parameters record)
! First 3 numbers are latitude (deg,min,sec)
! The next 3 are UTC date (year, month, day)
! The next 2 are Temp and Pressure
! The last 2 are Elevation and Rel. Humidity
!
MMT pointing data  9-1-2000
+31 41 19.6 2000 09 1 +14.0  970.0  2608.0 0.80

! These data are observed AZ/EL followed by Raw AZ/EL
! transformed AZ into tpoint convention (South = 0, East=90)

-4.90736190000001 52.6536137 -4.5903414 52.6554527

The complete files from the pointing run 9/1/2000 is here: in raw and tpoint formats. I use a small perl script to convert the raw file into tpoint format, and then hand edit the header information in place. Then I run tpoint with the following commands:

* indat mmt7.dat
* call altaz
* fit
* fit

       coeff       change     value    sigma

  1     IA         -0.001  +1188.83    9.961
  2     IE         +0.000     -5.53    0.764
  3     NPAE       -0.001     -8.35   11.574
  4     CA         +0.001     +3.13   14.232
  5     AN         -0.000     -3.49    0.872
  6     AW         -0.000    -26.83    0.863

Sky RMS =   8.46
Popn SD =   8.67

These 6 parameters are as follows: (All in units of Arc-seconds>

IA   = Azimuth axis index error (encoder offset).
IE   = Elevation axis index error (encoder offset).
NPAE = Az/El axis non-perpendicularity.
CA   = Elevation axis / Pointing axis non-perpendicularity.
AW   = E-W misalignment of Azimuth axis.
AN   = N-S misalignment of Azimuth axis.