WHT pointing-test instructions

At the WHT, a pointing test is typically required whenever the pointing rms (as reported by the CALIBRATEs) rises above ~ 2 - 3 arcsec at Cass or 3 - 4 arcsec at Nasmyth.

The pointing-test data will be used to constrain the values of ~ 18 coefficients in the WHT pointing model. This model relates requested and actual azimuths and elevations. Several of the coefficients have a physical basis, e.g. there is one to allow for tube flexure. The evolution in the values of the coefficients since 1990 is shown on the WHT pointing page.

~ 2 - 3 hours should be allowed for a full pointing test (80 - 100 stars). A pointing test can be done through cloud, and in seeing <~ 2 arcsec. The pointing test is usually carried out by the telescope operator.

Below are (A) instructions for carrying out a pointing test amd (B) instructions for carrying out a spot check of pointing accuracy across the sky.

(A) Instructions for carrying out a pointing test:

1. Plan roughly which stars to observe from the pointing grid (shown below). About 100 stars will be visible at any one time. It's necessary to observe ~ 80 - 100 stars, well distributed in azimuth and elevation, but concentrating on the zenith. Aim to observe half the stars at ZD < 45 deg. To ensure good coverage near the zenith, it may be helpful to observe the same star twice during the test. The order doesn't matter, but slewing time will be minimised by working broadly say, left to right across the grid, sampling the available declinations at each right ascension.
2. Choose the imaging detector to be used at the focal station of interest. If doing a pointing grid at Cassegrain, using the acquisition TV, 'direct-view' is preferred over 'slit-view' to minimise problems due to flexure of the slit-view probe.
3. Determine the rotator centre on the detector to be used (i.e. at the focal station of interest).
4. With ROT MOUNT 0, switch off the rotator tracking, to ensure that an error in the rotator centre enters only as a change in the collimation terms, and does not affect the other terms, or change the overall rms error.
5. Record the atmospheric pressure (mbar), outside air temperature (degC), relative humidity (fraction, between 0 and 1) and observing wavelength (micron), using the PRESSURE, TEMPERATURE, HUMIDITY and WAVELENGTH commands at the TCS. (If a mistake is made when entering these values, it can be corrected later, by whoever analyses the pointing test.)
6. Slew to the first star with e.g. GOCAT N11+82, centre it at the rotator centre (using the x,y mode of the handset), and type POINT NEW to log the position of the telescope as read from the encoders.
7. For each of the remaining stars, slew the WHT to the required position, centre up, and type POINT (= F20 on the keyboard). On a paper copy of the pointing grid (at the end of this page), cross off the star observed. It's helpful to also log the approximate azimuth and elevation of each star observed, e.g. on a copy of this diagram (provided by James McCormac):

   

   to monitor the fraction of stars being observed at high elevation. (NB the label at top left should read '315' not '305').
   Avoid observing too many stars at high declination, which are concentrated in one area of the sky.

8. Mail crb@ing.iac.es (Chris Benn) and fjg@ing.iac.es (Frank Gribbin) to let them know that a pointing test has been carried out, and is awaiting analysis.

Pointing grid:

   N01   N03   N05   N07   N09   N11   N13   N15   N17   N19   N21   N23 
   +82   +82   +82   +82   +82   +82   +82   +82   +82   +82   +82   +82  

N00   N02   N04   N06   N08   N10   N12   N14   N16   N18   N20   N22
+75   +75   +75   +75   +75   +75   +75   +75   +75   +75   +75   +75   

   N01   N03   N05   N07   N09   N11   N13   N15   N17   N19   N21   N23 
   +67   +67   +67   +67   +67   +67   +67   +67   +67   +67   +67   +67  

N00   N02   N04   N06   N08   N10   N12   N14   N16   N18   N20   N22
+60   +60   +60   +60   +60   +60   +60   +60   +60   +60   +60   +60 
   
   N01   N03   N05   N07   N09   N11   N13   N15   N17   N19   N21   N23 
   +52   +52   +52   +52   +52   +52   +52   +52   +52   +52   +52   +52  

N00   N02   N04   N06   N08   N10   N12   N14   N16   N18   N20   N22
+45   +45   +45   +45   +45   +45   +45   +45   +45   +45   +45   +45

   N01   N03   N05   N07   N09   N11   N13   N15   N17   N19   N21   N23 
   +37   +37   +37   +37   +37   +37   +37   +37   +37   +37   +37   +37  

N00   N02   N04   N06   N08   N10   N12   N14   N16   N18   N20   N22
+30   +30   +30   +30   +30   +30   +30   +30   +30   +30   +30   +30


S00   S02   S04   S06   S08   S10   S12   S14   S16   S18   S20   S22
+30   +30   +30   +30   +30   +30   +30   +30   +30   +30   +30   +30 

   S01   S03   S05   S07   S09   S11   S13   S15   S17   S19   S21   S23 
   +22   +22   +22   +22   +22   +22   +22   +22   +22   +22   +22   +22  

S00   S02   S04   S06   S08   S10   S12   S14   S16   S18   S20   S22
+15   +15   +15   +15   +15   +15   +15   +15   +15   +15   +15   +15 

   S01   S03   S05   S07   S09   S11   S13   S15   S17   S19   S21   S23 
   +07   +07   +07   +07   +07   +07   +07   +07   +07   +07   +07   +07  

S00   S02   S04   S06   S08   S10   S12   S14   S16   S18   S20   S22
+00   +00   +00   +00   +00   +00   +00   +00   +00   +00   +00   +00

   S01   S03   S05   S07   S09   S11   S13   S15   S17   S19   S21   S23 
   -07   -07   -07   -07   -07   -07   -07   -07   -07   -07   -07   -07  

S00   S02   S04   S06   S08   S10   S12   S14   S16   S18   S20   S22
-15   -15   -15   -15   -15   -15   -15   -15   -15   -15   -15   -15

   S01   S03   S05   S07   S09   S11   S13   S15   S17   S19   S21   S23 
   -22   -22   -22   -22   -22   -22   -22   -22   -22   -22   -22   -22  

S00   S02   S04   S06   S08   S10   S12   S14   S16   S18   S20   S22
-30   -30   -30   -30   -30   -30   -30   -30   -30   -30   -30   -30

   S01   S03   S05   S07   S09   S11   S13   S15   S17   S19   S21   S23 
   -37   -37   -37   -37   -37   -37   -37   -37   -37   -37   -37   -37  

S00   S02   S04   S06   S08   S10   S12   S14   S16   S18   S20   S22
-45   -45   -45   -45   -45   -45   -45   -45   -45   -45   -45   -45

   S01   S03   S05   S07   S09   S11   S13   S15   S17   S19   S21   S23 
   -52   -52   -52   -52   -52   -52   -52   -52   -52   -52   -52   -52  

(B) Instructions for a spot check of pointing accuracy:
The analysis of the full pointing test gives an estimate of the rms accuracy with which the WHT can be pointed using the new model, and a subsequent 7-star CALIBRATE (after the model has been updated in the TCS) provides a check of the actual pointing accuracy for a few stars near the meridian.

It's useful to measure the actual rms accuracy of pointing as a function of position on the sky, as follows:

• Last updated: 26/2/2021.