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:
- 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.
- 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.
- Determine the rotator centre on the detector to be used (i.e. at the
focal station of interest).
- 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.
- 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.)
- 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.
- 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.
- 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:
