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| Home > Astronomy > William Herschel Telescope > Quality Control > WHT optics log |
WHT optics logThis page logs numerical results from past Shack-Hartmann tests at the WHT.They were obtained by analysing (with Dave King's newhart program) the shifts in the x,y positions of the ~ 400 spots between star and lamp (artificial star) exposures. Star and lamp exposures look something like this:
Each data line below gives:
For estimates of FWHM, multiply the quoted rms values by 1.67, assuming the PSF approximates a 2-dimensional Gaussian I(x,y). On the basis of the same assumption, ee80 = 2.54 rms, or 1.52 FWHM. Orientation: the tests are usually (but not always) carried out with the Shack-Hartmann camera mounted on the WHT folded-Cassegrain ACAM port, with the TEK2 CCD as detector, with instrument = 'OWN' at the TCS and with mount PA = 0. On the CCD, azimuth and elevation directions then lie at approximately 45 deg to the rows and columns, with azimuth increasing to top right and elevation increasing to bottom right on the CCD. A result 'coma PA = 135' then implies that the coma tail points in the direction of decreasing azmiuth. Increasing mount PA rotates the azimuth and elevation axes anti-clockwise on the TEK2 CCD. Lines below starting 'CALIBRATE' report the azimuth, elevation and horizontal-collimation errors from a 7-star pointing calibration (CALIBRATE) carried out on the same night. Changes in telescope pointing, i.e. in the results from the CALIBRATE, are a sensitive diagnostic of misalignment. A coma extent of 1 arcsec at the Cassegrain focus will result from any of the following:
Coma extent = length of 'teardrop' PSF in Zemax spot diagram. (Predictions originally provided by Sue Worswick in 1996. Updated by Emilie Lhome and Neil O'Mahony, using Zemax, in July 2017.)
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9108 Secondary reglued
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Date Foc El Raw Raw Sub Sub Sph Coma Astig
rms 80% 80% rms rms rms ext rms
941117 Aux 88 .11 .38 .28 .08 .05 .05 .05
58 .10 .36 .27 .07 .02 .05 .04
30 .10 .37 .26 .07 .02 .06 .04
83 .10 .37 .26 .07 .02 .04 .06
(very similar to 12/93 results)
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9508 proprtional controller for mirror support
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951007 primary aluminised
951008 wrong spacer, no useful data
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951120 secondary aluminised
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Date Foc El Raw Raw Sub Sub Sph Coma Astig
rms 80% 80% rms rms rms ext rms
960318 Aux 88 .14 .51 .36 .10 .04 .05 .24 .07
57 .12 .47 .29 .08 .03 .07 .33 .06
30 .11 .41 .29 .08 .02 .06 .29 .04
83 .12 .43 .29 .08 .02 .07 .34 .05
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961002 primary aluminised, subsequent o/focus images show coma extent
of approx 1"
961017 mirror support definers tweaked to reduce coma extent < 0.3 arcsec,
as judged from o/focus images
961122 Aux 82 .12 .43 .32 .09 .01 .05 .26 .06
52 .12 .43 .32 .09 .04 .06 .30 .03
82 .12 .44 .32 .09 .04 .05 .25 .05
Cloudy, test terminated by bad weather.
Increase of tel AZ changes x,y of spot by -5.5, -3.5 /arcsec
EL -3.5 5.5
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971113 primary aluminised, coma ~ 1 arcsec evident from o/focus images
Date Foc El Raw Raw Sub Sub Sph Coma Astig
rms 80% 80% rms rms rms ext rms
971113 Aux 86 .23 .85 .42 .13 .00 .18 .95 .07
50 .20 .70 .38 .11 .01 .16 .83 .03
29 .23 .79 .43 .13 .01 .18 .95 .02
971119 primary-mirror axial definers tweaked
971119 Aux 83 .11 .38 .29 .08 .03 .03 .19 .06
(following tests to check optical quality at PF not compromised
by 97119 tweaking)
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Date Foc El Raw Raw Sub Sub Sph Coma Astig
rms 80% 80% rms rms rms ext rms
980414 PF 85 .13 .47 .40 .11 .02 .01 .06 .08
74 .26 .74 .65 .22 .02 .02 .13 .13
64 .16 .57 .43 .12 .02 .04 .18 .10
50 .14 .50 .48 .13 .00 .04 .22 .03
down to elev 17 (seeing too poor to use data),
now coming back up...
31 .15 .39 .34 .14 .01 .02 .10 .06
47 .14 .46 .46 .13 .01 .03 .21 .04
60 .13 .46 .33 .10 .04 .05 .23 .05
81 .24 .78 .64 .20 .02 .02 .13 .12
88 .22 .82 .62 .17 .01 .08 .40 .11
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9809 Primary aluminised (gauges measuring axial position
of mirror replaced since 1997 aluminising, hence no coma
expected)
980910 Aux 78 .06 .31 .07
62 .05 .51 .05
33 .02 .30 .06
77 .01 .38 .12
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990506 1-kg weight fell onto mirror. Chipped, but no change in CALIBRATE,
so mirror not moved.
990527 Cass TV images markedly astigamtic, slit has moved
990530 Aux 65 .10 .39 .25 .07 .03 .06 .29 .06
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000613 Primary aluminised
000616 Aux 77 .11 .37 .28 .09 .01 .05 .26 .05
50 .15 .36 .32 .12 .06 .06 .29 .02
31 .14 .40 .34 .11 .06 .07 .37 .03
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Date Foc El Raw Raw Sub Sub Sph Coma Astig
rms 80% 80% rms rms rms ext rms
001108 Aux 75 .14 .47 .34 .10 .01 .07 .46 .06
40 .12 .41 .32 .09 .00 .07 .42 .03
(suspected optical problem actually a feature of INGRID)
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010923 Secondary mount driven to (focus) limit, collision with
a cable connector introduces large coma.
Elevation CALIBRATE coefficient has changed by ~ - 40 arcsec.
011029 Secondary remounted.
Elevation CALIBRATE coefficient back to normal.
Shack-H mounted at aux port but seeing bad, reduction
software unable to match spots reliably.
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Date Foc El Raw Raw Sub Sub Sph Coma Astig
rms 80% 80% rms rms rms ext rms
020326 Aux 79 .22 .85 .36 .10 .03 .17 .83 .10
(vector plot similar to that of 011129, so this is probably
residual coma from the 010923 event)
020420 Tilting secondary from handset:
Transducer readings 1 2 3
Control-cabinet channels 7 8 9
Correspond to pivot Q motor P motor
Pivot is in elevation direction.
Q-, P+ on handset increases transducer readings 2,3
assumed to indicate movement in elevation.
Q and P speeds are 4.0 and 2.1 microns/sec.
TCS displays of transducers out of range at +-100,
so used LCD displays of LVDTs instead.
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
rms 80% 80% rms rms rms ext rms PA
020420 At start of night, star image slightly comatic on ShackH TV view.
020420 Aux 81 .17 .59 .34 .10 .05 .12 .81 .02 -6
36 .20 .69 .35 .11 .08 .13 .83 .05 -14
Move secondary transducers P and Q 50 microns and back.
67 .08 .12 .76 .06 -11
Secondary LVDTs now read -38 85 69 on LCD display.
Tilt secondary in elev to -74 135 119 i.e. 50 microns each on Q and P
81 .07 .12 .81 .03 -6
Tilt secondary in elev to -101 185 169 on LCDs i.e. another 50 microns.
77 .10 .10 .67 .03 -6
Tilt secondary in elev to -154 285 269 on LCDs i.e. another 100 microns.
70 .09 .06 .34 .06 47
Tilt secondary in elev to -199 285 469 on LCDs i.e. another 200 microns
on Q.
59 .07 .04 .22 .05 133
59 .06 .03 .18 .06 159
79 .06 .06 .37 .08 -172
79 .06 .07 .38 .10 161
35 .07 .10 .59 .13 140
35 .08 .05 .25 .09 118
82 .18 .62 .47 .14 .02 .04 .23 .10 138
NB PA of coma (from sharp to blunt end) on CCD is measured clockwise
from vertical. Stars move at PA -45 when elevation increased, PA -135 when
azimuth increased.
During reduction, inner and outer spots masked out but vanes invisible
(up-down, left-right) so not masked.
No evidence after tweaking for distortions of in-focus or out-of-focus
images on ShackH TV.
Net movement was +200 microns on P, +400 on Q, i.e. a tilt ~ 80 arcsec
(+- 20%).
A tilt of this magnitude should cause coma 0.6 arcsec, which is
consistent. NB we might have simply compensated for translation by
introducing tilt, which would degrade image quality off-axis,
but this will be < 0.2 arcsec at 2' radius (e.g. INGRID).
020420 UES CALIBRATE coeffs AZ EL HC have changed from -21 -140 70
to -25 -90 130. A change of 50 arcsec in the EL term
corresponds to secondary tilt 110 arcsec, broadly consistent.
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
rms 80% 80% rms rms rms ext rms PA
020425 Aux 66 .09 .32 .28 .08 .01 .05 .24 .02 123
Checking no change after top-end flipped.
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021218 Primary mirror replaced after first aluminising for 2.5 years
021218 Aux 79 .15 .49 .38 .13 .00 .06 .31 .05 79
Aux 45 .14 .47 .41 .13 .00 .05 .25 .02 135
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030409 o/focus image pair taken at aux port by AZ, no aberrations
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041116 secondary aluminised
041123 pointing ~ 1' out in elevation, focus at LIRIS, aux and GRACE
prob increased by 0.3 mm
o/focus image taken on LIRIS, aberrations < 0.5 arcsec
Shack-Hartmann OK:
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
rms 80% 80% rms rms rms ext rms PA
041123 Aux 84 .12 .42 .28 .09 .05 .03 .18 .06 137
The support of the secondary mirror was apparently too rigid,
due to a misplaced washer on one of the springs. This could
affect pointing due to flexure of the mirror backplane being
picked up by the LVDT transducers and fed back to the TCS,
without affecting optical alignment.
Support adjusted.
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
rms 80% 80% rms rms rms ext rms PA
041124 Aux 86 .12 .41 .35 .10 .05 .02 .13 .06 73
041124 Aux 57 .14 .46 .35 .12 .03 .05 .27 .04 116
041202 Primary aluminised
041211 Aux 76 .08 .28 .26 .07 .03 .01 .09 .02 -159
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070131 Primary aluminised
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
rms 80% 80% rms rms rms ext rms PA
070202 Aux 75 .13 .48 .36 .10 .05 .04 .21 .05 -88
70 .15 .53 .38 .11 .03 .07 .36 .07 -74
46 .14 .52 .47 .13 .04 .05 .24 .03 -137
33 .10 .38 .25 .08 .03 .03 .18 .05 174
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070405 Test of optical quality at the previously-unused broken-Cass focus,
in anticipation of the possible deployment there of the HARPS-N
fibre feed.
Out-of-focus white-light images were obtained (the Shack-Hartmann
camera could not be mounted) at telescope focus 92.0 and 104.0 mm (best
focus +- 5 mm), elevation 70 deg. Results:
COMA: < 0.3 arcsec (FWHM)
SPH. ABERRATION: < 0.2 arcsec (diameter of disc of least confusion)
ASTIGMATISM: < 0.3 arcsec (diameter at best mean focus)
These limits are consistent with the values measured using the
Shack-Hartmann camera at the Cass auxiliary port 2 months earlier
(070202 above), each aberration ~ 0.1 arcsec FWHM.
On an image of the globular cluster M3, the images are round,
with FWHM 1.3 arcsec, and no apparent changes in form across
the 60-arcsec field of the camera (0.06 arcsec/pixel).
An image taken at elevation 20 deg showed the (expected) ~ 2-arcsec
dispersion in the elevation direction, but otherwise nothing
unusual.
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091103 Primary aluminised (no subsequent Shack-Hartmann, because of bad weather)
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120208 Primary aluminised
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
rms 80% 80% rms rms rms ext rms PA
120209 Aux 82 .13 .43 .33 .09 .03 .01 .04 .08 -151
34 .12 .45 .34 .09 .01 .02 .10 .08 156
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140519 PF 87 .16 .55 .35 .10 .03 .10 .51 .07 -43
78 .16 .56 .25 .08 .07 .09 .46 .08 -44
63 .14 .50 .38 .10 .05 .08 .35 .01 -54
51 .15 .55 .36 .09 .02 .11 .51 .02 -63
47 .12 .43 .28 .08 .01 .09 .43 .04 -56
25 .16 .57 .29 .08 .05 .11 .62 .05 -27
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140915 Shack-Hartmann tests at bare prime focus
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160519 Primary aluminised. Transverse-definer pad T3 (on side of M1)
came unglued, re-glued.
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
160520 Aux 71 .28 .91 .30 .09 .02 .25 1.38 .09 -138 (mt PA = 0)
=> coma in azimuth
CALIBRATE AZ, EL, HC = -22, 6, 143
(cf median ~ -25, -22, 100 Jan 2015 - mid-May 2016)
160521 Aux 49 .26 .92 .35 .11 .08 .21 1.16 .06 40 (mt PA = -180)
33 .29 1.04 .34 .11 .05 .25 1.44 .10 37 ( " " )
=> coma in azimuth (+- few deg)
CALIBRATE AZ, EL, HC = -13, -46, 149
160523 Aux 74 .35 1.18 .36 .10 .02 .32 1.70 .08 -130 (mt PA = -3)
160524 M1 transverse definer T3 adjusted
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
Aux 59 .31 1.12 .34 .09 .00 .28 1.53 .09 -136 (mt PA = 0)
52 .28 .99 .37 .11 .03 .25 1.27 .08 -138 ( " " )
=> coma still in azimuth, slightly worse
CALIBRATE AZ, EL, HC = -28, -21, 165
160526 M1 translated 2.8 mm in GRACE-GHRIL direction
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
Aux 74 .27 .93 .56 .16 .10 .07 .45 .19 -178 (mt PA = 0)
38 .17 .57 .44 .13 .06 .09 .52 .03 168 ( " " )
Later same night:
77 .28 1.07 .69 .20 .06 .06 .36 .17 -168 ( " " )
=> coma much reduced, but small astigmatism at high elevation
CALIBRATE AZ, EL, HC = -25, -26, 115
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160613 Further investigations of aberrations at Cass
All data taken with mount PA = 0
CALIBRATE AZ, EL, HC = -19, -31, 110
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
160613 Aux 84 .10 .37 .29 .08 .01 .04 .25 .04 166
35 .13 .44 .33 .09 .04 .06 .36 .04 170
I.e. image quality now very good, although M1 has not been
touched since 160526 (although it might have been relaxing to
its current position under repeated telescope movements).
Point to zenith. Definer T3 relaxed.
Aux 71 .16 .57 .41 .12 .05 .05 .35 .08 180
36 .16 .52 .33 .10 .07 .05 .31 .08 179
Point to zenith. Definer T3 restored to start-of-night state.
CALIBRATE AZ, EL, HC = -32, -25, 115
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
160613 Aux 76 .13 .47 .32 .09 .03 .06 .35 .07 178
51 .16 .56 .45 .11 .03 .04 .26 .09 160
28 .10 .34 .32 .10 .03 .01 .07 .02 76
51 .13 .46 .32 .09 .01 .04 .21 .08 -159
77 .12 .45 .36 .10 .00 .04 .37 .06 -159
Measurements on next 2 nights (no adjustments to M1):
160614 two CALIBRATEs during night:
CALIBRATE AZ, EL, HC = -18, -33, 108
CALIBRATE AZ, EL, HC = -25, -34, 111
160614 Aux 72 .13 .47 .30 .08 .01 .06 .37 .07 -154
20 .12 .46 .34 .10 .00 .07 .42 .04 172
30 .10 .34 .32 .09 .01 .04 .18 .02 118
48 .18 .63 .40 .12 .04 .05 .26 .12 110
160615 Aux 51 .10 .34 .30 .08 .01 .00 .02 .05 110
72 .14 .47 .40 .12 .03 .06 .30 .04 141
34 .10 .37 .29 .08 .02 .04 .27 .04 180
48 .10 .34 .32 .09 .01 .02 .11 .02 -84
Re-ordering the results of the above 13 tests by elevation:
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
20 .12 .46 .34 .10 .00 .07 .42 .04 172
28 .10 .34 .32 .10 .03 .01 .07 .02 76
30 .10 .34 .32 .09 .01 .04 .18 .02 118
34 .10 .37 .29 .08 .02 .04 .27 .04 180
48 .10 .34 .32 .09 .01 .02 .11 .02 -84
48 .18 .63 .40 .12 .04 .05 .26 .12 110
51 .10 .34 .30 .08 .01 .00 .02 .05 110
51 .13 .46 .32 .09 .01 .04 .21 .08 -159
51 .16 .56 .45 .11 .03 .04 .26 .09 160
72 .13 .47 .30 .08 .01 .06 .37 .07 -154
72 .14 .47 .40 .12 .03 .06 .30 .04 141
76 .13 .47 .32 .09 .03 .06 .35 .07 178
77 .12 .45 .36 .10 .00 .04 .37 .06 -159
Conclusions from measurements over the last 3 nights:
(1) At a given elevation, the derived values for each of
spherical-aberration, coma and astigmatism rms are consistent
from night to night within ~ +-0.03 arcsec.
(2) Astigmatism is consistently lower at elevation ~ 30 deg than
at higher elevations. Coma may be slightly higher near the zenith than
at lower elevations.
(3) The high-elevation astigmatism has decreased from the night
160613 to the night 160615. The significance of this is not
high, but the change might reflect relaxation of M1 after the tweaking
of definer T3 on 160613.
(4) The quality of the delivered PSF is currently similar to that
delivered over most of the past 20 years, albeit with coma and
astigmatism towards the upper end of the usual range:
Typical, June 2016
1990s - 2012 (high elevation)
(arcsec) (arcsec)
Raw rms: .12 - .14 .13
Sph aberration rms: .01 - .04 .02
Coma rms: .04 - .07 .06
Coma extent: .10 - .30 .35
Astigmatism: .03 - .07 .06
The delivered FWHM is 1.67 * raw rms ~ 0.21 arcsec.
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160718 Prior to removal of the WHT top-end flip ring on 160719
All data for 160718 taken with mount PA = 0
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
160718 Aux 80 .15 .52 .42 .11 .01 .04 .27 .08 152
48 .11 .40 .29 .08 .00 .06 .36 .05 176
37 .10 .38 .31 .08 .01 .04 .23 .05 -164
23 .11 .41 .28 .08 .01 .07 .44 .03 177
CALIBRATE AZ, EL, HC = -24, -29, 112
160719 WHT top-end flip ring removed, and replaced the next day (test
for WEAVE).
160721 Aux 80 .11 .38 .34 .09 .00 .03 .14 .05 -67 (mt PA = -125)
=> 168 (mt PA = 0)
59 .11 .41 .32 .09 .01 .03 .15 .06 -106 (mt PA = -84)
=> 170 (mt PA = 0)
31 .11 .41 .30 .08 .01 .06 .34 .05 -147 (mt PA = -58)
=> 157 (mt PA = 0)
27 .10 .35 .29 .08 .02 .04 .25 .04 178 (mt PA = -7)
=> 171 (mt PA = 0)
CALIBRATE AZ, EL, HC = -34, -38, 110
Conclusion: No degradation in optical quality after removal and
replacement of flip ring. Sizes of coma, astigmatism and spherical
aberration fall well within ranges observed over the last two decades.
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Test at prime focus (with PF corrector in light path)
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
160921 PF 60 .11 .39 .33 .09 .00 .05 .24 .04 -109
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170407 After removal and replacement of the WHT top-end ring, both
intra-/extra-focal imaging with ACAM, and comparison of before and
after CALIBRATE index errors indicate coma extent < 0.3 arcsec.
(A Shack-Hartmann test was not possible because the ACAM port was
needed for science.)
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170619 After removal and replacement of the WHT top-end ring, ACAM images
taken in 0.6 arcsec seeing have excellent circular symmetry, no sign
of any coma. The alignment of pupil and central hole on an out-of-focus
image of a star confirms coma extent < 0.6 arcsec, and comparison of
before and after CALIBRATE index errors indicates coma extent
< 0.4 arcsec.
(The Shack-Hartmann camera was not available.)
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Tests carried out at bare prime focus, i.e. without corrector,
M1 only, by Emilie Lhome, Neil O'Mahony and Bram Lap.
Date Foc El Raw Raw Sub Sub Sph Coma Astig Coma
yymmdd rms 80% 80% rms rms rms ext rms PA
180307 PF 67 .12 .43 .29 .08 .01 .04 .28 .08 -27
PF 67 .11 .42 .28 .08 .01 .04 .30 .06 -1
PF 63 .12 .44 .30 .09 .03 .04 .21 .07 73
PF 52 .12 .43 .27 .08 .00 .06 .31 .06 61
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191125 After removal and replacement of the WHT top-end ring,
intra-/extra-focal imaging with ACAM indicates coma extent
<~ 0.3 arcsec.
(The Shack-Hartmann camera was not available.)
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