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ACAM filter transmitted wavefront errors (TWE)
Filters used in one of ACAM's two filter
wheels lie in a near-collimated beam, and the bundle of rays
from each star
(diameter 51 mm at wheel 1,
42 mm at wheel 2)
intersects a
substantial fraction of the total area of the filter.
If the rms transmitted wavefront error (TWE) of the filter
is >> wavelength/4, the
delivered PSF on the science detector will be significantly degraded
compared to the diffraction limit. It's not straightforward to
predict PSF from TWE when the latter exceeds wavelength/4, but in
practice we find a good approximation to be:
FWHM (arcsec) ~ TWE (waves) / 1.8 That is, filters with rms TWE > 0.9 waves will significantly degrade the best seeing at the WHT (0.5 arcsec). In early 2011, Annemieke Janssen and Neil O'Mahony measured the TWE of most of ING's filters using a Shack-Hartmann test setup (designed by Tibor Agocs). The rms TWE (in lambda), measured over the central 40-mm area are given below for each filter (ordered by filter type and number). The quality label in the fifth column is 'OK' for TWE < 0.9, '~' for 0.9 < TWE < 1.8 and 'Bad' for TWE > 1.8. The quoted TWE values exclude the focus term introduced by the filter (given in column 6) since it should be possible to take out most of this (but not all of it) by refocusing the telescope - see the ACAM filter focus offsets page. The quoted TWE also exclude the tip and tilt terms introduced by the filter (columns 7 and 8). (NB in ING's internal filter database the column 'TWS' records measured TWE including the focus term, i.e. bigger values than tabulated below). An overview of the results, by filter type, is given on the ACAM imaging page. Only a few of the old Hα filters have TWE > 1.8 waves rms (i.e. are particularly bad). Stromgren V (#179) introduces an unusually large focus term. On-sky images of stars were taken by Cecilia Fariña in August 2014, through several filters: #58 (H6559) ; #61 (H6570) ; #62 (H6553) ; #66 (H6645) ; #86 (SiII 6727) ; Sloan R . The on-sky FWHM for filters #58 and #66 (TWE rms = 3.9 and 1.1 lambda respectively) are 1.0 and 0.5 arcsec worse than ambient seeing (broadly consistent with the equation above), and the PSF for #58 is clearly distorted. Various factors contribute to a filter's total TWE, including polishing errors and inhomogeneities in refractive index. Polishing errors usually dominate. If they leave the nominally flat surface with irregularities D rms, the TWE will be (n1 - n2) * D / wavelength, where n2 ~ 1.5 and n2 = 1 are the refractive indices of glass and air, i.e. TWE ~ 0.5 D / wavelength per surface. Multiplying by sqrt(2) for 2 surfaces, this implies a total TWE ~ 0.7 D / wavelength. If a filter has been specified to have surface flatness D < 1/4 wavelength, one expects TWE <~ 0.18 wavelengths (many of the TAURUS filters satisfy this).
Filter No. Diam TWE TWE Defoc Tip Tilt
name mm rms qual.
Broad band:
RGOB1 8 50*50 0.5 OK 0.6 8.8 -13.1
RGOV1 11 50*50 0.5 OK 1.1 1.2 -0.7
RGOV1 12 50*50 0.4 OK 0.6 5.4 7.4
RGOR1 14 50*50 0.1 OK 0.4 -7.5 1.2
RGOR2 15 50*50 0.4 OK 0.3 2.4 5.2
RGOI1 17 50*50 0.1 OK 0.0 0.4 -6.6
RGOZ1 20 50*50 0.1 OK 0.1 1.6 1.8
RGOZ2 21 50*50 0.1 OK -0.1 1.4 -1.9
RGOZ3 22 50*50 0.1 OK -0.1 -1.1 -2.7
GunnZ 24 50*50 0.2 OK -0.8 8.1 -1.3
HarB1 25 50*50 0.3 OK -1.1 6.0 17.8
HarB3 26 50*50 0.7 OK -0.1 -0.4 1.9
HarB4 27 50*50 0.3 OK -0.5 -2.4 4.0
HarB5 28 50*50 0.3 OK 0.4 -0.8 -1.0
HarV1 30 50*50 0.7 OK 0.2 15.7 11.6
HarV2 31 50*50 1.2 ~ -0.8 1.2 2.7
HarV3 32 50*50 0.0 OK 0.1 -0.7 -2.2
HarV4 33 50*50 0.0 OK 0.0 -1.1 -0.3
HarR2 37 50*50 0.5 OK 0.4 -4.6 1.3
HarR3 38 50*50 0.0 OK 0.2 1.1 -1.8
HarR4 39 50*50 0.0 OK 0.1 0.0 2.1
HarI1 41 50*50 0.1 OK -0.7 7.9 -6.0
HarI3 43 50*50 0.0 OK 0.0 1.7 0.4
HarI4 44 50*50 0.0 OK 0.0 1.3 1.2
KPB2 47 50*50 0.7 OK -0.2 -2.8 4.6
KPB3 48 50*50 0.9 OK -0.2 1.2 -1.8
KPV1 49 50*50 0.5 OK 0.7 4.2 13.0
KPV2 50 50*50 0.3 OK -0.5 16.2 -1.2
KPV3 51 50*50 0.4 OK 0.6 -2.0 0.2
KPR2 53 50*50 0.5 OK 2.3 1.8 0.7
KPI1 55 50*50 0.2 OK 0.6 -7.4 5.4
KPI2 56 50*50 0.5 OK 0.1 3.8 3.5
KPI3 57 50*50 0.5 OK 0.8 1.6 -2.8
Hα:
H6559 58 50*50 3.9 Bad 2.0 -1.3 13.6
H6570 61 50*50 0.5 OK 0.3 -3.5 -4.1
H6553 62 50*50 1.0 ~ 1.1 8.2 -2.4
H6594 63 50*50 0.3 OK 0.3 5.2 -7.0
H6607 64 50*50 0.5 OK -2.0 -5.2 10.8
H6626 65 50*50 0.2 OK 0.0 3.1 1.5
H6645 66 50*50 1.1 ~ -1.6 3.2 -0.3
H6695 67 50*50 0.2 OK 0.7 6.7 1.8
H6686 68 50*50 0.3 OK -0.1 -1.0 0.3
H6695 69 50*50 2.9 Bad 1.9 6.0 1.3
H6712 71 50*50 0.3 OK -0.1 -2.4 -5.0
H6736 72 50*50 2.5 Bad 0.4 10.4 -0.2
H6785 73 50*50 1.6 ~ -2.3 0.4 -3.5
H6834 74 50*50 1.1 ~ -0.7 3.3 0.0
H6876 75 50*50 0.9 OK 0.9 6.1 5.2
H6923 76 50*50 0.9 OK -2.3 1.9 3.6
H7962 77 50*50 3.5 Bad 2.3 -5.9 -2.7
Misc. narrow-band:
He4685 80 50*50 0.1 OK 0.9 5.1 0.9
H4861 81 50*50 0.3 OK 1.5 2.9 -1.3
O5009 82 50*50 0.2 OK -0.3 -1.4 0.5
He5877 83 50*50 0.2 OK 0.6 0.6 1.9
O6298 84 50*50 0.2 OK 0.2 -0.7 0.2
N6584 85 50*50 0.2 OK 0.0 -0.1 0.3
S6727 86 50*50 0.1 OK -0.3 2.8 1.4
O7331 87 50*50 0.0 OK 0.7 0.3 1.5
S9077 88 50*50 0.0 OK 0.0 1.1 -2.4
S9540 89 50*50 0.0 OK -0.3 0.3 -1.0
TAURUS narrow-band:
T4550 97 76 0.2 OK 0.4 5.0 3.0
T4570 98 63 0.1 OK 0.2 0.4 -0.4
T4770 99 63 0.1 OK 1.1 0.2 -0.1
T4868 100 63 0.4 OK 0.9 4.1 -2.0
T4880 101 76 0.1 OK 1.5 0.0 -0.1
T4883 102 63 0.4 OK 1.0 -0.1 1.4
T4898 103 63 0.4 OK 1.0 0.3 1.7
T4912 104 63 0.3 OK 1.1 0.2 1.1
T4962 105 76 0.2 OK -0.4 -0.1 1.9
T4974 106 76 0.2 OK -0.2 1.1 -0.8
T5000 107 63 0.1 OK -0.1 -0.3 0.6
T5009 108 76 0.2 OK -0.5 14.1 -1.9
T5012 110 50 0.9 OK -0.7 8.4 -2.5
T5021 111 76 0.3 OK -0.4 -1.2 -4.8
T5032 112 50 0.3 OK -1.1 17.0 5.5
T5033 113 76 0.2 OK 0.0 6.8 -9.5
T5045 114 76 0.4 OK -0.3 -4.0 3.6
T5052 115 50 0.5 OK -0.9 2.3 0.9
T5065 116 76 0.1 OK -0.3 0.2 1.1
T5072 117 50 0.3 OK -0.4 1.7 7.2
T5092 118 50 0.3 OK -0.2 22.9 -9.2
T5111 119 76 0.2 OK -0.2 -0.8 0.2
T5129 120 76 0.1 OK -0.2 0.8 0.5
T5145 121 76 0.1 OK -0.1 0.8 0.8
T5162 122 76 0.1 OK -0.2 0.8 2.0
T5175 123 76 0.1 OK 0.0 0.6 0.3
T5196 124 76 0.2 OK 0.0 -0.9 0.5
T5214 125 76 0.1 OK -0.1 2.2 -0.1
T5232 127 76 0.1 OK -0.1 -0.3 2.4
T5250 128 76 0.1 OK -0.1 -0.8 -1.0
T5340 130 76 0.0 OK 0.2 -0.5 -0.2
T5700 133 63 0.1 OK 0.9 -0.8 -1.3
T5905 134 76 0.1 OK 0.7 1.1 -0.5
T5690 135 63 0.1 OK 0.7 -0.5 -0.6
T6240 136 63 0.1 OK 0.5 0.9 -0.5
T6303 137 63 0.1 OK 0.2 0.5 -0.6
T6565 139 76 0.3 OK -0.1 5.5 0.1
T6568 140 50 0.2 OK -0.2 7.9 -1.0
T6577 141 76 0.4 OK 0.4 17.6 2.1
T6589 142 76 0.3 OK -0.1 -8.3 -3.4
T6589 143 76 0.7 OK -0.1 2.2 0.0
T6590 144 76 0.0 OK 0.0 0.1 0.2
T6610 146 50 0.2 OK 0.0 5.6 -1.9
T6613 147 76 0.2 OK -0.3 3.2 5.6
T6631 148 50 0.2 OK 0.0 0.2 2.4
T6637 149 76 0.1 OK -0.1 -0.9 -0.7
T6652 150 50 0.4 OK 0.2 5.7 14.7
T6673 151 76 0.0 OK -0.1 0.1 -0.8
T6673? 152 76 0.2 OK 0.1 13.9 12.5
T6685 153 76 0.1 OK -0.1 0.8 -0.4
T6689 154 63 0.0 OK 0.0 -1.0 0.5
T6697 156 76 0.1 OK -0.1 1.4 1.2
T6709 157 76 0.1 OK -0.1 -0.2 1.0
T6721 158 76 0.1 OK -0.1 -1.0 0.2
T6733 160 76 0.0 OK -0.1 0.5 0.7
T6745 161 76 0.1 OK -0.2 -0.1 0.4
T6757 162 76 0.0 OK -0.2 -0.4 1.4
T6769 163 76 0.0 OK -0.2 -0.9 -0.5
T6770 164 50 0.1 OK 0.0 0.2 1.6
T6781 165 76 0.0 OK -0.1 0.5 1.0
T6793 166 76 0.0 OK -0.3 -1.0 -1.4
T6805 168 76 0.1 OK -0.2 -0.9 -1.6
T6817 169 76 0.1 OK -0.2 0.8 -0.4
T6829 170 76 0.1 OK -0.2 1.4 0.3
T6841 171 76 0.0 OK -0.2 -2.1 -1.8
T6853 172 76 0.1 OK -0.3 -0.6 -1.2
T6865 173 76 0.1 OK -0.2 0.5 1.3
T6877 174 76 0.1 OK -0.3 0.3 -1.3
T6889 175 76 0.1 OK -0.3 1.2 -0.9
T4686 189 76 0.3 OK 0.5 4.8 7.1
Narrow-band:
O6310 176 50*50 0.1 OK 0.2 0.7 -0.6
Stromgren:
StromV 179 50*50 0.3 OK -5.1 3.5 3.3
StromB 180 50*50 0.2 OK 1.2 0.5 0.1
StromY 181 50*50 0.3 OK -0.3 -4.6 -2.2
StromHbW 182 50*50 0.2 OK 1.6 -4.7 -2.6
StromHbN 183 50*50 0.4 OK 0.7 -4.9 6.0
Sloan (new):
SlnU 700 60 0.2 OK 0.1 0.1 0.3
SlnG 701 60 0.0 OK 0.0 1.5 1.6
SlnR 702 60 0.2 OK 0.1 0.2 0.9
SlnI 703 60 0.2 OK 0.2 0.5 0.0
SlnZ 704 60 0.1 OK 0.0 0.3 0.2
Sloan (old):
SloGunR 216 50 0.1 OK 0.3 11.0 7.5
SloGunI 217 50 0.0 OK 0.4 0.2 0.8
SloGunG1 218 50 0.0 OK 0.4 1.0 1.2
SloGunG2 219 50 0.1 OK 0.4 0.7 0.4
Bessell:
BesB 231 50*50 0.2 OK 0.0 2.0 -0.6
BesV 232 50*50 1.7 ~ 1.7 -1.0 3.3
BesR 233 50*50 0.1 OK 0.4 1.2 0.4
BesI 234 50*50 0.0 OK 0.1 0.4 -0.1
Order-sorting filters:
GG395A 705 76 0.3 OK -0.3 2.3 9.1
GG495A 706 76 0.0 OK 0.1 0.7 1.6
The rms errors in the Shack-Hartmann-measured TWE and defocus terms depend strongly on wavelength, and are estimated by Annemieke to be as follows:
Band Rms error in Rms error in
defocus term TWE
B (3800-4900) 3.0 0.4
V (4900-5600) 0.5 0.16
R (5600-7300) 1.0 0.14
I (7300-8500) 0.7 0.03
Z (8500-9500) 0.5 0.03
The U filters have not yet been measured - the Shack-Hartmann setup used is not sensitive at these wavelengths. However, the new Sloan u filter (#700) has been used to image a focal-plane pinhole in ACAM itself, and the PSF is very good (FWHM < 2 pixels). (Because of the variation of focus with wavelength within ACAM, it's not currently straightforward to carry out the pinhole-imaging test for filters redder than U/B.)
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