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When you intend to use different filters during your observing run, you should focus the telescope without any filter in the light path. When you put in a filter you add the corresponding offset given in tables below, using the command foc-del nn, where nn is the offset value. Under good seeing conditions (0.5 arc sec), repeated focus measurements indicate an uncertainty of around 5-10 focus units. As the seeing gets worse, the variation between measurements increases, and can under bad conditions result in uncertainties of up to 25-30 focus units.

Note:
The use of the 1/2 wave and 1/4 waveplates in polarimetry also introduces an offset in telescope focus. See FAPOL table below.
Focus offsets measured in good seeing conditions with an accuracy of 5-10 steps.Focus offsets for filters measured in mediocre seeing conditions have uncertainty of 25 steps.Focus offsets for filters measured in quite bad seeing conditions. If you have better than 1.5" seeing, then please re-measure yourself: 42, 50, 68, 113
The calculated offset is based on a correlation between the thickness of the filter and the known TCS focus offset (+19 TCS units per mm).
If you have better seeing than for which the offset was measured, you may want to make your own offset measurement. If the seeing is worse you can safely use the given offsets.

Please note that these tables are not complete. If the filter you are using has 0-value here, you will have to find the focus offset yourself. We will of course greatly appreciate that you share such measurements with us.

ALFOSC
IDNameThicknessOffsetAccuracy
7 U_Bes 362_60 -70 5-10
11 I_Bes 728_LP 5 -5 25
12 i_int 797_157 -5 5-10
74 B_Bes 440_100 -85 5-10
75 V_Bes 530_80 -10 5-10
76 R_Bes 650_130 -80 5-10
92 550_275 -10 5-10
100 GG420 416_LP 3 -90 25
101 GG475 480_LP 3 -105 10

FAPOL
IDNameThicknessOffsetAccuracy
620 lambda/2 290 5-10
621 lambda/4 80 10-20

FASU (ALFOSC/MOSCA)
IDNameThicknessOffset (Measured)Offset (Calculated)Accuracy
7 U_Bes 362_60 0 0
11 I_Bes 728_LP 5 0 95
12 i_int 797_157 0 0
14 r_Gun 680_102 5.4 113 103 10
15 u_Str 351_34 5.8 80 110 10
16 v_Str 411_18 4.8 80 91 10
17 b_Str 467_18 4.8 95 91 10
18 y_Str 547_22 4.8 90 91 10
19 Hbet 485_4 5.8 90 110 10
20 Hbet 485_13 5.7 90 108 10
21 Halp 656_3 6.2 130 118 10
22 Halp 658_18 6.3 120 120 10
23 [SII] 403_3 10.1 160 192 10
24 407_3 10 190 190 calculated
25 [SII] 1020_10 10 190 190 calculated
26 1033_10 10.1 190 192 calculated
30 [OII] 373_5 4.6 133 87 10
31 [NeIII] 387_4 6.2 150 118 15
32 Hdel 410_4 6.2 100 118 15
33 Hgam 434_3 6.2 120 118 calculated
34 HeI 448_6 6.2 110 118 25
35 HeII 470_6 6.2 79 118 10
36 b_Str 472_20 6.2 120 118 calculated
37 Hbet 487_5 6.2 126 118 10
38 Hbet 489_10 6 110 114 calculated
39 Hbet 487_4 6.1 105 116 25
40 [OIII] 501_4 6.2 100 118 25
41 506_5 6.2 148 118 10
42 511_5 6.2 140 118 10
43 515.9_4 6.1 135 116 25
44 y_Str 552_18 6.1 120 116 calculated
45 HeI 588_6 6.2 120 118 calculated
46 Eu 619_2 6.2 120 118 calculated
47 [OI] 631_4 6.3 120 120 calculated
48 635_4 6.1 120 116 calculated
49 661_5 6.3 135 120 10
50 665_5 6.2 125 118 re-measure
51 670_4 6.3 125 120 10
52 [SII] 672_6 6.1 120 116 10
53 674_5 6.1 135 116 25
54 679_4 6.3 135 120 25
55 684_5 6.2 140 118 re-measure
56 688_6 6.2 120 118 calculated
57 692_5 6.2 120 118 calculated
58 696_5 6.2 140 118 25
59 HeI 727_2 6.2 120 118 calculated
60 751_5 6.1 120 116 calculated
61 892_5 6.1 120 116 calculated
62 948_5 6 110 114 calculated
63 v_Str 412_19 6 110 114 calculated
64 Halp 656_5 6.1 135 116 10
65 504_4 9.2 170 175 25
66 510_4 9.9 205 188 25
67 521_10 9.2 185 175 25
68 664_4 9.6 180 182 re-measure
69 670_3 9.9 190 188 calculated
70 667_5 9.8 210 186 5-10
73 GG385 380_LP 2 40 38 25
74 B_Bes 440_100 0 0
75 V_Bes 530_80 0 0
77 662_4 4.9 95 93 15
78 [NII] 658_4 4.7 95 89 10
79 HeII 469_3 4.8 90 91 calculated
81 [SIII] 953_3 4.7 90 89 calculated
82 u'_SDSS 350_65 5.2 100 99 calculated
83 g'_SDSS 480_145 5 90 95 25
84 r'_SDSS 625_140 5 110 95 25
85 i'_SDSS 770_160 5.1 100 97 calculated
87 [OI] 630_3 5.4 100 103 calculated
88 [OII] 372_3 5.1 60 97 10
89 [NII] 575_3 4.7 90 89 calculated
90 [OIII] 501_3 4.7 80 89 10
91 [SII] 673_1 5.1 100 97 calculated
93 GG375 375_LP 4 70 76 25
94 WG345 356_LP 4 85 76 25
95 674_LP 3.1 60 59 calculated
96 776_LP 3 60 57 calculated
97 357_2 8.8 170 167 calculated
98 358_2 9 180 171 25
99 369_4 7.2 110 137 25
100 GG420 416_LP 3 55 57 25
101 GG475 480_LP 3 50 57 10
102 OG515 518_LP 3 60 57 10
103 OG575 566_LP 3.1 60 59 25
104 U_Bes 361_62 5.3 140 101 15
105 B_Bes 428_108 5.1 80 97 25
106 V_Bes 536_89 5 120 95 15
107 R_Bes 646_124 5.2 80 99 10
108 I_int 817_163 5 80 95 10
109 u'_SDSS 353_55 5 60 95 10
110 r'_SDSS 618_148 5.1 95 97 10
111 i'_SDSS 771_171 5 100 95 10
112 z'_SDSS 832_LP 5 80 95 10
113 494_5 5.9 100 112 re-measure
114 u_Str 347_38 8.4 170 160 10
115 v_Str 412_18 4 70 76 10
116 b_Str 468_19 6 110 114 10
117 y_Str 547_21 6.5 135 124 10
118 Hb_Str 485_4 6 120 114 10
119 Hb_Str 487_14 6.6 100 125 10
120 g'_SDSS 480_145 5.2 90 99 25
121 406_6 8 180 152 15
122 363_6 8 150 152 calculated
123 [SII] 672_5 7 130 133 calculated
124 Sort13 554_166 5.2 100 99 calculated
125 633_26 7.4 150 141 10
127 515.6_4 8 150 152 10
128 551_4 8 150 152 10
129 489_5 6.35 110 121 25
130 381_8 8 150 152 calculated
131 410_6 8 185 152 15
132 463_248 1.9 40 36 25
133 401_7 8.1 130 154 15
134 391_7 7.6 130 144 15
135 458_66 8 170 152 15
136 397_6 6.8 90 129

IAC filters

Focus offsets for IAC filters.
IAC# offset mounted in
12 +150 FASU
15 +105 FASU
17 +90 FASU
19 +140 FASU
20 +150 FASU
24 +155 FASU
31 +110 FASU
36 +140 FASU
39 +150 FASU
40 +120 FASU
41 +120 FASU

Andrew Cardwell's filters

Focus offsets for ANDV filters. Note, offsets for ANDV7432 and ANDV8348 are not tested
Filter offset mounted in
ANDV7432 +210 FASU
ANDV7435 +210 FASU
ANDV8346 +210 FASU
ANDV8348 +210 FASU

Back to top Last modified: December 29 2011