Focus offsets for NOT filters

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 1, 2 and 3 below, using the command foc-del nn, where nn is the offset value. Under good seeing conditions (0.5"), 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.

In the tables below, the filters are listed in order of increasing NOT identification number. The focus-offsets for the filters in table 1 was obtained in 0.5"-0.7" seeing, while the offsets in table 2 was measured in around 1" seeing. The offsets in table 3 were measured in around 1.5" seeing, so they are not as accurate. 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 is not listed here, you will have to find the focus offset yourself. We will of course greatly appreciate that you share such measurements with us.

Table 1. Focus offsets for various NOT filters measured in good seeing conditions. Accuracy is +/- 5-10 steps.
NOT# description offset mounted in
7 Bessell U -70 ALFOSC
12 interference i -5 ALFOSC
15 Strömgren u +80 FASU
16 Strömgren v +80 FASU
17 Strömgren b +95 FASU
18 Strömgren y +90 FASU
19 H-beta narrow +90 FASU
20 H-beta wide +90 FASU
21 H-alpha narrow +130 FASU
22 H-alpha wide +120 FASU
23 [SII] Miller blue +160 FASU
37 H_beta +126 FASU
49 H_alpha rs 5nm +135 FASU
51 H-alpha rs 15nm +125 FASU
52 [SII] +120 FASU
64 H_alpha +135 FASU
70 H-alpha rs 11nm +210 FASU
74 Bessell B -85 ALFOSC
75 Bessell V -10 ALFOSC
76 Bessell R -80 ALFOSC
78 [NII] +95 FASU
88 [OIII] +60 FASU
90 [OIII] +80 FASU
109 u' SDSS +60 FASU
110 r' SDSS +95 FASU
111 i' SDSS +100 FASU
114 u Stromgren +170 FASU
115 v Stromgren +70 FASU
116 b Stromgren +110 FASU
117 y Stromgren +135 FASU
118 Hbeta narrow Stromgren +120 FASU
119 Hbeta wide Stromgren +100 FASU
125 H_alpha blue continuum +150 FASU
127 515.6BP4 +150 FASU
128 550.7BP4 +150 FASU
 

Table 2. Focus offsets for various NOT filters measured in mediocre seeing conditions. Uncertainty is +/- 25 steps.
NOT# description offset mounted in
11 BESSEL I -5 ALFOSC
30 OII +75 FASU
34 HeI +110 FASU
39 H_beta +105 FASU
40 [OIII] +100 FASU
43 [OIII]rs 15nm +135 FASU
53 H_alpha rs 20nm +135 FASU
54 H_alpha rs 25nm +135 FASU
55 H_alpha rs 30nm +140 FASU
58 H_alpha rs 45nm +140 FASU
65 [OIII] +170 FASU
66 [OIII] rs 10nm +205 FASU
67 [OIII] +185 FASU
73 GG385 cut-on +40 FASU
83 g' +90 FASU
84 r' +110 FASU
93 GG375 cut-on +70 FASU
94 WG345 cut-on +85 FASU
98 Fynbo 357.8 +180 FASU
99 Fynbo 369.4 +110 FASU
100 GG420 cut-on -90 ALFOSC
120 g' SDSS +90 FASU

Table 3. Focus offsets for various NOT filters measured in quite bad seeing conditions. If you have better than 1.5" seeing, then please re-measure yourself.
NOT# description offset mounted in
42 [OIII] +100 FASU
50 H_alpha rs 10nm +125 FASU
68 H_alpha rs 8nm +180 FASU
113 H_beta redshifted +100 FASU

IAC filters

Focus offsets for IAC filters measured by observers in various athmospheric conditions:

Table 4. 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