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 |
|
