Filter OffsetsBack to Filters page
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 |
|---|
| ID | Name | Thickness | Offset | Accuracy |
|---|
| 12
| i_int 797_157
|
| -5
| 5-10
|
| 76
| R_Bes 650_130
|
| -80
| 5-10
|
| 7
| U_Bes 362_60
|
| -70
| 5-10
|
| 74
| B_Bes 440_100
|
| -85
| 5-10
|
| 92
| 550_275
|
| -10
| 5-10
|
| 75
| V_Bes 530_80
|
| -10
| 5-10
|
| 100
| GG420 416_LP
| 3
| -90
| 25
|
| 101
| GG475 480_LP
| 3
| -105
| 10
|
| 77
| 662_4
| 4.9
| 0
| 10-15
|
| 84
| r'_SDSS 625_140
| 5
| 0
| 25
|
| 11
| I_Bes 728_LP
| 5
| -5
| 25
|
| 14
| r_Gun 680_102
| 5.4
| 10
|
|
| 19
| Hbet 485_4
| 5.8
| 0
| 5-10
|
| 52
| [SII] 672_6
| 6.1
| 0
| 5-10
|
| 22
| Halp 658_18
| 6.3
| 19
| 5-10
|
| FASU (ALFOSC/MOSCA) |
|---|
| ID | Name | Thickness | Offset (Measured) | Offset (Calculated) | Accuracy |
|---|
| 75
| V_Bes 530_80
|
| 0
| 0
|
|
| 12
| i_int 797_157
|
| 0
| 0
|
|
| 92
| 550_275
|
| 54
| 0
| 5-10
|
| 7
| U_Bes 362_60
|
| 0
| 0
|
|
| 74
| B_Bes 440_100
|
| 0
| 0
|
|
| 132
| 463_248
| 1.9
| 40
| 36
| 25
|
| 73
| GG385 380_LP
| 2
| 40
| 38
| 25
|
| 96
| 776_LP
| 3
| 60
| 57
| calculated
|
| 101
| GG475 480_LP
| 3
| 50
| 57
| 10
|
| 102
| OG515 518_LP
| 3
| 60
| 57
| 10
|
| 100
| GG420 416_LP
| 3
| 55
| 57
| 25
|
| 103
| OG575 566_LP
| 3.1
| 60
| 59
| 25
|
| 95
| 674_LP
| 3.1
| 60
| 59
| calculated
|
| 94
| WG345 356_LP
| 4
| 85
| 76
| 25
|
| 115
| v_Str 412_18
| 4
| 70
| 76
| 10
|
| 93
| GG375 375_LP
| 4
| 70
| 76
| 25
|
| 30
| [OII] 373_5
| 4.6
| 133
| 87
| 10
|
| 78
| [NII] 658_4
| 4.7
| 95
| 89
| 10
|
| 90
| [OIII] 501_3
| 4.7
| 80
| 89
| 10
|
| 81
| [SIII] 953_3
| 4.7
| 90
| 89
| calculated
|
| 89
| [NII] 575_3
| 4.7
| 90
| 89
| calculated
|
| 18
| y_Str 547_22
| 4.8
| 90
| 91
| 10
|
| 79
| HeII 469_3
| 4.8
| 90
| 91
| calculated
|
| 16
| v_Str 411_18
| 4.8
| 80
| 91
| 10
|
| 17
| b_Str 467_18
| 4.8
| 95
| 91
| 10
|
| 77
| 662_4
| 4.9
| 95
| 93
| 15
|
| 112
| z'_SDSS 832_LP
| 5
| 80
| 95
| 10
|
| 84
| r'_SDSS 625_140
| 5
| 110
| 95
| 25
|
| 108
| I_int 817_163
| 5
| 80
| 95
| 10
|
| 82
| u'_SDSS 350_65
| 5
| 100
| 95
| calculated
|
| 106
| V_Bes 536_89
| 5
| 120
| 95
| 15
|
| 139
| [NII]_658_4_OSN
| 5
| 126
| 95
| 10
|
| 111
| i'_SDSS 771_171
| 5
| 100
| 95
| 10
|
| 109
| u'_SDSS 353_55
| 5
| 60
| 95
| 10
|
| 83
| g'_SDSS 480_145
| 5
| 90
| 95
| 25
|
| 11
| I_Bes 728_LP
| 5
| 0
| 95
|
|
| 138
| Halp_656_3_OSN
| 5
| 127
| 95
| 10
|
| 110
| r'_SDSS 618_148
| 5.1
| 95
| 97
| 10
|
| 88
| [OII] 372_3
| 5.1
| 60
| 97
| 10
|
| 85
| i'_SDSS 770_160
| 5.1
| 100
| 97
| calculated
|
| 91
| [SII] 673_1
| 5.1
| 100
| 97
| calculated
|
| 105
| B_Bes 428_108
| 5.1
| 80
| 97
| 25
|
| 124
| Sort13 554_166
| 5.2
| 100
| 99
| calculated
|
| 120
| g'_SDSS 480_145
| 5.2
| 90
| 99
| 25
|
| 107
| R_Bes 646_124
| 5.2
| 80
| 99
| 10
|
| 104
| U_Bes 361_62
| 5.3
| 140
| 101
| 15
|
| 87
| [OI] 630_3
| 5.4
| 100
| 103
| calculated
|
| 14
| r_Gun 680_102
| 5.4
| 113
| 103
| 10
|
| 20
| Hbet 485_13
| 5.7
| 90
| 108
| 10
|
| 15
| u_Str 351_34
| 5.8
| 80
| 110
| 10
|
| 19
| Hbet 485_4
| 5.8
| 90
| 110
| 10
|
| 113
| 494_5
| 5.9
| 100
| 112
| re-measure
|
| 144
| [SII]_672_1_OSN
| 5.9
| 110
| 112
| calculated
|
| 62
| 948_5
| 6
| 110
| 114
| calculated
|
| 118
| Hb_Str 485_4
| 6
| 120
| 114
| 10
|
| 63
| v_Str 412_19
| 6
| 110
| 114
| calculated
|
| 116
| b_Str 468_19
| 6
| 110
| 114
| 10
|
| 38
| Hbet 489_10
| 6
| 110
| 114
| calculated
|
| 43
| 515.9_4
| 6.1
| 135
| 116
| 25
|
| 48
| 635_4
| 6.1
| 120
| 116
| calculated
|
| 64
| Halp 656_5
| 6.1
| 135
| 116
| 10
|
| 39
| Hbet 487_4
| 6.1
| 105
| 116
| 25
|
| 53
| 674_5
| 6.1
| 135
| 116
| 25
|
| 44
| y_Str 552_18
| 6.1
| 120
| 116
| calculated
|
| 60
| 751_5
| 6.1
| 110
| 116
| 25
|
| 61
| 892_5
| 6.1
| 90
| 116
| 25
|
| 52
| [SII] 672_6
| 6.1
| 120
| 116
| 10
|
| 59
| HeI 727_2
| 6.2
| 120
| 118
| 25
|
| 34
| HeI 448_6
| 6.2
| 110
| 118
| 25
|
| 50
| 665_5
| 6.2
| 125
| 118
| re-measure
|
| 41
| 506_5
| 6.2
| 148
| 118
| 10
|
| 57
| 692_5
| 6.2
| 120
| 118
| calculated
|
| 143
| [SII]_671_1_OSN
| 6.2
| 120
| 118
| calculated
|
| 32
| Hdel 410_4
| 6.2
| 100
| 118
| 15
|
| 55
| 684_5
| 6.2
| 140
| 118
| re-measure
|
| 46
| Eu 619_2
| 6.2
| 120
| 118
| calculated
|
| 37
| Hbet 487_5
| 6.2
| 126
| 118
| 10
|
| 35
| HeII 470_6
| 6.2
| 79
| 118
| 10
|
| 42
| 511_5
| 6.2
| 140
| 118
| 10
|
| 58
| 696_5
| 6.2
| 140
| 118
| 25
|
| 33
| Hgam 434_3
| 6.2
| 120
| 118
| calculated
|
| 21
| Halp 656_3
| 6.2
| 130
| 118
| 10
|
| 40
| [OIII] 501_4
| 6.2
| 100
| 118
| 25
|
| 56
| 688_6
| 6.2
| 120
| 118
| calculated
|
| 31
| [NeIII] 387_4
| 6.2
| 150
| 118
| 15
|
| 45
| HeI 588_6
| 6.2
| 125
| 118
| 10
|
| 36
| b_Str 472_20
| 6.2
| 120
| 118
| calculated
|
| 22
| Halp 658_18
| 6.3
| 120
| 120
| 10
|
| 51
| 670_4
| 6.3
| 125
| 120
| 10
|
| 49
| 661_5
| 6.3
| 135
| 120
| 10
|
| 47
| [OI] 631_4
| 6.3
| 120
| 120
| calculated
|
| 54
| 679_4
| 6.3
| 135
| 120
| 25
|
| 129
| 489_5
| 6.35
| 110
| 121
| 25
|
| 117
| y_Str 547_21
| 6.5
| 135
| 124
| 10
|
| 119
| Hb_Str 487_14
| 6.6
| 100
| 125
| 10
|
| 136
| 397_6
| 6.8
| 110
| 129
| 20
|
| 137
| 410_7
| 6.9
| 110
| 131
| 20
|
| 141
| [HeII]_469_1_OSN
| 7
| 125
| 133
| calculated
|
| 142
| [OII]_373_1_OSN
| 7
| 125
| 133
| calculated
|
| 123
| [SII] 672_5
| 7
| 130
| 133
| calculated
|
| 140
| [OIII]_500_7_OSN
| 7
| 125
| 133
| calculated
|
| 99
| 369_4
| 7.2
| 110
| 137
| 25
|
| 125
| 633_26
| 7.4
| 150
| 141
| 10
|
| 134
| 391_7
| 7.6
| 130
| 144
| 15
|
| 127
| 515.6_4
| 8
| 150
| 152
| 10
|
| 122
| 363_6
| 8
| 150
| 152
| calculated
|
| 130
| 381_8
| 8
| 150
| 152
| calculated
|
| 128
| 551_4
| 8
| 150
| 152
| 10
|
| 135
| 458_66
| 8
| 170
| 152
| 15
|
| 131
| 410_6
| 8
| 185
| 152
| 15
|
| 121
| 406_6
| 8
| 180
| 152
| 15
|
| 133
| 401_7
| 8.1
| 130
| 154
| 15
|
| 114
| u_Str 347_38
| 8.4
| 170
| 160
| 10
|
| 97
| 357_2
| 8.8
| 170
| 167
| calculated
|
| 98
| 358_2
| 9
| 180
| 171
| 25
|
| 67
| 521_10
| 9.2
| 185
| 175
| 25
|
| 65
| 504_4
| 9.2
| 170
| 175
| 25
|
| 68
| 664_4
| 9.6
| 180
| 182
| re-measure
|
| 70
| 667_5
| 9.8
| 210
| 186
| 5-10
|
| 66
| 510_4
| 9.9
| 205
| 188
| 25
|
| 69
| 670_3
| 9.9
| 190
| 188
| calculated
|
| 24
| 407_3
| 10
| 190
| 190
| calculated
|
| 25
| [SII] 1020_10
| 10
| 190
| 190
| calculated
|
| 23
| [SII] 403_3
| 10.1
| 160
| 192
| 10
|
| 26
| 1033_10
| 10.1
| 190
| 192
| calculated
|
| ALFOSC POLARIMETRY |
|---|
| FAPOL |
|---|
| ID | Name | Thickness | Telescope offset | Accuracy |
|---|
| 620
| lambda/2
|
| 290
| 5-10
|
| 621
| lambda/4
|
| 80
| 10-20
|
| CALCITE |
|---|
| ID | Name | Instrument_offset | Telescope offset | Accuracy |
|---|
| 605
| Calcite
| -870
| 200
| 10
|
| 604
| Calcite
| -870
| 200
| 10
|
| 604
| Calcite
| -870
| 200
| 10
|
| 510
| Pol_1.8"
| -870
| 200
| 10
|
| 509
| Pol_1.4"
| -870
| 200
| 10
|
| 508
| Pol_1.0"
| -870
| 200
| 10
|
| 605
| Calcite
| -870
| 200
| 10
|
| 605
| Calcite
| -870
| 200
| 10
|
| 604
| Calcite
| -870
| 200
| 10
|
| 604
| Calcite
| -870
| 200
| 10
|
| 510
| Pol_1.8"
| -870
| 200
| 10
|
| 509
| Pol_1.4"
| -870
| 200
| 10
|
| 508
| Pol_1.0"
| -870
| 200
| 10
|
| 605
| Calcite
| -870
| 200
| 10
|
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 |
More information for IAC filters can be found here
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 |
|
