Filters
Page last updated: Feb. 13, 2024
There are two filter wheels in each LBC. Each wheel has 5 positions, and since 1 must remain free, at most 8 filters can be available in LBCB and 8 in LBCR at any one time. The filter wheels and the cryostat are all mounted on the derotator, which tracks the sidereal rotation during an exposure. There are 6 filters for LBC Blue and 10 for LBC Red. The TiO_784 and CN_817 filters are installed in LBC Red only by request. It is important that the partner coordinators and observers communicate the need for these filters to LBTO staff well in advance of the run, to allow scheduling of a filter swap.
The tables below list some of the properties of the filters installed in LBC Blue and Red. Click on the filter name to see the plot of filter transmission and on the “ascii” link for the table. The plots show the curve based on the ascii file (solid black), which for all filters are now the actual measured transmission data.
These transmission data were measured in a collimated beam, normal to the filter surface except for the TiO and CN filters where the beam was at a 5 degree angle of incidence. For interference filters, the transmission in the convergent beam is blueshifted and the FWHM broadened from that measured in a collimated beam. The dashed lines in the transmission plots for the Sloan filters illustrate this effect, where the dashed black lines plot the predicted transmission in a convergent f/1.45 beam. Note the values for the 50% cut on and cut off wavelengths for the inteference filters were corrected for a convergent beam. For F972N20, both the transmission as measured in a collimated beam and the modeled transmission in an f/1.45 beam are plotted side-by-side, and for the TiO and CN filters, the calculated shift in the peak is printed on the plots. For Uspec, the shape of the transmission curve depends on where it is measured and three curves are shown, corresponding to points 15mm, 40mm and 60mm from the filter center.
| Filters in LBC Blue | ||||
|---|---|---|---|---|
| Position1 | Filter Name | 50% cut on [Angstroms] | 50% cut off [Angstroms] | Peak Trans[%] |
| 11 | empty | |||
| 12 | U-BESSEL ascii |
3432 | 3773 | 65.7 |
| 13 | V-BESSEL ascii |
4881 | 6015 | 93.1 |
| 14 | SDT_Uspec ascii_R15 ascii_R40 ascii_R65 |
|||
| 15 | B-BESSEL ascii |
3879 | 4569 | 65.7 |
| 21 | empty | |||
| 22 | r-SLOAN4,5 ascii #1 ascii #2 |
5520 | 6860 | 95.3 |
| 23 | empty | |||
| 24 | pinhole | |||
| 25 | g-SLOAN4,5 ascii #1 ascii #2 |
3970 | 5500 | 94.4 |
| Filters in LBC Red | ||||
|---|---|---|---|---|
| Position1 | Filter Name | 50% cut on [Angstroms] | 50% cut off [Angstroms] | Peak Trans[%] |
| 11 | empty | |||
| 12 | r-SLOAN5 ascii |
5533 | 6866 | 98.3 |
| 13 | I-BESSEL ascii |
7139 | 8803 | 96.8 |
| 14 | i-SLOAN5 ascii |
6972 | 8369 | 96.5 |
| 15 | z-SLOAN2,5 ascii |
8300 | N/A2 | 97.0 |
| 21 | empty | |||
| 22 | R-BESSEL ascii |
5755 | 6716 | 78.6 |
| 23 | V-BESSEL ascii |
4940 | 5716 | 87.3 |
| 24 | F972N20 ascii_meas ascii_model |
9636 | 9825 | 87.6 |
| 25 | Y-FAN5 ascii |
9525 | 11107 | 96.3 |
| TiO_7843 ascii |
7750 | 7941 | 98.3 | |
| CN_8173 ascii |
8082 | 8275 | 98.4 | |
1. The first digit denotes which wheel, 1 or 2, and the second the position, 1 through 5, in that wheel.
2. The Sloan z’ filter has no red cutoff, it is limited by detector QE which reaches ~0 at 1100nm
3. Only installed upon request.
4. There are two copies of the g-SLOAN and r-SLOAN filters in LBCB. The one marked #1 is installed and #2 is the spare.
5. These interference filters were measured in a collimated beam, but the cut on and cut off values in this table were corrected for a convergent beam as is the case for the LBCs.
