Filters

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. 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.