Large Filters for CCD Imaging

We are slowly but steadily increasing our complement of large filters for direct imaging. 4 × 4 inch filters can be used for imaging with large format, single CCD systems at the 0.9-m, 1.5-m and 4-m. This size will severely vignette mosaic imagers, however. The BTC Mosaic Imager can accommodate 6 × 6 inch filters, although purchases made by CTIO are slightly smaller to be compatible with the NOAO Mosaic Imager. Although these filters are expensive and we have a limited filter budget, suggestions for new filter purchases are always welcome. We hope to purchase some Ha and O[III] filters for the BTC later this year.

New large filters recently received are:

• Johnson-Cousins UBVRI set, 146 × 146 mm and 12 mm thick, for use with the 4-m prime focus BTC Mosaic Imager, and eventually with the NOAO Mosaic.

The BVR filters are identical to the filters used with the NOAO Mosaic at KPNO. The I band filter is an interference filter to the same specification as our 4 × 4 I filters. The U filter does not match Johnson U very well, and has a narrower passband rather similar to Sloan u. The throughput is rather disappointing, with a peak transmission of only 50%. Later this year we hope to replace this filter with one that more nearly matches Johnson U.

• Sloan Digital Sky Survey (SDSS) ugriz set, 4 × 4 inch format (see AJ, 111, 1748, 1996 for a description of the SDSS photometric system).

Nominal central wavelengths and FWHM in Angstroms are u (3513/628), g (4759/1430), r (6265/1483), i (7734/1500), and z (9100/1400). The long wavelength cut-off for the z filter is CCD dependent.

• Calcium II H and K filter, 4 × 4 inch.

This filter supplements our Stromgren set, and is intended to permit Caby photometry (see AJ, 101, 237, 1991). The filter has center wavelength and FWHM 3960/100Å, with peak transmission of 50%.

Alistair Walker