Technical aspects of the new AAO/UKST H interference filter

The UKST H survey and the need for a new filter

With the successful introduction of Tech Pan film at the UKST in 1992 a series of experiments with narrow band H imaging was performed using existing H filters. This was because Tech Pan has a sensitivity peak around H lending itself to the possibilities of a new H survey. However serious defocussing/imaging defects were seen.The old mm AAO656 filter (FWHM 190 Å, central wavelength 6560 Å) had become locally delaminated giving gross defocussing over large areas. Other remaining UKST H filter mosaics (e.g. as used by Meaburn & Rovithis, 1977) generally had fields of view too small for survey work, were very old and suffered from cosmetic defects, blemishes and poor image quality. Such effects are expected due to the nature of narrow-band interference filters used in fast converging beams, especially if they are a mosaic and not of extremely tight specification. Imaging imperfections are also more noticeable with Tech Pan's high resolution compared to the coarse grained 098-04 emulsion used for most previous UKST H exposures. Existing full field 4 and 16 element H mosaics also gave stripey cosmetic defects and de-focussed areas due to small filter component alignment problems as well as having quite broad passbands (80-200 Å).

Clearly, if we wish to properly exploit the excellent imaging qualities of Tech Pan to obtain deep, wide-field, UKST H imaging then we needed an interference filter of exceptional specification and quality and one that can image a substantial fraction of the UKST's large field. We thus sought an unusually large custom made narrow-band interference filter that would give the best achievable imaging over the widest area. This was necessarily coupled with a choice of filter central wavelength and bandpass that would work effectively under the constraints imposed by use in the UKST's fast f/2.48 converging beam.

© Copyright Astronomical Society of Australia 1997