Photographic Enhancement of H Films

Tech Pan film

The most technically advanced of these photographic materials, Eastman Kodak's Tech Pan, has been in use on the UK Schmidt Telescope (UKST) since 1992. It has an effective pixel size of about 5m, well matched to the 15m/arcsec image scale of this telescope. The detective quantum efficiency of the optimally hypersensitised material appears to be about 10 percent, significantly higher than the comparable IIIa-F material used hitherto. These advantages come at no cost in terms of exposure time, which is essentially the same as optimally hypersensitised IIIa-F, but the resulting negatives show fainter stars and extended objects when exposures are limited by the night sky brightness. Such `sky-limited' exposures are terminated where the subsequent developed photographic density due to sky is about unity.

However, the Tech Pan emulsion is only effective when coated on a film (Kodak 'Estar') base. Despite extensive testing (unpublished) at the Anglo-Australian Observatory, glass-based Tech Pan does not appear respond well to hypersensitisation. This may be the result of the anti-abrasion overcoat applied to film-based products.

Though large-format plates have been available for many years, it was only when the film-based material became available in 1992 that Tech Pan could be considered for use on large telescopes. As a result, much effort has been expended to design a system that could effectively use a film-based material in a hypering system, telescope and processing line designed for glass plates. These difficulties have now been overcome and use of hypersensitised film with good hypersensitised speed is a routine procedure at UKST.

An important feature of Tech Pan is a legacy of its origins as a solar patrol film. The spectral sensitivity in a narrow region around 640 to 670nm is considerably higher than elsewhere in the red part of the spectrum. This gives Tech Pan its excellent H sensitivity compared with the normal red-sensitive material used on Schmidt telescopes. This implies that a relatively narrow interference filter could be used to exploit this sensitivity enhancement and discriminate H from red continuum with practical exposure times. The excellent imaging properties of the film also contribute significantly to the usefulness of such exposures since star images under good conditions are very small. This is very important for the detection of extended emission in crowded Galactic Plane fields.

These features have encouraged us to undertake the first complete H survey of the Galactic Plane and the Magellanic Clouds (Parker and Phillipps, these proceedings). To this end a large format H filter was constructed by Barr Associates in the USA and first tests confirm that the filter-film combination works very well (Parker and Bland-Hawthorn, these proceedings).

In this short paper I will make some comparisons between H images taken on earlier generations of plates (and filters) and suggest ways in which faint emission features can be further enhanced against the sky background.

© Copyright Astronomical Society of Australia 1997