Subsections

• Implementation of Tech Pan film hypering and processing procedures at the UKST
• Mounting Tech Pan film in the UKST

Practical considerations for UKST Tech Pan film use

The ability to use film-based emulsions in the UKST was seen as a cost-effective addition to normal operation in the late 1980s and was encouraged by Colin Humhpries and Ann Savage, successive Astronomers In Charge. However, though hypering and processing tests revealed its astronomical potential, several practical difficulties had to be overcome before hypered Tech Pan could be used routinely in the telescope. Specifically, modifications were required to the hypersensitisation plant, storage tins and handling frames to accommodate film, and other mechanical problems with mounting film in the telescope had to be addressed. Early experiments with mounting film (not Tech Pan) in the UKST met with only limited success (Humphries & Morgan 1988). It was the work of Russell et al. (1992) that finally demonstrated the viability of mounting Tech Pan successfully in the UKST.

Implementation of Tech Pan film hypering and processing procedures at the UKST

Once it was clear that the practical problems could be overcome, routine hypering of Tech pan required accurate temperature control of the hypering process at much higher temperatures than usually required for IIIa materials. The entire hypering system was refurbished in 1994 to achieve this and the normal six-plate hypering tins were modified to enable up to 12 films to be hypered simultaneously. The films themselves are supported on aluminium inserts which prevents contact between films while helping to maintain their flatness. The films are generally stored at C in dry nitrogen until they are required.

Special film handling frames were constructed to allow use of the standard plate processing line. Identical processing chemistry is used although Tech Pan film is developed for 10 minutes, twice as long as plates. Once loaded into the handling frames the films can be treated in the same manner as glass plates. The processed films are susceptible to dust, and the Estar support scratches easily, so films are placed into clear plastic storage sleeves after processing to minimise dust accretion and then, together with a film stiffener sheet, inserted into normal plate storage envelopes. Films are only removed from their plastic sleeves for machine digitisation or photographic copying. Careful handling of the films is required as they can `kink' leaving permanent indentations in the Estar which makes copying and machine scanning of the original films more difficult.

Mounting Tech Pan film in the UKST

The focal surface of the UKST is part of a sphere of radius 3.05m. Normally 1 mm thick glass plates are conformed to this surface by mechanical pressure and are held against a curved mandrel by a vacuum when the plate holder is inside the telescope. Because films are inherently more flexible they are difficult to simultaneously stretch and compress uniformly to the entire curved focal surface. For Tech Pan the film mounting problem eased when thick (m) Estar base became available in mm formats. The thicker base is much less flexible than the m variety used initially facilitating easier handling and mounting. Nevertheless, initial trials still produced grossly de-focused images. A number of modifications were implemented from 1991 onwards and the film mounting system has been steadily improved so that all stages of film handling are now as simple as handling plates.

Though numerous and detailed, these minor plate-holder modifications should be applicable to any large Schmidt telescope equipped with vacuum backed plate-holders (e.g. the ESO schmidt, Reipurth 1996).

© Copyright Astronomical Society of Australia 1997