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Photographic plates

Hypersensitising

The hypering facilities at the Schmidt do not normally concern users, since plates are routinely hypersensitised by Schmidt Telescope staff. In some circumstances, staff can hypersensitise plates for use in other telescopes on Siding Spring Mountain.

Kodak IIIa-J and IIIa-F emulsions are hypersensitised by soaking in nitrogen gas for three months, followed by a few hours soak in hydrogen gas. IV-N plates are soaked in dilute silver nitrate solution and then dried, just before exposure. Speed gains of about a factor of 10 are achieved. Other emulsions are not usually hypersensitised.

Plate uniformity

All IIIa emulsions are exposed in the telescope in an atmosphere of dry nitrogen to remove differential desensitisation. The vignetting function for Schmidt plates has been discussed by Dawe and Metcalfe (1982, Proc ASA 4 (4) 466) and Campbell (1982 The Observatory 102, 195). It is slightly different for hypered and unhypered plates.

Calibration

For relative intensity calibrations of direct photographs, two uniformly illuminated step wedges are projected onto each photograph through the filter. The illumination is automatically switched on and off when the telescope shutter is opened and closed, so exposures of the wedges have the same duration as the main sky photograph and are made under exactly the same conditions of temperature and humidity. The bodies of the projectors provide a shielded area so that the calibration is put onto clear plate. In this way, the calibration is well determined to a level below that of the lowest sky density on the plate and contamination problems from stars and nebulosity are avoided.

The step wedges only give a relative calibration of the plates, and the problem of determining a zero point remains. One way is to use the background sky density in star-free regions of the photograph, and when this is done carefully it can give photometry accurate to 0.1 or 0.2 magnitude.

A small night-sky photometer with an EMI type 9502/B (S11) photomultiplier tube is used to measure the sky brightness during each exposure. Observations are made in the U, B and V wavebands and calibrated with a beta-light (tritium/phosphor) source. The nominal B sky brightness is also noted on the observing card records. To establish a photometric zero point, however, two numbers are needed: the night sky emissivity and the extinction. A night-sky photometer reading cannot provide both, and for high precision photometry the only satisfactory method is to relate photographic measurements to known photoelectric measures of stars or galaxies on the plate.

Ghosts, spikes and haloes

Internal reflections within and between telescope components (corrector, spiders, filter and plates) produce spurious or `ghost' out-of-focus images of real stars. The Schmidt Telescope Handbook describes and illustrates the various ghost images which may be seen. Most are only visible for bright stars (i.e. brighter than magnitude 6-7), though small ghosts arising from internal reflections within the filter can be seen for stars as faint as magnitude 11 on sky survey plates. These are eccentric except at the field centre. The scratch-like ghosts formed opposite very bright stars by grazing incidence reflections of spiders and other internal structures within the telescope are particularly insidious in dense star fields, and have several times led to the reported `discovery' of new edge-on galaxies, minor planets or comets. On prism plates, additional ghosts (some of which may be formed by stars outside the area of the plate) arise from reflections within the prism and between the prism and other optical components.

The vertical and horizontal spikes superimposed in the form of a cross on star images brighter than about 16th magnitude are diffraction spikes formed by the four spider arms supporting the plateholder.

Haloes arise from reflections within the photographic plate when light penetrating through the emulsion is reflected from the back of the plate. The halo is centred on the primary image, and has an inner radius of about 1.5mm. Halation rings are only visible on the brighter stars, and are worse on red-sensitive emulsions. All plates taken at the Schmidt are coated with an antihalation backing to reduce the intensity of these haloes.

Photographic film

Photographic emulsions coated onto a film base are stable enough for many astronomical purposes, and may offer significant advantages in both cost and quality over the traditional coated glass plate. Tests of film in the telescope are being carried out by Schmidt staff, from whom further details may be obtained. Hypersensitising is possible, just as for glass plates, and the finer grain of some emulsions available on film offers improved resolution along with the absence of the reflection ghosts seen on conventional glass plates.

While film is significantly cheaper than glass plates, it does have some disadvantages: it may not give a true astrometric replication of the sky, and may not be so convenient to copy.



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Next: The FLAIR multi-object Up: The Schmidt Telescope Previous: The telescope



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