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The telescope

The Schmidt Telescope is housed in its own building about 600m from the AAT. The building also contains offices, a plate and film inspection facility, darkrooms, workshops, an aluminising plant and hypersensitising facilities.

The dome is a hemispherical steel shell with an inner lining of polyurethane insulation and aluminium and a ventilated air gap between. Sideways opening shutters give a clear aperture of 2.74m. The dome also has a windscreen, and fans for ventilation.

The telescope's equatorial mounting is a symmetrical fork. Mechanical jacks allow the elevation of the polar axis to be adjusted to minimise the effects of differential refraction on field rotation. The pointing accuracy is about 6" rms, which means that guide stars can be acquired quickly and efficiently since they usually lie within the aperture of the guide telescope after a slew. Wallace and Tritton (1979 MNRAS ,115) discuss both the pointing and field rotation.

There are two 254mm aperture guide telescopes fitted to the main tube, and an X-Y stage at the focus of each allows stars to be found over a  +- 25'  range. The crosshead, which carries the autoguider, can also be driven at a controlled rate to provide differential motions for tracking solar system objects or widening objective prism plates.

The autoguider is based on a quadrant dissector photomultiplier. Light from a star is focused onto a pair of prisms which then, via a Fabry lens, produce four spots on the photomultiplier tube: two for hour angle and two for declination. Changes in the intensity of opposing spots are then treated as motion of the telescope. A chart recorder trace showing the (integrated) intensity of the guide star during an exposure is useful in checking for thin cloud. A new CCD-based autoguider has been successfully tested on the telescope.

Optics

The telescope is an f/2.5 classical Schmidt camera of 1.24m clear aperture. The general layout is shown in Figure 3.1, and some details in Table 3.1.

 
Figure 3.1: The Schmidt Telescope


Table 3.1: Details of the Schmidt Telescope

The corrector plate is a full aperture achromatic cemented doublet with elements of UBK7 and LLF6 Schott glass. It gives images with FWHM less than 1" at all photographic wavelengths from 3100Å to beyond 10,000Å, in the absence of seeing or photographic effects. The smallest images visible on IIIa-type emulsions are about 20um in diameter. Table 3.2 shows the UV transmission of the corrector as a function of wavelength.

 
Table3.1: UV transmission of doublet corrector

Until April 1977 (plate UR3148P), a single element corrector plate was used. Made of Schott BK7 glass, it gave images of less than 2" diameter over a 1000Å bandwidth centred on the corrected wavelength of 4200Å.

Objective prisms

The Schmidt has two full-aperture objective prisms, with apex angles of 44' and 2.25°. The prisms can be mounted either singly or together, and the complete assembly can be rotated to place the dispersion in any desired orientation (the default is with dispersion north-south). Prism configurations are denoted by P1 ( 44' prism), P3 (2.25° prism), P2 (P1 & P3 mounted opposed) and P4 (P1 & P3 mounted in parallel).

Table 3.3 lists dispersions for the various prism combinations at several wavelengths. More details are given in the Schmidt Telescope Handbook.

 
Table 3.3: Dispersions and effective resolutions for the four prism combinations

Before November 1984, objective prism spectra were widened using a tilting block image displacer in front of the autoguider. Widening is now done with the comet tracking mechanism, which is more flexible and allows the spectra to be dispersed in any direction. The widening rate can also be changed with time.

Sub-beam prism and objective grating

A sub-beam prism can be fitted in front of the corrector for photographic calibration, and a full aperture coarse objective grating is also available for use with photometric or astrometric programs.

The sub-beam prism and objective grating are described in detail in the Schmidt Telescope Handbook, but in practice they have rarely been used since CCD photometers became widely available for establishing calibration sequences.

Filters

Table 3.4 shows the Schott glass broadband filters available and the emulsions with which they are normally used. All filters are uncoated. The Earth's atmosphere and the telescope optics combine to give a short wavelength cut-off at about 3200Å. The AAT interference filters listed in AAO UM 5: Photography with the AAT can also be used, but their characteristics will be somewhat different in the f/2.5 beam of the Schmidt.

 
Table 3.4: Broadband filters available

HN32 polaroid sheets can be used as filters for linear polarization measurements. A 250mm diameter sheet, which can be rotated, is held in a special frame with a fixed sheet for reference purposes.

Several other filters, mainly interference filters designed and built by individual astronomers, are (semi-)permanently stored at Siding Spring and may be available on request. Details can be obtained from the Schmidt staff. In particular, a 15-inch square Halpha mosaic filter designed and built by John Meaburn is available for general use.

Only one filter at a time can be loaded into a plateholder, and it is not possible to make two exposures through different filters on the same plate.



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