The UK Schmidt Telescope (UKST)



The UK Schmidt Telescope (UKST) is situated at Siding Spring Observatory (SSO), New South Wales, Australia where it began its work in 1973. During its first 15 years, the UKST was operated by staff of the Royal Observatory, Edinburgh, on behalf of the UK Science and Engineering Research Council (formerly the Science Research Council (SRC)). Since June 1988, the UKST has been run, together with the 4-m Anglo-Australian Telescope (AAT), as part of the Anglo-Australian Observatory (the UK funding now originating from the Particle Physics and Astronomy Research Council).

The initial task of the UKST was to construct a photographic survey of the entire southern sky. The telescope still takes some 700 plates a year - about half for current surveys and the remainder taken at the request of research astronomers around the world. To date the UKST has taken over 17,000 plates, the plates are stored in the Plate Library at the Royal Observatory, Edinburgh (ROE) and represent a huge source of data for the astronomical community. Some 300 active research programmes make use of UKST plate material. Many plates are copied in the ROE Photolabs and sold as Sky Atlases or Teaching Packages. In addition to its photographic role the UKST also has a multi-object fibre spectroscopy system known as FLAIR.


Siding Spring Observatory (SSO), New South Wales

Latitude -31.27  Longitude +149.07  Altitude 1130m


The telescope is a classical Schmidt with the following parameters:

Mirror diameter                     1.83m
Aperture diameter                   1.24m
Focal length                        3.07m
Radius of curvature at focal plane  3.07m
Plate scale                         67.12 arcsec/mm
Photographic plate size             356mm square, covering
                                    6.4 x 6.4 degrees of sky
Photographic plate thickness        1mm
Unvignetted field radius (nominal)  2.7 degrees

The corrector plate is a full aperture achromatic cemented doublet. It gives images with a half maximum intensity width of better than 1 arcsecond at all photographic wavelengths in the absence of seeing or photographic effects.

There are two 254mm aperture guide telescopes, which are normally fitted with photoelectric offsetting autoguiders. Although normally fixed, the offset can be programmed to vary during an exposure. This facility allows the telescope to track at a non-sidereal rate and is used primarily for obtaining untrailed images of comets.

The control system allows most of the telescope functions except plate and filter changing to be controlled from a console in a separate room within the dome. The dome azimuth automatically follows the telescope attitude.

The polar axis has to be adjusted for optimum off-axis image quality at different declinations, to counter differential atmospheric refraction. This is done for each exposure by means of a motorised jacking system.

Filters and Emulsions:

The earth's atmosphere and the telescope optics together produce a short wavelength cut-off at about 320nm. The following uncoated Schott glass broadband filters are available.

Name         Thickness    Approximate     Wavelength at transmittances of
                (mm)     Transmittance     1/10 and 9/10 maximum values
356mm square
UG1              2     320 to 390nm            315,390       330,375*
WG305            3     longward of 305nm         290            320**
GG385            2         "       385nm         360            400
GG395            2         "       395nm         365            415 
GG455            2         "       455nm         440            465
GG495            2         "       495nm         480            510
AAOV                        V-band  (approx 4.6 degree fov)
OG590            3     longward of 590nm         580            600
RG630            3         "       630nm         620            640
HA659                    H-alpha band (approx 5 degree fov)
RG715            4     longward of  715nm         690            730

180 mm square

RG830            4         "       830nm         800            860
RG1000           4         "      1000nm         910            980***

*   This filter has a red leak longward of 680nm 
**  used to allow "unfiltered" plates to be exposed in th dry
    nitrogen-filled platehoders.  
*** Maximum transmittance assumed to be 0.34 at 1000nm

The above transmittance characteristics are interpolated from graphs given in the Schott Colour Filter Glass Catalogue. Users who need to know the exact values of transmittance are advised to consult this catalogue.

A number of smaller-sized narrow-band interference filters purchased for the 3.9m Anglo-Australian Telescope can also be used with the UKST. They are:

            Approximate      central*      bandpass*
             size (mm)    wavelength (nm)  fwhm (nm)

H-alpha     130 square         656            10
[S II]          "              672            10
[O I]           "              630             9
He I            "              587             9
[O III]         "              501             8
H-beta          "              486             7
He II           "              468             7
H-gamma         "              434             6
[O II]          "              373             6
continuum       "              460            20
continuum       "              540            40
continuum       "              643            10

* Please note that these bandpasses and central wavelengths were
  measured for the f/3.3 beam of the Anglo-Australian Telescope, and
  will be slightly different on the f/2.5 UKST.
Other specialised filters are available on request (see UKST handbook).

All emulsions except 4415 (an estar-based film) are coated on to 356mm square glass, 1mm thick. The principal emulsions available are the following Eastman-Kodak types:

Emulsion  Grain type  Spectral sensitivity  Code letter
IIa-O     medium         uv to 500nm             B
IIIa-J    fine           uv to 540nm             J
IIa-D     medium         uv to 650nm             V
098       coarse         uv to 690nm             R
IIIa-F    fine           uv to 690nm             R
IV-N      fine           uv 400nm to 540nm       I
                         and 670nm to 900nm
I-Z                      uv to 570nm and         Z (hardly ever used)
                         830nm to 1120nm
4415      very fine      uv to 690nm             R  (tech pan film)

The tech pan emulsion (4415) can be applied to several wavebands (eg U, R and H-alpha) where its superior resolution, low grain noise and cost offer substantial advantages over the glass alternative.

Please note that the IIa-O, IIa-D and O98 emulsions are no longer available; details are given as many plates taken on these emulsions are available for loan.

Most plates are coated with an antihalation backing, with the exception of I-Z plates and IV-N plates taken before 1989.

The IIIa-J, IIIa-F, 4415 and IV-N emulsions are normally used after hypersensitisation.

Each new batch of plates is subjected to an extensive series of tests to determine the optimum hypersensitising procedure, and the progress of hypersensitising is monitored by the routine exposure of test samples in a laboratory spot sensitometer.

Hypersensitised IIIa-type emulsions lose sensitivity when exposed to air of high humidity. The UKST is now equipped (December 1982) with three plateholders which may be uniformly flushed with dry nitrogen to suppress this desensitisation. It has been regular practice since then to nitrogen-flush all IIIa-type plates in this way. Furthermore, since that date, it has been the practice to store exposed plates in an atmosphere of dry nitrogen until they are processed.

Some of the common filter and emulsion combinations and their limiting magnitudes are give below.

Emulsion  Filter      Equivalent      Sky limit     Approx limiting
                     photoelectric   exposure time      magnitude
                    waveband/prefix  (nominal mins)

IIa-O     UG1              U             180              21.0
IIIa-J    UG1              U             180              21.0
4415      UG1              U             180              21.0
IIa-O     GG385            B              60              21.0
IIIa-J    GG395          Bj(J)            60              22.5
IIa-D     GG495            V              60              21.0
IIIa-F    OG590           OR              60              21.5
IIIa-F    RG630            R              90              21.5
4415      OG590           OR              60              22.5
4415      HA659       HA (H-alpha)       180              21.5
IV-N      RG715            I              90              19.5

Users should note that exposure times of greater than about 120 mins can be significantly affected by trail. Plates can be photographically or digitally stacked to achieve greater depth.

Sky limit refers to a density of about 1.0 above chemical fog (i.e. a total ASA diffuse central sky density in the range 1.2 - 1.5). The exposure time given is only a nominal value. The actual exposure time depends on the speed of plates which varies from batch to batch.

The limits for all the above combinations with the exception of the `infrared' emulsion/filter combinations (I-Z, IV-N) refer to exposures taken with a fully dark sky. The infrared photographs are normally taken in some (but not full) moonlight; slightly deeper limits can be obtained with longer exposures on moonless nights, but the gain is not great at infrared wavelengths. Reducing the exposure times to about a third of the quoted times results in a plate with limiting magnitude about 1 magnitude brighter than those quoted. In this case some moon can be tolerated with no further loss of information. The limiting magnitudes quoted above apply when the total seeing is less than 3 arcsec (45 micron images on the plate); in seeing conditions of 4 arcsecs the limiting magnitude will be about 0.5 mag brighter.

Relative intensity calibration of direct photographs is achieved by projecting two uniformly illuminated step wedges on to each photograph through the filter (see UKST handbook).

H-alpha filter and survey:

In 1997 a new (single element) H-alpha filter was purchased. Mounted on a full size (356mm) substrate this new filter, HA659, has a clear aperture of approximately 300mm diameter (5 degrees). This filter combined with the very fine grain 4415 emulsion is being used to carry out a survey of the Milky Way and Magellanic Clouds in H-alpha.

More details on the filter and survey can be found here here.



Objective Prisms

Two full aperture objective prisms are available, one with an apex angle of 44 arcmin (2400 A/mm at 4300 A) an the other with an apex angle of 135 arcmin giving very nearly three times the dispersion. The prisms can be mounted singly or together on the telescope, and the complete assembly can be rotated to give the dispersion direction in any required position angle (the default is to have the dispersion north-south). The various prism configurations are denoted by P1 (44 arcmin prism), P3 (135 arcmin prism), P2 (P3 and P1 mounted opposed) and P4 (P3 and P1 mounted parallel); the numbers indicate the dispersion relative to the original low dispersion prism.

Objective prism spectra can be widened by slowly changing the autoguider offset during the exposure Users are advised to specify the minimum widening practical for their needs. Before 1984 spectral widening was achieved by the use of a tilting block image displacer in front of the autoguider.

Limiting magnitudes using the Objective Prisms:

`Waveband'  Emulsion  Filter   Prism       Sky limit       Approx 
                             dispersion  exposure time  limiting mag

    UJ       IIIa-J   WG305      1            45            20.0 
    UR       IIIa-F   WG305      1            20            18.5 
    UR       IIIa-F   WG305      3            20            17.5 
    YR       IIIa-F   GG455      3            35            18.5 
    UJ       IIIa-J   WG305      3            45            18.0 
     J       IIIa-J   GG395      3            60            18.5 
    YJ       IIIa-J   GG455      3            90            19.5

The 4415 emulsion can be used instead of IIIa-F giving up to a one magnitude gain in depth in good conditions.

Widening the spectrum on an objective prism plate to about 130 micron makes the limiting magnitude about 1.5 mag brighter.

While emission lines can be fairly easily detected near the plate limit, the useful magnitude range for objects which can be studied in detail on prism plates is in the range 1-3 magnitudes above the plate limit. Note that the total usable range is much less than on direct plates, since the spectra of objects more than three magnitudes above the plate limit are saturated. Also the wavelength variation of the prism dispersion often restricts the wavelength range in which a spectrum can be well exposed.

Sub-beam prisms

A sub-beam prism, of approximately 250mm aperture, can be fitted in front of the corrector for photographic calibration producing single secondary images to the side of each star with a magnitude drop of about 3 mag (see UKST handbook).

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