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The auxiliary focus photometer

A photoelectric photometer is also available for use at the auxiliary Cassegrain focus of the AAT. It can be used to obtain quick photometric measurements of stars and galaxies or carry out high speed photometry of rapidly variable sources (e.g. pulsars).

The photometer is a single channel instrument with no chopping; telescope offsets to sky are required. Its great advantage is that it can be accessed within a few seconds while another instrument (such as the RGO spectrograph) is at the main Cassegrain focus. It is closely parfocal and on axis. No remote aperture viewing is available, so source acquisition via MAXIM (see § 2.6.8) is recommended.

The photometer accepts interchangeable 12-position filter wheels. Broadband (Cousins), DDO and Strömgren filter sets are available. An extended S20 and a GaAs photomultiplier tube are available; both are mounted in thermo-electric cold boxes. The maximum count rate for the GaAs tube is 100kHz and must not be exceeded, otherwise irreparable damage will be done to the tube. The maximum count-rate for the S20 tube is 20MHz, although hardware constraints limit this to 4MHz. Aperture wheels provide a choice of 8 apertures ranging in diameter from 0.67 to 7.8 mm.

Photometer functions (e.g. aperture and filter selection) may be performed locally in the cage or remotely in the control room by a program on any of the Interdata computers (normally the CCS) called PHOSET. Software packages for the recording of data are also available on the Interdata. The HSP3 and HSP4 programs are intended for use with high-speed photometry and automatically write data to tape. The HSP3 program allows time resolutions from 1ms to 10s, while the HSP4 program gives a fixed time resolution of 1us with a count rate limitation of 8kHz. The AUXPHOT program is available for normal photometry. Output from AUXPHOT is recorded on the Printronix. There is also provision for strip chart records, on logarithmic or linear scales. Some reduction programs are available (e.g. FFTs with HSP modes) but at present these are unsupported and potential users should contact the AAO for more information.

Typical count rates for a 12th magnitude star observed with the S20 tube are given in Table 7.3.

 
Table 7.3: Count rates for star of 12th magnitude with the S20 auxiliary photometer



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