The Peoples' Photometer is a two-channel photoelectric photometer designed to have interchangeable optics so as to be suitable for a variety of photometric and polarimetric applications. Figure shows a schematic optical layout, indicating the location of the most important components of the system. As can be seen, the two channels are orthogonal; channel 1 (Romeo) is along the optical axis of the telescope, while channel 2 (Juliet) is fed via one of several different 45 degree reflectors.
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Figure: Optical layout of the Peoples' Photometer. The top eyepiece is the old TV position (not now used) and the lower eyepiece is the present TV position.
The aperture slide holds three aperture plates which may be selected from a large set. Plates with double apertures are for use with the star/sky prism, the light through each aperture being directed into a separate channel. The apertures are 12.5 mm apart, corresponding to 172 and 66 arcsec when used on the JKT and INT respectively. Plates with single apertures are intended for use with the beam splitters or with the Foster beam analyser for polarimetry. The intensified TV views the field through these apertures. Thus it is best to leave one of the three aperture positions open for the TV to use for acquisition. Once the target star has been acquired the working aperture can be moved in and the star centred precisely.
The photometer contains a retractable glass wheel with 100 mm wide slits located every 10 degrees, approximately 5 mm apart. This wheel can scan the aperture at a rate of 150 scans per second, and is intended for analysis of strip-brightness distributions. To match the slit width with the Rayleigh disc of the JKT it is necessary to use the magnification box with a 10 microscope objective. This box is a module which bolts above the Peoples' Photometer and cannot be mounted or dismounted except in daylight, when the telescope can be re-balanced. Microscope objectives of 3 and 2 are also available.
Each channel has a dedicated slide holding up to six filters; filters can be square (25.4 mm 25.4mm) or circular (25.4 mm in diameter) and up to 25 mm thick. Two sets of ready-loaded filter slides are available, standard UBVRI and Stromgren uvby, H. Several narrow band filters are also available. Users requiring particular narrow-band filters should contact RGO staff.
A removable rotating ring (refer to Figure ) is available for polarimetric work. The ring accepts a quarter-wave or a half- wave retarding plate which intercepts the beam before it reaches the aperture. The quarter-wave plate is usually mounted in ISIS, and prior arrangement must be made to use it with the photometer. Only one plate at a time can be mounted on the ring; changing plates is a daytime job carried out by support staff. The wave-plates are ``super-achromatic'' as supplied by the German firm Halle. No significant departure from 100 per cent modulation efficiency has been found over the UBVRI wave-bands. There are however significant differences in the fast direction of the plates between U and I, and observers should determine the zero point of position angle independently for each wave-band they use.
A dedicated high speed photometry module, ``AVG'', designed for synchronous photometry, is available. Using a frequency synthesizer, the module cycles the addresses in a CAMAC-resident integrating memory which is external to, but shared by, the processor. The memory can be organised as required with variable resolution (up to 16 k bins per cycle in single-channel use), twin channel options and memory paging. Non-synchronous photometry may also be handled; the maximum photon rate is about 2 10 counts s. The module has a substantial degree of autonomy from the processor and may be operated remotely by CAMAC commands. It is intended for use in high resolution pulsar work and in a number of other applications, such as occultation and flicker studies.
The performance of the PP is such that with an EMI 9658 photomultiplier, a star of m = 7 should give about 810 counts s through the B filter on the JKT and about 510 counts s on the INT. It is extremely important to note that stars brighter than m = 6 should not be observed through broad-band filters on the JKT, as this would damage the photomultipliers. The corresponding limit for the INT is m= 8.
Further details are contained in the Peoples' Photometer User Manual (La Palma User manual III). There is a further manual for the Peoples' Photometer Acquisition Software (La Palma User manual IX). The AVG system is described by Dick and Jones (Journal of Physics E, vol 21, p853, 1988).