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Optec SSP-5 Photomultiplier Tube Photometer (Generation 2) w/Motorized 6-Filter Slider

The SSP-5 Photometer is the central part of a complete stellar magnitude measurement system as shown in Figure 1. A complete set of precision filters, photomultiplier tubes, data acquisition interfaces, aperture sizes, and telescope adapter options are available to fit any need.

Light enters the photometer through the 1¼ inch telescope adapter and is directed either to the focusing eyepiece or the photomultiplier tube (PMT) by means of a flip-mirror. The focusing eyepiece consists of a 1 inch focal length Ramsden and an illuminated reticle with a precisely scribed ring that defines the aperture field of view. After a star is centered in the ring, the flip mirror is rotated to allow light to pass through an aperture stop and Fabry lens and fall upon the PMT.

A two position filter slider is mounted between the flip-mirror and the field aperture stop. Any selected pair of Johnson standard UBVR, Strömgren uvby, or Clear filters may be factory mounted in a slider. Since the slider is easily pulled out of the photometer, sliders with other combinations of filters may be inserted.

The signal from the PMT is converted to a frequency for counting and reading on the 4-digit LED display. This method of signal conversion is superior to the traditional system of DC amplifier and meter. The integration time of 1 or 10 seconds and voltage-to-frequency converter gain of 1, 10 and 100 are easily set by toggle switches on the front panel. For most variable star work, readings from the LED display are recorded in a notebook for later reduction. A TTL level frequency output is available for interface to a computer or other data logging device.

If a PMT photometer is accidentally pointed at a very bright object such as the moon or even room lights, the large photocurrent generated at the anode stage of the PMT could damage the tube. The SSP-5 has circuitry which will shut the high voltage off when the anode current exceeds a safe level. The SSP-5 is reset by turning the instrument off and then back on.

To fill the need of APT (Automatic Photoelectric Telescope) users, the SSP-5A is available with a motorized 6 or 10-position slider. Combined with the SSPCARD IBM-PC interface, a complete automated photometry system is easily configured.

The sensitivity of the SSP-5 and SSP-3, which uses a silicon photodiode detector, is shown in figure 2. The display output is expressed in counts per second vs. magnitude using the Johnson B & V filters. It should be noted that these are the approximate display counts, and that accurate magnitude should be determined using the accepted techniques of astronomical photometry. Each line on the chart represents the approximate relationship between stellar magnitude and counts from the photometer. The noise counts were determined by taking the standard deviation of 10 consecutive readings using either 1 or 10 second integration times. The intersection of the diagonal B & V lines with the horizontal noise lines determine at what magnitude the signal-to-noise ratio is 1. The star Zeta Hercules, spectral type G, was used to make this chart.

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