The SSP-3 Photometer is the central part of a complete stellar magnitude measurement system as shown in figure 1 above. A complete set of precision filters, data acquisition interfaces, detector size and adapter options are available to fit any need. Light enters the photometer through the 1.25 inch telescope adapter and is directed either to the focusing eyepiece or the detector by means of a flip-mirror. The focusing eyepiece consists of an 1 inch focal length Ramsden design and a reticule with a precisely scribed ring that defines the detector field of view. A green LED illuminates the reticule from the side. After a star is centered in the ring, the flip-mirror is rotated to expose the detector. It is important to note that, unlike a photomultiplier system, the image of the star falls on the detector plane; therefore, a Fabry lens is not used and the photometer can be used with any focal length telescope.
A two position filter slider is mounted between the flip-mirror and the detector. Any pair of Johnson standard UBVRI, Clear, or MA-MB-MI (part of the Wing 8-color system for late type stars) filters can be factory mounted in a slider. Since the slider is easily pulled out of the photometer, sliders with other combinations of filters can be inserted. For most variable star observations, a single slider with B and V filters is recommended to begin with.
Powered by a 9-volt NiCd rechargeable battery, the SSP-3 offers unencumbered use at the telescope without annoying power cords. A single night's charge gives approximately 5 hours of operation, and if the observing session demands it, the battery charger can be used to give continuous power during use.
The signal from the detector is converted to a frequency for counting and reading on 4-digit LED display. This method of signal conversion is superior (higher signal-to- noise ratio) than 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 - a must for use on winter nights with gloved hands. 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.
To fill the needs of APT (Automatic Photoelectric Telescope) users, a motorized 6-position slider can be added to the SSP-3 making it an SSP-3A. Combined with the IBM interface card SSPCARD a completely automated photometry system is easily configured.
The sensitivity of the SSP-3 is shown in figure 2 below. The display output is expressed in counts per second vs. magnitude using the various 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 SSP-3. 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 UBVRI and CLEAR 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.