Specifications

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SECTION 2.0
THEORY OF OPERATION
2.1 BASIC PHYSICAL CHARACTERISTICS
Figure 2-1 shows a cross-sectional view of the SSP-4 photometer. 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 a 1-inch focal length
Ramsden type optic and a reticule with a precisely scribed ring that defines the detector field of
view. A green LED illuminates the reticule from the side. A reticule diameter of 0.3mm is
standard for the instrument but a 1.0mm is available for the optional 1.0mm detector. 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 photo multiplier system, the image of the star falls on the detector plane; a Fabry
lens is not used.
A two-position filter slider is mounted between the flip mirror and the detector. Any pair of
filters selected by the user before delivery can be mounted in the slider. Since the slider is easily
pulled out of the unit, sliders with other combinations of filters can be inserted. For most variable
star work, a single slider with J and H filters is recommended. Because of the wide and deep
atmospheric absorption bands that lie within the spectral sensitivity of this detector, observing
without a filter in place is not recommended.
The detector and electrometer are rigidly mounted behind the filter slider on an X-Y adjustable
dovetail bracket. Accurate alignment of this bracket on an optical bench insures that the sensitive
area of the detector matches the stellar field as defined by the reticule in the focusing eyepiece.
Detector diameters of 0.3 and 1.0 mm are available with the 0.3 mm size considered best for
most telescopes with focal lengths in the range of 50 to 150 inches.
The electrometer amplifies the current from the photodiode by a factor of 1 x 10
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and is
analogous to the operation of a photo multiplier tube. From the electrometer amplifier, the signal
is then routed to the voltage-to-frequency converter for final processing into counts based on a 1,
5, 10 or 30 second gate time interval. The resulting count is then read from the four-digit readout
on the front panel. Under PC control using SSP4DATA.EXE nearly any integration time can be
chosen with 0.01 second resolution from 1 to 60 seconds.
Control of the instrument is possible either through the front panel two-button interface with
output shown on a 4-character display or through a serial connection to a PC running our
SSP4DATA.EXE interface program. Most all functions of the instrument are accessible with the
two-button interface so that observations can be made with only a data logging pad and pencil.