Optec SSP-5 User manual

MODEL SSP-5
PHOTOELECTRIC PHOTOMETER
TECHNICAL MANUAL FOR
THEORY OF OPERATION AND OPERATING PROCEDURES
OPTEC, Inc.
OPTICAL AND ELECTRONIC PRODUCTS 199 Smith St.
Lowell, MI 49331
U.S.A.
[email protected] [phone]
http://www.optecinc.com [phone] FAX
*** IMPORTANT ***
PLEASE READ THIS MANUAL
THOUROUGHLY BEFORE ATTEMPTING
TO OPERATE THE PHOTOMETER

2
Figure 1-1. SSP-5 Shown with Meade 8-inch Telescope.

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TABLE OF CONTENTS
Revision 3 - March 1995
Section Page
1.0 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
1
2.0 THEORY OF OPERATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Photometer Head. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
3
2.2 Detector - PMT Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
4
2.3 Field Aperture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
5
2.4 Fabry Lens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
5
2.5 Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
6
2.6 High Voltage Supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
6
2.7 Preamp & Low Pass Amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
8
2.8 Main Board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
8
2.9 Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
10
3.0 OPERATING PROCEDURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
11
3.1 Check-Out List. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
11
3.2 Using the Photometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
12
3.3 Calibration and Adjustment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
13
4.0 TROUBLE-SHOOTING GUIDE. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
15
5.0 SPECIFICATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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17
6.0 JOHNSON FILTERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
19
7.0 STRÖMGREN FILTERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
24
8.0 SSP-5A OPTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
26
8.1 Physical Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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26
8.2 Procedure for Determining Filter Position. . . . . . . . . . . . . . . . . . . . 26
8.3 Connecting and Operating Stepper Motor. . . . . . . . . . . . . . . . . . . .
.
27
8.4 Using the Signetics SAA1027 Stepper Driver. . . . . . . . . . . . . . . . . 28

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LIST OFFIGURES
Figure Page
1-1 SSP-5 Shown with Meade 8-inch Telescope. . . . . . . . . . . . . . . . . . . . . . . . . . . Cover
1-2 The SSP-5 photometer system and accessories. . . . . . . . . . . . . . . . . . . . . . . . .
.
2
2-1 Cross-sectional view of the SSP-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
3
2-2 Power supply and signal processing function diagram. . . . . . . . . . . . . . . . . . . . 7
2-3 Performance of the SSP-5 compared with the SSP-3. . . . . . . . . . . . . . . . . . . .
.
10
3-1 Identification of controls and features of the SSP-5. . . . . . . . . . . . . . . . . . . . .
.
11
8-1 Typical stepper control circuit using the SAA1027. . . . . . . . . . . . . . . . . . . . . . 28
LIST OF TABLES
Table Page
6-1 Physical characteristics of the Optec filters. . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
19
6-2 Standard UBVRI response functions according to Johnson. . . . . . . . . . . . . . .
. .
20
6-3 R4040 normalized response with Optec UBV filters. . . . . . . . . . . . . . . . . . . . .
.
21
6-4 R4457 normalized response with Optec UBVR filters. . . . . . . . . . . . . . . . . . .
. .
22
6-5 Transmission of the Optec SSP-5 UBV and SSP-3 UBVR filters. . . . . . . . . . .
.
23
7-1 Optical specifications of Strömgren filters. . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
24
7-2 Physical specifications of Strömgren filters. . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
25
8-1 Specifications for Model PF35-48C stepper motor from Nippon. . . . . . . . . . .
. .
26
8-2 Function and color code of stepper motor wires. . . . . . . . . . . . . . . . . . . . . . . .
. .
27
8-3 Pin number and function for the motor control cable connector. . . . . . . . . . . .
. .
28

1
SECTION 1.0
INTRODUCTION
Optec has developed a high-precision stellar photoelectric photometer which makes use of a
miniature photomultiplier tube for sensitivity and faster time response. Using the same design
philosophy encompassed in the SSP-3, the SSP-5 looks and operates in very much the same
way. The use of a photomultiplier tube (PMT) allows fainter stars to be measured accurately and
the enhanced response time, as fast as 1 ms, allows fast events such as lunar occultations to be
recorded with greater time resolution.
With the R4040 PMT option, the SSP-5 can exhibit an S-5 response similar to the original 1P21
photomultiplier tube. For researchers interested in greater low light sensitivity and extended red
response to 830 nm, the R4457 PMT option is available along with Johnson UBVR filters. Each
PMT is a 9-stage, side-on, low-noise photomultiplier housed in a small ½-inch diameter
enclosure.
In spite of the fact that a PMT based photometer is more sensitive to damage from bright lights or
rough handling, a great deal of effort has been expended to make the SSP-5 nearly as survivable
as the SSP-3. The SSP-5 will allow the researcher to measure both bright and faint stars in the
UBV spectral region with the degree of precision and reliability associated with the venerable
SSP-3.
The SSP-5 Photometer is the central part of a complete stellar magnitude measurement system as
shown in Figure 1-2. A comprehensive set of precision filters, data acquisition interfaces,
aperture stops, and adapter options are available to fit any observing program.

2

3
SECTION 2.0
THEORY OF OPERATION
2.1 PHOTOMETER HEAD
A cross-sectional view of the photometer head is shown in Figure 2-1. 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 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 the aperture stop which separates the star from the background. A Fabry lens then
projects an image of the primary mirror/lens onto the photocathode of the PMT.

4
2.2 DETECTOR - PMT OPTIONS
Optec currently offers two different models of side-on photomultiplier tubes (PMT's) which can
be used in the SSP-5. These PMT's are manufactured by Hamamatsu Corporation and measure
only 40 mm long by 14 mm in diameter. In this small package is a well designed 9-stage
photomultiplier which can operate at -1000 VDC. At the operating voltage of -850 VDC and
after several hours of warm-up, the model R4040 PMT has a typical measured dark current of
around 8 pA at 70° F ambient temperature.
The model R4457 is an extended red response PMT which can allow measurements into the
Johnson R band. The overall gain and quantum efficiency (QE) of the R4457 is much better than
the R4040. The QE at the center of the Johnson V band, 540 nm, is approximately 2% for the
R4040 and 16% for the R4457. Because of this increased sensitivity, when the R4457 PMT is
specified the operating supply voltage is set to -750 VDC rather than -850 VDC. At this voltage,
the measured dark current is typically less than 2 pA at 70° ambient temperature.
The spectral response of the R4040 is S-5 which is identical to the S-4 response of the traditional
1P21 tube except for extended response in the UV. This extended response to about 185 nm is
due to the special UV transmitting glass used for the tube. The spectral response of the R4457
extends from the same 185 nm at the blue end to 830 nm at the red end. A Fabry lens of B270
glass and 2.9 mm center thickness cuts the transmission to 50% at 315 nm and to 0 at 300 nm.
This small loss of UV transmission near 300 nm should not adversely affect the U filter
transformation to the standard system.
The R4040 tube is powered by an -850 VDC power supply to give a current amplification of
about 1x106. This value was chosen after measuring the signal to dark-noise ratio at several
applied voltages and using the voltage giving the highest value. Once set, the operating voltage
cannot be changed by the user. When the R4457 PMT is specified, the voltage is turned down to
-750 VDC. This lower voltage setting is needed to allow the user to observe brighter stars
(brighter than 4th magnitude) without saturating the detector. The current amplification of the
R4457 at -750 VDC is about 6x105
A mu-metal shield of high permeability is placed around the PMT to reduce the effect of external
magnetic fields (dome motors, the earth's field, conductors inside the SSP-5, etc.) on the path of
photoelectrons in the tube. See Figure 2-1. In addition the shield is brought to the same potential
as the photocathode, so that photoelectrons are not drawn to the glass tube. To protect against
shock hazard, the shield is connected to the high voltage supply through a 22 M ohm current
limiting resistor.
As discussed by Miles1(1986) the dark current of a PMT is decreased substantially by
dehumidifying the tube in a desiccating chamber. This apparently reduces current leakage around
the tube pins and socket connectors to a minimum. Before assembly, the tube and socket used in
the SSP-5 are placed in a vacuum desiccator for several days before they are installed into the
SSP-5. To insure continued dry operating conditions, a rechargeable silica gel canister is used as
an access cover on the side of the unit. The silica gel pellets are dark blue when activated and
turn pale pink when its drying ability is diminished. At that time, the silica gel canister/cover can

5
be recharged by removing the unit (only 4 screws to take out), and baking it for 4 hours at 250°
F. Additional access port covers with desiccant are available (stock #17555) for a small charge.
The internal cavity of the photometer head is sealed by using silicon sealant on the non-movable
parts and O-ring seals on those parts which are removable, such as the silica gel canister/access
port cover.
2.3 FIELD APERTURE
As in the SSP-3, a single field aperture of either 0.5, 0.75, 1.0 or 2.0 mm diameter is placed at
the focus of the telescope inside the photometer head. This aperture is not removable or
changeable after assembly. Experience with the SSP-3 has indicated that the need for adjustable
field apertures is not great for the vast majority of applications which usually consist of stellar
brightness measurements. The focusing eyepiece contains an illuminated reticle with a scribed ring
which precisely coincides with the boundary of the field aperture. Normally, the star is centered
within the ring of the focusing eyepiece and then a measurement is started by rotating the flip
mirror to the measurement position - a turn of about 180 degrees.
2.4 FABRY LENS
Because the photomultipler tube's cathode surface has very poor response uniformity, a Fabry lens
is needed to fill a large area of the cathode uniformly without regard to where the star is
positioned within the field aperture. Considering an f/10 cone of light as produced by a Celestron
or Meade telescope, the Fabry lens used in the SSP-5 will image the telescope's entrance pupil
slightly past the wire mesh screen in front of the photocathoe with a spot having a diameter of
2.5 mm. Telescopes with f-ratios in the range of 7 to 20 should work with the 25.4 mm focal
length Fabry lens without difficulty. The Fabry lens has a plano-convex shape and a diameter of 9
mm.
2.5 FILTERS
In addition to the standard UBVR Johnson filters, the four color (uvby) Strömgren and Hydrogen
ß wide and narrow filter sets are available. See Section 6.0 and 7.0 for Johnson and Strömgren
filter specifications.
Filters are mounted in two-position sliders which are inserted through a side port. A spring
plunger screw keeps a slight amount of pressure on the slider to keep it in place and to locate one
position by a detent machined in the slider. The other position is found by pushing the filter slider
in until it stops. Filters, or clear windows of identical thickness, have to be used in order to keep
the focus in the same position as determined by the focusing eyepiece. Using no filter at all
will move the focus up by about 3.5 mm. A black felt light seal is used on the filter slider port to
1Miles, R. 1986, IAPPP Communication 24, 6.
prevent external lights from being picked up by the PMT. Experience at Optec has shown that
even bright office lights do not effect the output when a slider is in place and the flip mirror is in

6
the viewing position. Removing the filter slider does cause stray light to be picked up, but not
enough to trip the over voltage protection circuit to be discussed later.
The Model SSP-5A is supplied with a motorized filterslide which allows any of six or ten filters to
be selected by computer control. Coupled to the SSPCARD IBM interface, complete automatic
control and data acquisition of the photometer is possible. Filter selection cannot be made
manually since the filter covers and geared drive to the stepper motor prevent any other selection
except by proper activation of the stepper motor. Control of the stepper motor is discussed in
Section 8.0.
2.6 HIGH VOLTAGE POWER SUPPLY
For stable PMT operation an extremely well regulated and low noise high voltage power supply is
needed. The gain of the PMT used in the SSP-5 is proportional to the 7th power of the applied
voltage. Thus, for small values a percent change in gain is equal to 7 times the percent change in
the applied voltage. For exam ple, a 1 volt change at -850 VDC is equal to a .8% change in
PMT gain, or nearly a 0.01 magnitude error. Following a 30 minute warm-up time, the voltage
stability for the SSP-5 is ±0.2 V for periods of at least 15 minutes. In the bandwidth of 1 to 0.05
Hz, the voltage noise is less than 0.1 V.
The high voltage supply (see Figure 2-2) uses a 20 kHz oscillator to drive a pot core transformer
with a 1 to 55 turn ratio. A voltage doubler and rectifier circuit produces a maximum voltage of
about -925 VDC when fully powered. The output is regulated down to the working voltage of -
850 VDC or -750 VDC (with the R4457 PMT) by feeding a fraction of the output to a high-gain
difference amplifier which compares the output to a very stable voltage reference (10ppm T-C)
and amplifies the difference as a correcting voltage to the pot core voltage driver. A failure of the
feedback mechanism will not overdrive the PMT causing an expensive tube replacement since the
maximum voltage that can be produced is approximately -925 VDC, which is within the maximum
ratings of the PMT.
In addition to the base requirement of stability, the high voltage circuitry must fit within the
available space of the control box and use a minimum of power. The oscillator, voltage driver
and regulator are mounted on the daughter board attached to the main circuit board and occupy
about 3 square inches of space. For safety and noise constraints, the pot core transformer, high
current TMOS switcher and rectifier circuit are built into a small steel enclosure with approximate
dimensions of 1 x 1 x 2 inches which is mounted under the main circuit board. Power
consumption of the high voltage supply is about 1.5 watts.

7
If the PMT is exposed to bright lights such as the moon or a bright planet, permanent damage
could result to the anode stage of the tube. To prevent this, the output of the preamp is
connected to a protection circuit which will turn off the high voltage within a few milliseconds
when the preamp output nears its saturation point. Thus a 2nd magnitude or brighter star when
observed with the V filter and 11 inch aperture telescope will trip the protection circuit and turn
on an LED near the power switch. This circuit can only be reset by turning the SSP-5 completely
off and then back on after a few seconds wait.
2.7 PREAMP & LOW PASS AMPLIFIER
Even with the million fold gain of the PMT, the output current is still very small, on the order of
picoamperes for dim stars. This current has to be amplified sufficiently for the V/F converter to
work properly without introducing gain instabilities or noise to the output signal.
The preamp used in the SSP-5 is divided into two stages. The first stage is a current-to-voltage
amplifier with a gain of 7.9x106. The output voltage is related to the input current from the PMT
by the following equation:
Eout = 7.9x106·Iin

8
The amplifier used is an Analog Devices model 549K electrometer which has a bias current of
less than 0.1 pA and noise currents much smaller. The second stage is a low pass amplifier of 1.5
gain which also inverts the signal since the V/F converter needs a negative voltage level. The
total measured voltage drift is 6µV/°C and output noise is less than 0.1µV in the 0.05 to 350 Hz
bandwidth of interest. The response time is defined as the time taken for the output signal to go
from 10% to 90% of its final value. The response time of the preamp and low pass amp
combination is 1 mS. This rapid response allows for the measurement of stellar angular diameters
by observing lunar occultations.
2.8 MAIN BOARD
The main circuit board used in the SSP-5 is essentially the same one used in the SSP-3 with some
small modifications. Thus the V/F converter, integration timer and counter work in the same way.
See Figure 2-2.
The gain or scale of the V/F converter is set by a panel mounted toggle switch to the values of 1,
10, and 100. These values are set by internal pots and have a precision of about 0.5%. The gain
linearity of the V/F converter is hard to measure since the input voltage has to be measured to a
greater precision but is estimated to be better than 0.03% in the range of 10 Hz to 10 kHz at any
one scale setting. The output of the V/F converter is also available as an output to interface with
the Optec SSPCARD (IBM computer interface card) or other computer interface.
Since the SSP-5 is operating in a DC mode as compared to other PMT photometers which
operate in a photon (pulse) counting mode, it is important to distinguish the difference between
the pulses coming from the V/F converterand the photons of light. They are not the same. This
becomes very important when considering dark count. With the flip mirror in the viewing
position, the 'dark' count of the SSP-5 is the sum total of PMT dark current and V/F amplifier
offset voltage, which is many times higher than the dark current. The offset voltage is set high in
order that there is some negative voltage going into the V/F converter at all times resulting in a
count. The counter will not count backward if input noise voltages change polarity; that is, go
from a negative to a positive voltage.
An Epson programmable clock generator IC is used to set the gate times for the counter chip.
The unique aspect of this chip is its completeness. No other elements, either passive or active, are
needed for it to function. Programming the IC requires setting the levels of 6 pins to either high or
low. The precision of the gate time is 0.0005% at 25° C. A front-panel toggle switch sets the
integration time to either 1 or 10 seconds.
The output of the V/F converter is fed to a CMOS counter chip that directly drives a 4-digit LED
display. Each time the display is updated with a new count, an LED mounted next to the display
will flash briefly. If the count exceeds the capacity of the 4-digit display (exceeds a count of
9999), the same LED will turn on until the count is within the range of the display once again. A
blank display indicates an over-range condition that is exceeding a count of 12999.
2.9 PERFORMANCE

9
Testing has shown that the SSP-5 is substantially more sensitive in the UBV bands than the SSP-3
which uses a silicon photodiode. Figure 2-3 compares instrument responses in B and V. The
display count is expressed in counts per second vs. magnitude for the various filters using an 11
inch aperture telescope. It should be noted that these are approximate display counts, and that
accurate determinations of magnitude should be made using the accepted techniques of
astronomical photometry. The lower noise limits were measured by taking the standard deviation
of 10 consecutive readings at either 1 or 10 seconds of integration with no light incident on the
detector. With its lower sensitivity, the SSP-3 low-light-limit shown in Figure 2-3 approximates
the operational limit when using the instrument on a star since the contribution of sky and signal
noise (shot noise from the star and background sky) is small compared to the instrument noise.
However, these terms will effect the low-light-limit of the SSP-5, thus the true low-light-limit will
be slightly higher than what is indicated.

10
SECTION 3.0
OPERATING PROCEDURES
3.1 CHECK-OUT LIST
Consult Figure 3-1 for identification of the various controls and features of the SSP-5.
IMPORTANT: The use of a photomultiplier tube in this photometer presents a serious shock
hazard if the user is careless. If the rear cover is removed for any reason, PLEASE UNPLUG the
power supply first. In addition to the possible damage that the PMT can do to the user, be
attentive to the fact that serious damage can be brought on the PMT by the same carelessness.
The tube should be shielded from bright lights including room lights at all times even when the
power is off. NOISY TUBES AND BURNT ANODE STAGES WILL NOT BE COVERED
UNDER WARRANTY TERMS. When not used, the photometer should be stored in a dry
place away from heat, vibration, lights and kids.

11
Remove the filter slider and clean filters, only if necessary, with a cotton swab dipped in alcohol
or lens cleaning fluid. Do NOT rub hard.
Before taking readings, plug the wall mounted power supply into a AC outlet with suitable
voltage and connect the 2.5 mm mini-plug into the jack located on the control box near the
bottom. Turn the photometer on and allow at least 30 minutes warm-up time at ambient
observatory temperature. Make sure the a filter slider is in position and the flip mirror is down so
no incident light reaches the photomultiplier tube.
After warm-up, observe that the count on the 4-Digit Display with the mirror down (no light on
detector) is within the range of 3 to 7 with SCALE and TIME set at 1. If reading are not within
this range, adjust the OFFSET POT with a small screw driver until the correct reading is obtained.
With the U, B and V filters, the common low wattage red lightsfound in most observatories
which provide low level illumination will not affect the reading if some precautions are taken to
shield the telescope optics from direct illumination. However, when using R4457 PMT, the R
filtercan allow more than 100 times more light energy from red light to irradiate the detector.
Even interior observatory surfaces which are illuminated by the red light and near the front of the
telescope will affect the reading. Before taking important readings with the R filter, it is
recommended that the user experiment with the illumination in his observatory to gauge the effect
it has on the SSP-5 count.
3.2 USING THE PHOTOMETER
With the mirror down, site the star or sky region in the center of the reticle. After confirming that
the telescope is tracking properly, carefully flip the mirror up and record the readings.
Because of the extreme low-light sensitivity of the SSP-5, care must be taken to insure that field
stars just beyond the visual limit are not within the field aperture during the measurement process
as this may cause errors. For example, when using an 11 inch aperture telescope, a 15th
magnitude star should be beyond the visual limit. However, if it is within the field aperture, nearly
a 0.1 magnitude error could result in the measurement of a 12th magnitude star. A careful check
of a photographic star atlas should be made to determine if the aperture field is clear of dim stars.
To take a star or sky brightness measurement, three consecutive readings of 10 seconds
integration time (TIME set for 10) each are normally taken. Always ignore the first reading since
the mirror can never be flipped up exactly at the beginning of a new 10 second cycle. After seeing
the first count displayed, record the next three.
After the reading sequence is completed, return the mirror to the viewing position and confirm
that the star is still centered properly in the reticle. Normally, if the star is within a circle of 0.35
radius of the scribed ring center, more than 99% of the star's light has been collected by the
detector. This rule of thumb does depend on seeing quality.
At this point, the user has successfully completed making a reading and is referred to other
sources of information about stellar photometry. The user is advised to obtain a good working

12
knowledge of the data reduction process. However, a number of computer programs are
available to simplify the data acquisition and data reduction process. For instance, Optec offers
the RPHOT Automated Data Acquisition and Reduction Software Package for Aperture
Photometry. RPHOT is a complete package for performing Johnson standard photometry using
either "all-sky" or differential techniques. Contact Optec for additional information on RPHOT.
Ask for stock number 17170.
3.3 CALIBRATION AND ADJUSTMENTS
For precise determination of stellar magnitude, the filters used in the SSP-5 must be calibrated
with standard stars. The procedure for doing this is beyond the scope of this manual and the user
is referred to the IAPPP organization and standard texts on photometry. Simple determination of
filter correction factors can be made by using a close pair of stars with a wide color temperature
difference. This method is used by many members of the IAPPP and is recommended for novice
users. Optec also sells a number of data reduction packages for various computers. Contact
Optec for descriptive information.
The reticle and detector are critically aligned at the factory no adjustment should be made by the
user. If the eyepiece is removed from its mount it may be necessary to realign the detector which
will probably have to be done at Optec. If dust on the reticle is troublesome, remove it by
blowing air (canned air for camera cleaning is suggested) through the 1¼ inch telescope adapter.
If the reticle ring needs to be adjusted in brightness, the circuit board mounted potentiometer
located on the bottom right corner of the main circuit board can be adjusted. Use a small
screwdriver to turn the control counterclockwise to brighten the reticle or clockwise to dim it.
If the photometer is exposed to a bright star, moon or other source of illumination which exceeds
the safe threshold level of the tube's anode current, the high voltage will be turned off and the red
LED near the power switch will be switched on. After appropriate measures are taken to reduce
the illumination levels to the photomultiplier tube, turn the power off and then back on to reset
the high voltage power and continue use.

13
SECTION 4.0
TROUBLE-SHOOTING GUIDE
The following common problems and solutions have been collected over the years from our
customers and our attempts to solve their instrument problems. Before calling us, read through
these and relevant sections of this manual to see if an easy solution exists for your errant
photometer.
1) Unit has been turned on and left to warm up for the proper time but no display is seen
unless light is incident on the detector.
With a small screw driver adjust the offset control pot on the front panel. With no light incident
on the detector, rotate the control pot CCW for at least 20 turns to make sure a display value is
seen and then rotate CW until a count of 5 is obtained with the GAIN and TIME set for 1
2) The dark count (no light on the detector) appears to drift with time and temperature
beyond what you have been accustomed to.
This could be a moisture problem especially for observatories located in humid locations.
Moisture contamination of the PMT socket pins and around the first preamp stage could cause
erratic changes in the count. The photometer should always be stored in a warm dry place when
not used. The desiccant contained in the side access port of the photometer head may have to be
reactivated. Remove the four mounting screws holding the cover on and check to see if the small
pellets still have a deep blue color. If they are a pale transparent blue or pink, bake the cover
containing the pellets at a temperature of 250° F for about 4 hours. Make sure you do the work
on the photometer head in a dimly lit room so as not to expose the photomultiplier tube to bright
lights. Exposure to bright lights even with the power off can cause the noise counts from the tube
to increase for several days. Allow the unit to stabilize for several hours at ambient temperature
before measuring the drift. If the problem does not disappear or reduce to acceptable levels, it
will have to be returned to Optec for repair.
3) When you first turn the unit on, a display count is seen momentarily and then disappears.
Adjustments to the offset do not work. The integration LED flashes in the usual manner
and the battery is good.
The most common chip to fail on the main circuit board is the ICL7217 counter chip from Intersil.
Experience has shown the above symptom occurs when this chip fails in the usual way. Since the
chip is socketed, the part can be easily replaced. Contact Optec for a replacement.

14
4) The unit appears to give much higher than expected counts every once in a while.
Any surge in the power line may cause an increase in the number of counts for the integration
interval. Refrigerator compressors, dome motors, and telescope position servos could easily be
the cause of this problem. Vibration and strong radio signals from nearby stations may also cause
similar symptoms. Use a surge protector with RF filtering to solve the power surge problem.
There is no solution for nearby radio stations.
5) The night appears clear but the star count is diminishing with time faster than expected
due to changing extinction conditions.
A common problem, especially with Celestron and Meade telescopes, is that a nearly invisible film
of condensed water will develop on the corrector plate or main mirror during the night if the dew
point is high enough. Usually this fog film can only be seen when a strong light is projected down
the front of the telescope and the optics carefully inspected. A hair dryer is the only cure.
6) As the star approaches the edge of the detector the count begins to fall but it looks as if the
star is still completely within the ring as seen in the eyepiece.
The stellar light profile (energy vs. radius from the center of the star) is much larger than what is
seen. On a good night a seeing disk could appear to be about 2 arc second in diameter. However,
to capture over 99% of the energy a detector diameter (field aperture) of over 20 arc seconds is
needed. A hazy night or a night with much greater turbulence could increase the stellar profile
many more times. Thus, care must be taken to keep the star from drifting near the edge of the
detector since some of the incident energy will be lost.
7) An external counter or computer (not the SSPCARD) is connected to the SSP-5 and the
count displayed on the photometer is significantly different when compared to the count
recorded on the external device.
It is possible for extra noise counts to be picked up on long signal cables if proper techniques are
not followed. The SSPCARD (Optec's IBM interface card) uses a Schmidt trigger input gate on
the interface card to prevent small amounts of noise on the cable from affecting the count. Make
sure that your external device has similar input noise protection. A shorter or shielded cable may
also have to be used.

15
SECTION 5.0
SPECIFICATIONS
DETECTOR (R4040 option)
Type 9-stage side-on photomultiplier tube
Model R4040 from Hamamatsu
Photocathode Sb-Cs
Spectral Range 185 - 650 nm (S-5)
Cathode Sensitivity 35 mA/W at 360 nm (Johnson U band)
10 mA/W at 550 nm (Johnson V band)
Quantum Efficiency QE=14% at 360 nm
QE=2% at 540 nm
Operating Voltage -850 V
Gain
≈ 1x10
6
at -850 V
Rise Time 1.4 ns (PMT only)
Dark Current
≈ 8 pA at -850 V and 25° C
DETECTOR (R4457 option)
Type 9-stage side-on photomultiplier tube
Model R4457 from Hamamatsu
Photocathode Multialkali
Spectral Range 185 - 830 nm
Cathode Sensitivity 60 mA/W at 360 nm (Johnson U band)
70 mA/W at 550 nm (Johnson V band)
20 mA/W at 700 nm (Johnson R band)
Quantum Efficiency QE=20% at 360 nm
QE=16% at 540 nm
QE=3.5% at 700 nm
Operating Voltage -750 V
Gain
≈2x10
5
at -750 V
Rise Time 1.4 ns (PMT only)
Dark Current
≈2 pA at -750 V and 25 C
PREAMP/LOW PASS AMPLIFIER
Type Current-to-Voltage for 1st Stage
Bias Current .15 pA Max.
Offset Voltage <.25 mV
1st Stage Gain
7.9x10
6
2nd Stage Gain 1.5
Input Voltage Noise 4 µV(p-p) (.1 to 10Hz)
Input Current Noise .003 pA (.1 to 10Hz)
Maximum Output Voltage -4.0 V
Operating Frequency Range 0.05 to 350 Hz
Response Time 1 msec

16
A/D CONVERTER
Type Voltage-to-Frequency
Full Scale Frequency 10 KHz (13 KHz maximum)
Full Scale Input Voltages -66 mV (100 SCALE)
-660 mV (10 SCALE)
-6.6 V (1 SCALE)
Linearity <0.3%
Offset <.5 mV (adjustable to 0)
COUNTER/DISPLAY
Integration Times (Gate) 1 and 10 seconds
Timer Quartz crystal programmable timer
Timer Accuracy +/-5ppm at 25° C
Display 4-digit (9999)
Character Height/Color .11 inch - Red
POWER SUPPLY
Type AC to DC wall mount
Input Voltage 110-120 VAC U.S. Model
110-240 VAC International Model
Output Current 200 mA rated maximum
EYEPIECE
Focal Length 25 mm
Optical Design Ramsden
Reticle Illumination Green LED
Field of View 0.4 degrees at 2000 mm focal length
MECHANICAL
Body Material Aluminum 6061-T6 alloy
Finish Bright Dip Black Anodized
Overall Length 9 inches (tip to tip)
Weight 3 lbs. 6 oz.
Telescope Coupler 1.25 inch (standard)