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.

i
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
17
6.0 JOHNSON FILTERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .
19
7.0 STRÖMGREN FILTERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
24
8.0 SSP-5A OPTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
26
8.1 Physical Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
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

ii
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.