MODEL SSP-4 SOLID-STATE INFRARED PHOTOMETER TECHNICAL MANUAL FOR THEORY OF OPERATION AND OPERATING PROCEDURES OPTEC, Inc. OPTICAL AND ELECTRONIC PRODUCTS sales@optecinc.com http://www.optecinc.com 199 Smith St. Lowell, MI 49331 U.S.A.
Figure 1-1. SSP-4 Solid-State Infrared Photometer shown with TCF-S Focuser.
TABLE OF CONTENTS Revision 1 – June 2010 Section 1.0 2.0 3.0 4.0 5.0 6.0 7.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 1 THEORY OF OPERATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Basic Physical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The Detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.
LIST OF FIGURES Figure 1-1 Page cover 1-2 SSP-4 Solid-State Infrared Photometer with TCF-S focuser . . . . . . . . . . . . .SSP-4 . . . . .System . . . . Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 Cross-sectional view of the SSP-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2-2 6 2-3 Detector/Electrometer Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Functional Signal Processing Diagram . . . . . .
SECTION 1.0 INTRODUCTION – a personal history by Gerald Persha The SSP-4 IR photometer has a long history at Optec. In 1984 a prototype unit was first designed and constructed using an uncooled Lead-Sulfide photoconductive cell from Hamamatsu. The system used a chopper to stabilize the temperature drift and allow for long integration times. Considering the available electronics and IR detectors at the time, this instrument held much promise for small observatories attempting JHK photometry.
Fig. 1-2.
Fig. 2-1.
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.
2.2 THE DETECTOR The detector used in the SSP-4 is a model G5851 manufactured by Hamamatsu Corporation. This unit can be classified as an InGaAs-PIN photodiode, which offers the highest possible shunt resistance value (Rs) resulting in a very low noise current when used in the photovoltaic mode. See Appendix A for complete description of this detector. The detector is normally operated in a cooled state down to –40 C using a two stage thermoelectrically Peltier cooler.
Of course, the gain stability of this circuit is highly dependent on the resistance stability of the feedback resistor. Fluctuations of its value due to temperature changes could cause gain errors during observations, which are especially objectionable when all-sky photometry is done. The special resistor used has a temperature coefficient of 200 ppm/°C which is extremely low for a resistor of its size and Figure 2-2. Detector/Electrometer Circuit rating.
linear, resulting in laboratory measured correlation coefficient using a least squares regression of r = 0.99995. There is an offset adjustment on the front control panel that allows the user to select the output count for zero input light or dark count. It is important to set this to a positive count of around five a gain setting of 1 and a 1 second integration time. This insures that if the electrometer amplifier drifts slightly, the dark count will still be above 0.
SECTION 3.0 OPERATING PROCEDURE 3.1 CHECK-OUT LIST Remove the filter slider and clean filters, if necessary, with a cotton swab dipped in alcohol or lens cleaning fluid. Do NOT rub hard. Before taking readings, turn the photometer on and allow at least 10 minutes warm-up time at observatory temperature. The green TEMP LED will light up in approximately 1 minute after the unit is turned on. This indicates that the detector is at the default operating temperature of -25 C.
Light energy from any observatory red light will be a serious source of error for this photometer. Tungsten lamps are a rich source of IR in the spectral band that this instrument is most sensitive to. Even interior observatory surfaces which are dimly illuminated by the red light and near the front of the telescope will affect the reading substantially. What might appear as a good black paint in the visible spectrum may appear gloss white in the J and H bands.
Figure 3-2. 2-Button Control Menu Flow Chart 3.4 CALIBRATION AND ADJUSTMENTS For precise determination of stellar magnitude, the filters used in the SSP-4 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.
difference. This method is used by many members of the IAPPP and is recommended for novice users. The reticule and detector are critically aligned at the factory and the user should make no adjustment. If the eyepiece is removed from its mount it may be necessary to realign the detector that will probably have to be done at Optec. If dust on the reticule is troublesome, remove it by blowing air (canned air for camera cleaning is suggested) through the 1.25-inch snout.
SECTION 4.0 PC SERIAL INTERFACE AND COMMANDS 4.1 PC (RS-232) SERIAL COMMUNICATIONS The SSP-4 provides a serial communications feature as standard for this model. Using simple ASCII commands, the SSP-4 can be commanded to select gain, integration time, detector operating temperature and report the count. The RS-232 interface operates at 19.2 K baud with 8 data bits, one stop bit and no parity. Optec has developed a user interface program called SSP4DATA.
Command Return SSTART ! CR LF Function: This command initializes the serial connection between the PC and the SSP-4. SSTART causes the SSP-4 program to enter into the main serial loop which checks for additional inputs along the serial interface. A successful return from this command (!) is required before the SSP-4 will accept any other user commands. Warning: If “!” is not returned repeat SSTART. There is a possibility that the command will timeout.
Command Return SIwxyz none Function: This command sets the integration time for acquiring a count from the voltage-tofrequency converter. The four-digit integer value of wxyz is equal to the total integration time in units of 0.01 seconds. Thus, a value of 0100 will set the integration time to 1.00 seconds. Warning: Values less than 0100 (1.00 seconds) are of limited value because the rise time and noise of the system electronics demand longer integrations.
Command Return STEMxy none Function: This command sets the detector operating temperature in degrees celsius below 0. Thus, a xy value of 05 will set the detector to -5 C. Warning: Only integer values from 00 to 40 are acceptable. Table 4-5. Select Detector Temperature Command Return SFTEMP F=xy.z CR LF Function: This command gets the detector operating temperature in degrees centigrade below 0 and sends it on the serial interface. Resolution is approximately 0.
SECTION 5.0 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 exist 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.
SECTION 6.0 SPECIFICATIONS DETECTOR (G5851-203) Type NEP InGaAs PIN-photodiode -14 2x10 W/ Hz (typical) at –35 C 0.3mm diameter (1.0mm diameter optional) 900 to 1850nm >0.2G (typical) at –35 C 1.2 A/W at 1750 nm Detector Size Spectral Range (5% points) Shunt Resistance Peak Response ELECTROMETER (AD8552) Type Bias Current Offset Voltage Open Loop Gain Closed Loop Gain (Rf) Current-to-Voltage 10 pA typical 1 V typical 120db Min. 9 1x10 0.32 µV(p-p) (0 to 1Hz) 2 f /Hz1/2 (10Hz) 2.
SECTION 7.
Figure 7-1. J & H Filter Specifications.
Appendix A InGaAs PHOTODIODE A-1
A-2
A-3
A-4
A-5
Appendix B WIRING DIAGRAM B-1
B-2
Appendix C COMPONENT LAYOUT & CIRCUIT DIAGRAMS Figure C-1. SSP-4 Circuit Board Component Layout.
C-2
C-3
Appendix D SAMPLE DATA ENTRY FORM The sample report form printed on the next page is useful for recording data when using the differential photometry technique. Basically, this method is to compare the brightness of the variable star to that of a nearby comparison star, which is known to have no variability. No attempt is made here to educate the user in all aspects of proper observing procedure and the associated data reduction.
VAR __________________________________ DOUBLE DATE _______________________ COMP._____________________________ VAR.
