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A computer controlled low light photometer for nightglow observations
A computer controlled low light photometer for  nightglow observations.
 
 Hardware units of portable photometer for low light measurements, especially night glow emission is described. The spectral region for measurements is chosen using separable interference filters.
The photomultiplier FEU-138-1 with GaAs photocathode and photon counting method is used. The amplifier-discriminator have a minimal threshold of 1 mV and under, output pulse is 50 ns - 1 mks (variable) duration and 4 V amplitude with 20 m coaxial cable.
The control system is based on an  PC compartable computer with the interface unit. Interface card contain three channels of 24 bit counter, 4 channels of 12 bit ADC and the buffer bus for the controlled motors of the altazimuthal fork mounting and filters wheel, the position encoders.
 

HARDWARE

Photodetector unit.
The prototypes of the photometers were designed for measurements of plasma emissions in the APEX project. The same instruments were used for groundbased observations. A scheme of the photodetector unit of the photometer is shown in Fig. 1.
 
Fig.1 Photodetector unit configuration:

1- light baffles; 2- diaphragms; 3- objective lens; 4- diaphragm FOV; 5- electronics of PMT protection with photodetector 6; 7- LED for light calibration; 8,9,10 - filters wheel and step driver with electronics; 11- Fabri lens; 12- PMT in magnetic screen 13; 14- amplifier-discriminator; 15,16 - PMT’s and electronics power supply.

 Technical parameters of photometers are as follows: objective lens - diameter 70 mm, focal distance - 360 mm; max. FOV - 5 deg., photomultiplier - FEU-138 with GaAs photocathode, spectral range - 250-900 nm, photon counting mode is used, light sensitivity is 10-30 counts per sec for l =630 nm for Dl =3 nm; dark current is <100 1/s for +20 deg. C and <10 1/s for +5 deg. C; general efficiency is 0.03 - 0.05 counts/photon. Dimensions: diameter is 100 mm, length - 420 mm; weight - 2.2 kg; power supply - voltage 22-32 V, current 0.15 A.

In "WARNING" project working wavelengths of the photometers can selected by changing of the interference filters: P1 is equipped with 3 filters - l1=557.7 nm , l2=630.0 nm [OI] and l3=761.9 nm O2 emission; P2 is equipped with filters for l1=630 [OI] and l2=656.3 nm [Ha] emissions. The green line of oxygen has max. of emission for the altitudes of 100-110 km, and max. for red emissions - 250-300 km, therefore nadir photometers can give measurements of disturbances in ionosphere for different altitudes. Such a choice of the wavelengths provides and opportunity to study Earth's hydrogen corona. As continuous operation of TV camera is not possible because of telemetry limitations, the zenith photometer besides the measurements of emissions of outer ionosphere can give the additional information on orientation of the vehicle by observation the stars in its FOV with a special diaphragm.
PMT may be TE cooled.
 

Counter and interface for PC unit
New PC comparable board consists three 24 bit counters, 12 bit four channels ADC, two bit I/O bus and used 8/16 bit ISA interface.
Counters based on ACTEL ACTTM Family field programmable gate arrays. One chips consist three independent 24 bit counters with 16 bit out bus its, 8/16 bit multiplexed out bus (for PC ISA bus) and control logic. Input pulse width is 20 ns minimum, maximum count rate - 50 MHz. May be use 8, 16 or 24 bit count, 1, 2 or 3 counters.

Mounting

To simplify construction and in the light of specifications for its use, we chose an altazimutal mounting. The mounting structure is made of aluminum and bronze (gears). Overall dimensions are 65 x 25 x 25 cm, weight 10 kg. Azimuth and elevation worm gear drive rotate by of small, low powered DC motors. Rate of angular motion is 8 (elevation) and 12 (azimuth) degree in second. Movements controlled by precision  resistance transducer.

SOFTWARE

The software for groundbased observation sets the following parameters: date, time, observation point and its coordinates, target object and its coordinates, storage time, duration of pause between measurement, the total measurement time, the number of averaged measurements, program for filter changing, program driving the turning device (increment and span), and the program for calibration using stars as secondary spectrometry standards. Graphics interface used.
 
Calibrations
 The calibration of the photometer made in the laboratory of the Base Observatory Bulgarian Academy of Science in Stara Zagora, 1990 and 1991. Used secondary standard with calibrated lamp.
Measurement light flux was be calculated:

Where w=7.6 10E-4 sr is solid angle field of view, A=38.4 sq. sm. is area of the input aperture, Dt - integration time (1 second), k0=10E6 photons/(sq. sm. s) = 1 Rayleighs (R), B0() - standard brighten in Rayleighs/Angstrom, K1() - collimator transmission, K2(l) - photometer's optic transmission, K3(l) - filters transmission, 2 - 1- spectral band of PMT.
The sensitivity of photometer is: 
Where N() is output signal photometer generated input light flux F(l)
We measurement transmissions of optic and filters and had assume:

The filter parameters is: =5570 A - ef=44.9 A; =6300 A - eff=50.2A; =7775A - eff=70A.
Threshold flux is:

Where Nn is dark current.
The results of photometer calibrations are given in tables 1 and 2.
Table 1. Data of calibrations of photometer with light standard used
PMT # 599 599 599 599 599 599 586
l, Angstrom 5570 6300 6300 6300 6300 6300 7775
B0l, R/A 0.9 1.4 1.4 1.4 1.4 1.4 4.3
Nn, 1/s 109.8 59.6 108.9 147.3 232.8 359.7 276.5
Nn+s, 1/s 660.2 671.8 731.6 791.0 796.8 914.4 1983.4
N, 1/s 550.3 612.2 622.7 643.7 562.3 554.7 1706.9
Sl, 1/(s R) 20.6 13.4 8.6
Sl, 1/(s erg) 2.5 E9 1.8 E9 1.4 E9
Fnl, R 0.51 0.78 1.93
Fnl, erg 4.2 E-9 5.7 E-9 1.2 E-8
* N=Nn+s - Nn;
* PMT #599 have been measurement at the different temperature on 630 nm.
 
The photometer also was calibrated using stars - spectrophotometric standards (aLyr, aBoo, aUMa, hUma, ...) in the Observatory Kyiv University 1990-1995.
Table 2. Data calibrations of photometer with stars - spectrophotometry standards used
PMT # Filter central wavelength, 
A		 Filter 
Transmission, maximum		 eff, 
A		 Nn, 
1/s		 S, 
1/(R s)		 W, deg
1 586 5579  0.62 23  110 35.6  2.8
2 " 6299  0.39  10 " 24.3 " 
3 " 6801 0.68 120 " 16.6 "
4 "  7775 0.72 70  "  10.4 "
5 599  6300  0.65 50 120  13.5 "
6 602 7776 0.79 60 480 7.2 2.5
7 611 7781 0.77 62 140 0.6 2.5
8 695 7778 0.81 58 175 16.8 2.5
Photodetector unit and 8 bit ISA PC card.
  Last update 17 september 1999
Spectrometer CCD table PMT table   Nightglow emissions