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The star light photometry is a suitable method for the determination of the spectral aerosol optical depth (δ A ( λ)) during night time when photochemical processes don't take place.
The method may be used to complete an extensive δ A ( λ) monitoring over the whole day in midlatitudes and especially for the δ A ( λ) observation in high latidude area during the polar night.
The Star photometer SPSTAR 3 can be applied to evaluate the aerosol optical depth during night based on measurements both according to the so-called two-star-method (TSM) and according to the so-called one-star-method (OSM).
Both procedures allows automatically un-manned observation after correct alignment. It is highly recommended to install the system inside a Radom (at least 2.5 m inner diameter) for protection against rain and snow.
The basic equation of stellar photometry for the estimation of the spectral optical depth δ A ( λ) is Bouger-Lambert-Law, which describes the weakening of radiation passing a medium. Because of stars in different air masses, a relative measurement between two stars (two-star-method - TSM) can be used for the estimation of δ A ( λ). In this case, a calibration of the system is not necessary. Prerequisite is a homogeneous horizontal distribution of the absorber in the atmosphere, which is often not fulfilled.
Nevertheless, measurements with high precision can be carried out, when only one star (one-star-method - OSM) is used. For this, the extraterrestrial signal of the instrument for the used star has to be known, which can be found using the two-star differential method under very good atmospheric conditions.
The principle of the measuring system is based on the optical coupling of a mirror telescope MENISCAS 180 (aperture 180 mm, focal length 1800 mm), an optomechanical block which contains a special focussing optics, an optical gate, a CCD line as a sensor, an automatic focusing and a controll electronics.
The used sensor is a CCD sensor S7031 with a sensitive spectral range from 400 to 1100 nm.
In front of the CCD line is placed an optical grating for the spectral splitting of the incoming light and a special optics to the focusing of the complete stellar light allows a completely new, technically solution in the field of the atmospheric photometry, where instead of solar irradances the much lower intensities (magnitudes mi( λ)) of selected fixstars with knowledge of the extraterrestrial magnitudes m0i( λ) is measured. This technology is completed by an innovative tracking system, which automatically secures both the exact orientation of the measuring system towards the star during the measuring with the help of two CCD cameras as electronic visors and the necessary background measuring.
The star light is focussed by the mirror telescope MENISCAS 180 in a measuring diapragm with a diameter of 0.5 mm. The telescope is pointed roughly at the star chosen for the measuring with the help of visor unit. The first direction justification is carried out with the help of a viewfinder telescope with a CCD camera. The exact direction justification is carried out with the help of a CCD camera which is in the starphotometer using a part of the star light (app. 10%), refracted by a beam splitter.
In connection with the control unit of the star photometer the program system STMHWin32 running on the PC controls the measuring procedure and the operation of the parallactic tracking unit by using a CCD camera. The star light is so conducted over a special optics that the complete light energy of the select star reaches the receipt of a fiber optic..
By help of the fiber optic the light is led to the gate where the spectral splitting and the illustration of the spectrum on the CCD line is carried out. With a controll unit the contents of every single CCD cell are evaluated and changed into an electrical equivalent. These values are available as raw measurements and are passed on to a PC for the further processing.
The PC is connected to the controll electronics of the starphotometer about a RS232 / USB interface.
In connection with the control unit of the star photometer the program system STMHWin32 controls the measuring procedure and the operation of the parallactic tracking unit by using of two CCD cameras, one in connection with a additional refractor for finding the star in the sky
A parallactic tracking unit can be a GTO 900 (ASTROPHYSICS) or GM11 (LOSMANDY). This unit is used for the orientation of the star photometer towards the measuring star. It is assembled on a tripod column or pier and lined up horizontally exactly. Then the exact orientation towards the sky pole must carried out according to the known proceedings.
If the orientation was carried out successfully the tracking unit can be also moved by the hand contol box or with the help of a computer unit to preselected extraterrestrial targets (sun, moon and stars) with the precision corresponding to its parameters.
With the program system STMHWin32 the complete control of the starphotometer SPSTAR 3 is carried out. Among others it makes possible the execution of the following operations:
| Optical system | MAKSUTOV-Telescope |
| Measurement range | 410 nm ... 1050 nm |
| Measuring wavelength | 10 (412 nm, 442 nm, 500 nm, 532 nm, 610 nm, 672 nm, 778 nm, 862 nm, 964 nm, 1024 nm) |
| Tolerance to central wavelength | ± 2 nm |
| Light diffraction | grating |
| Spectral bandwidth (FWHM) | < 6 nm |
| Sensor system | CCD sensor S7031 (Hamamatsu) |
| Number of pixels | 1024 x 58 (1044 x 64 total), 24,6 µm2 |
| Quantum efficiency | 90% peak |
| Duration of a measuring | < 2 min for OSM |
| Accuracy | 0,003 ≤ Δ δ A ≤ 0,011 |
| Guiding system | two CCD-cameras |
| Interface | RS232 |
| Power supplyS | 220 V/12V (3 A) |
| Weight without telescope | 7 kg |
| Weight of telescope | 10 kg |
| Operating temperature range | up to -80 ° C (with additional temperature insulation and heating) |
| Tracking system | parallactic mount (GTO900 or GM11 or similar) |
| Measuring stars | up to the magnitude 3 |
| Time resolution | 20 - 30 min, mean value of 10 single spectra |