Televizní a video meteory
Many professional and amateur groups around the world use so called low light level television (LLLTV) technique to study meteors and meteor showers (Ceplecha et al.1998). The sensitivity of this technique lies between the photographic and radar observations. A typical TV or video camera with an image intensifier is able to detect stars up to 9th magnitude and moving objects as meteors up to 8th magnitude.
In case of double station observation on baselines from 25 to 150 km, it is possible to obtain heights with accuracy up to 0.2 km, velocities up to about 2 km/s and the radiant position up to 0.4 deg. Typical time resolution is 25 - 30 images per second. One of the biggest limitations of this technique is a small field of view, which is typically within 20 deg. Also ratio signal/noise for very faint meteors is very low. Finally the processing of these records is time consuming despite the advance in computation technique (Hawkes 1993).
Observations at the Ondrejov observatoryTV observations of meteors began at the Ondrejov observatory in 1990. There are observations in the integral light and of meteor spectra. The observations are performed in the periods of activity of the major meteor showers. The nightvision TV camera model HT 11-22/SIT equipped by vidicon tube + photocathode detector and lens Meopta-Stigmar f/0.75, f=50 mm is used. For spectral observations it is coupled with a spectral grating Milton-Roy 300 groves/mm blazed to 490.0 nm with dispersion 1.0 nm/pixel in the first order.
The double station observation programme started in 1998. The commercial videocameras Panasonic S-VHS equipped with a Russian made, second generation Dedal 41 image intensifiers and Arsat lenses 1.4/50 mm are used. The diameter of the field of view depends on the zoom of the camera. Typically it is about 25 deg. The observations are made on the baseline Ondrejov - Kunzak (vlozit mapku ci tabulku se souradnicemi)
The limiting magnitude of our instrumentation is about +6 for moving objects as meteors are. The corresponding mass for this brightness is approximately 10^-5 to 10^-6 g and our sample extends to meteors of about 10^-2 g. (fotografie aparatury by take nebyla marna)
The third system, usually located in Ondrejov, is equipped with a spectral grating and is used for meteor spectra detection. Recently, the fourth system began operate. This one is outfitted by wide-field of view lens xxx. All cameras are placed on tripods during observation.
The video signal is recorded using the Super VHS PAL system, giving time resolution 0.04 second. Time authority is provided by DCF77 signal receiver and time is directly included into the video record.
Data processingThe records are searched by either using automatic software Metrec or by human inspection and the recorded data are digitalized with a Miro DC-30 framegrabber on a PC computer. Each frame is transformed into 768 x 576 pixel, 8-bit monochrome image and stored as a sequence in the non-compressed AVI format.
A software PHOTOMET was developed to measure the digitalized records. The source code of this software is written in Fortran 77 language and contains more than 3400 language statements now. The user interface is written in English, so it is possible to use this software worldwide. Description of the method used is given in Koten (1999).
The processing of the records can be divided into two important steps. The identification of the stars and the calibration of the image is the first step and the measurement of the position and brightness of meteor is the second one.
First step includes flat-fielding, dark image subtraction and reduction. The identification of the stars follows. It is useful for a calibration curve construction and for an image coordinate system determination. The signal of the star is computed as the sum of pixel intensities in the area of interest. The calibration curve is computed as a plot of the stars cataloque brightness against their signal intensities logarithm. Linear response of the image intensifier is assumed, which proved to be true.
Similarly the signal of the meteor is measured as the sum of pixel intensities
in the area of interest on each frame of the record. The background value
is subtracted and the apparent magnitude of the meteor is determined
using the calibration curve. The light curves of several Perseid meteors
are shown in Figure (odkaz na obrazek).
Atmospheric and orbital trajectories were computed by means of our standard procedures as well as absolute magnitudes (apparent ones converted to the distance of 100 km) and masses of meteoroids. The errors of all parameters of the meteor trajectory were computed, too.
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|Poslední úprava stránky: 29. října 2001||Auhor: Pavel Koten|
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