International Occultation Timing Association * European Section * |
Abstract For the use in stellar occultation work a fast, transportable and relatively cheap camera system has been developed. It is based on the TC245 frame transfer chip (Texas Instruments). The control program running in a PC allows real time monitoring of the images as well as data storage on harddisk. For small image sizes, up to 18 images per seconds can be recorded. The readout noise of the camera is around 25 e- at a chip temperature of about -30 deg C. The timing of the images is done by receiving the DCF 77 time signal radio station. In the report recordings and analysis of lunar occultations, of the occultation of nu2 SGR and of PPM269153 by Jupiter will be shown to demonstrate the versatility of the system. First tests on photometric measurements of planetary satellites will be reported. Introduction Most astronomical objects require long exposure times (up to many hours) to collect enough photons for a sufficient signal to noise ratio. Their light intensity remains constant over time or their variation is slow compared on a hourly or daily time scale. However, there are objects with changing structure and brightness on a much smaller time scale. Variable stars such as fast eclipsing binaries, cataclysmic variables or flare stars can change their brightness sometimes in a few seconds of minutes by a factor of ten or more. Occultations of stars by the moon, planets and asteroids can only be observed by CCD systems with a time response in the subsecond range. For the mutual events of satellites of the big planets (Jupiter, Saturn etc.), the necessary time resolution is a little lower, but if possible a minimum resolution of 1 second should be achieved. Most CCD systems used in astronomy up to now are very specialized to extremly high sensitivity light detection. They are large area CCD's (up to 7000 times 9000 pixels!) with readout times of up to many minutes. Many of them can not be used in a fast recording mode, even if smaller subareas can be selected. Sometimes the rotation of the earth can be used, to smear out the image on the chip in the direction of rectascension and to have a better time resolution. But the 2-dimensional information is lost, and severe problems may rise, if a high background (or a lunar of planetary limb) is close to the object. On the other side, there exist a large number of systems with normal video readout, commercially available video cameras or camcorders. Up to now, these systems deliver a analog read out video signal, which gives problems for correct intensity calibration and linearity. The images (50 half frames per second) have to be digitized and transfered to a computer. With careful linerity checks, they can be used in some cases, but if the object intensity is small or the wavelength range to be used is restricted (p.e. the 20nm width methane band range around 890nm to suppress the jovian light scatter for mutual event measurment) large telescopes may be needed. The exposure time per image can not be increased, however by adding more than one frame to an image after digitization the effective exposure time can be longer. There exist a large number of CCD cameras designed for the amateur or public observatory as well as for small university observatories, which have smaller sized chips, thermoelectric cooling (instead of liquid nitrogen) and are connected to a standard PC. However, most of theese cameras are not designed for image acquisition rates faster than 1 image per second. This situation has provoked us to design a small area chip CCD camera with an image acquisition rate of up to 20 images per second for small subareas, with thermoelectric cooling, direct computer interface with a analog/digital converter board, exact timing by a radio time signal station and a real time software to display the images immediatly during acquisition. In our report we discuss the technical data of the system and show some application from occultation work. Camera Design, Technical Data and Software The IOTA Occultation Camera (IOC) has been developed around a commercially available CCD chip for use in vido cameras in order to keep the price of the system as low as possible. This allows a broad distribution of these systems among amateurs, public observatories and small professional observatories as well. The system is mainly focussed on the observation of occultations, but is nevertheless suitable for long time exposures up to many minutes. In order to have a high quantum efficiency (up to 65%) combined with a good blue wavelength response without coating or thinning, a chip from the Texas Instruments series in virtual phase technology with full frame transfer has been selected. Balancing price and performance, the TC245 has been chosen. It has an area large enough even for the deep sky observers under the amateur astronomers as well. It has an on-chip implemented double correlated sample and hold circuit (DCSH) and a full frame store section, to allow a fast transfer of the image for read out and at the same time to give a high quantum efficiency. Due to the masked image storage area, a mechanical shutter is not necessary. The chip is cooled with a single stage thermoelectrical cooler to give a temperature roughly 50 deg lesser than the outside temperature of the housing. A glas window in front of the chip closes the housing to prevent the formation of ice on the chip. The housing can either be cooled by free convection, with a small fan (enforced air cooling) or with water or an other coolant. The read-out noise of the chip is around 25 electrons, a value which is not too good compared to more modern CCD arrays, but gives a reasonable compromise. The electronic has to be designed to be of high linearity and reproducibility. The output signal is amplified from about 4aV per electron to about 100aV. Together with the A/D converter card (12 Bits, 7asec conversion time, for the ISA Bus system) it results in approx. 10 electrons per ADU. The chip is controled directly by the computer -any PC from 386SX onwards can be used- without any microcontroler. Only the voltage levels have to be converted, from TTL level (0V and +5V) to the more complex CCD chip levels. This is done by analog switches (CD4053), which provide high speed switching in about 100nsec from -9V to +2V. By controling the chip entirely by the PC, the software can be changed easily. Even complex operations, such as single line spectroscopy with a rate of up to 1000 samples per second can be programmed with ease. The system time of the computer is set by an external radio time signal receiver of the DCF77 at a radio frequency of 77.5 kHz connected to the serial port of the PC. Every minute an internal routine compares the received signal with the system time of the PC and corrects it if necessary. This works in an radius of about 1000km to 1500km around Frankfurt/Main, Germany. For other countries in the world, special arrangements may be with GPS reception have to be done. The software written in Fortran 77 and Assembler (time critical parts of the software) allows the control of the camera during image recording. The size and position of a subarea on the CCD chip can be freely selected. The smaller the actual readout area, the faster the image can be recorded. For an array of about 50 times 50 pixels (far more than necessary in lunar occultations) up to 18 pictures per second can be recorded. During program execution, simple measurements can be done, giving the mean value in a selected area and its standard deviation in order to control the chip performance. A single frame mode can be selected to store images in FITS mode. The following table summarizes the main technical data of the IOC system: Sensor: TC245 made by Texas Instruments Pixelsize: 8.5um x 19.75um Pixels: 755 x 242 pixels = 6.68mm x 4.82mm Pixelsize 2x: 19.0um x 19.75um by binning: 3x: 27.5um x 19.75um Technology: Frame transfer in virtual phase technology# Sensitivity: 390nm - 1050nm Q.E.: max 65% Readout: Double correlated sample and hold on chip Output ampl: 4uV per electron Readout noise: 20 to 30 electrons Cooling: Single stage thermoelectric cooler Typical temp: Chip temperature -30deg C Cooling of housing by free convection, forced convection or liquid coolant Preamplifier: Operational ampl., DC coupled, 0.1mV per electron A/D board: 12-Bit AD774 (Analog Devices) 7asec conv. time Separate Sample and Hold, ISA-Board Conversion: 10 electrons per ADU SOFTWARE: Max. acquisition rate: 25asec per pixel Size of subarea free selectable Position of subarea on chip free selectable Internal or external binning selectable Timing: DCF-77 receiver via RS232 interface, update of system time every full minute Image rate: 50 x 50 pixels at 18 images per second Examples and Applications of the IOC The occultation of nu2 Sagittarii by Jupiter on the 6th of March 1996 was one of the most spectacular occultation events in the last years. The IOC camera system has been used to record this event from the US west coast. In order to suppress the brightness of Jupiter compared to the 4m4 K0 star a methane band filter has been used. It is centered around 890nm with a HWB of 20nm and a maxcimum transmission of 87% (Dr. Hugo Anders, Gesellschaft f. Dunne Schichten, Nabburg, Germany). It improves the contrast of the star relativ to Jupiter by approx. a factor of 20. The occultation was recorded at the Anza observatory of the Orange County Astronomers (about 10 miles inland from Mount Palomar Observatory) with their 55cm Kuhn telescope. In order to extract the star intensity at the limb of Jupiter the following approach was done: After dark image subtraction and flat fielding an unsharp masking filter was applied to all images in the recorded data file. The parameter of the mask was choosen not to change the intensity of the stellar image but to sufficiently reject most of the remaining Jovian image. The position of the star was marked by visual inspection of all the images and an automatic process was used to determine the background around the stellar image and subtract it from the star. The intensity of the star was calculated for each image and transfered to a data analysis program to analyze time series data (Beisker, 1994). The data were normalized for full star intensity set to 1 and zero star intensity set to 0. Fig.1 shows the reappearence of the star behind the Jovian limb. A similar approach was done for the occultation of the 8m7 star PPM269153 observed from the Cuno Hoffmeister Observatory in Namibia (a C14 on a fixed site) and at the Specola Vaticana (60cm Cassegrain telescope) in Castel Gandolfo (Italy). The camera clearly recorded the occultation of this very faint star, the disappearence as observed in Namibia is shown in Fig. 2. Besides theese two events, numerous lunar occultations have been recorded with up to 18 images per second to give a timing accuracy of up to 0.055 seconds. After the PHEMU97 meeting in Catania, an expedition was sent in July 1997 to Australia to successfully record at three independent stations the occultation of a 12m7 star (event TR176) by Triton. As first attempts to observe mutual events for the PHEMU97 campaign, 11 events have been recorded so far.