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The minimum requirements for BRITE photometry are to observe one star field, with up to 15 primary target stars, during at least 15min per satellite orbiting period. In a Low-Earth-Orbit (LEO) of about 800km elevation the orbital period of the satellite is about 100 minutes. The point-to-point scatter (rms noise) of the collected photometric data during that 15min collection time will be about or below 0.001 mag for stars brighter than 4th magnitude.
The BRITE photometry will be conducted with a CCD detector from KODAK, a 4008 x 2672 pixels device fed by a 3 cm aperture five-lens telescope. The plate scale is ~30 arcsec per pixel. A filter mounted before the entrance aperture of the telescope and a baffle is also placed in front of the optics (see instrument page). The typical camera exposure time for bright stars will fall in the range of 1-60 sec. The images will be defocussed as shown in the figure below.
The figure shows a simulated Point Spread Function (PSF) of the BRITE-AUSTRIA (blue bandpass) instrument. The image shape is a ring in the central parts of the Field of View (FOV) with a diameter of about 8 pixels. The UniBRITE instrument with a red bandpass (see Filter page) has a very similar PSF.
For each of the primary target stars in the field a subraster section (image data) of the CCD containing the Point Spread Function (PSF) and sky background will be transmitted to the ground. For other stars in the field, on-board data processing will be performed to collect photometric data, thereby reducing the limited data stream.
The limiting magnitude will depend on the exposure time, which is constrained by the primary (or brightest) target stars. The figure below shows the expected photometric performance of the BRITE instruments (BLUE and RED) as a function of magnitude and exposure time.
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The diagram shows the simulated photometric point-to-point of scatter in magnitudes (blue lines) for different exposure times and as function stellar brightness (magnitudes).These simulations have been conducted for a one minute sampling time. The yellow lines show the respective photon noise limits.
An example to illustrate the potential of the BRITE instruments is as follows: for primary target stars of apparent visual magnitude 2 the optimal exposure time would be about 8 seconds, which means that per minute 5 exposures can be collected. This includes overhead time due to data extraction from the CCD. With this setup a 2nd mag star will have a point-to-point scatter (noise) of about 0.0004 magnitudes (0.4 milli-mag) which is more than a factor of two better than the basic requirements. Even for stars which are as faint as 6th mag (with the same exposure time) the photometric noise will better than 0.01mag, hence the performance will us allow to study the variability of much fainter stars our fields in addition to the selected primary targets, providing they are not crowded by other targets.
Any field on the sky can be observed with the BRITE instruments. There are no restrictions concerning the direction of the observation other than that the boresight of the instrument should not be closer than 90deg from the Sun. In general, fields will be observable with BRITE up to 180-200 days continuously (for at least 15 min during each orbit). When a given field can be observed depends in general only on its Right Ascension. However, there are small areas at the celestial poles which can be monitored essentially all year respectively independent of seasonal restrictions.
The time series of the BRITE photometry may look like the illustrations below, where light curves from the MOST satellite are shown. This satellite often observed two fields alternatingly during each satellite orbit (101min period).
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The light curves of two variable stars in two different fields observed with the MOST satellite are shown in the pictures above. The upper panel shows the light curves over a few weeks. The lower figure is a blow up into the time series to show the observing sequence more clearly for each of the two fields. Even though both time series have a reduced duty cycle, with respect to the 101min orbit period of the satellite, the observations sample the stellar variability well. Very similar photometry, even in two bandpasses even, can be expected from the BRITE satellites.
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