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Дата изменения: Tue Nov 18 00:20:13 1997 Дата индексирования: Sat Dec 22 14:49:30 2007 Кодировка: Поисковые слова: photosphere |
The levels, spatial distribution, and time variation of the principal sources of background are discussed below to help you decide whether the background on your images might be astronomically interesting or is merely an instrumental effect. For a more thorough discussion, see the FOC Instrument Handbook.
For O I 1304, the background is less than 5 x 10-5 counts/sec/pixel for solar zenith distances (ZDs) of more than 90 degrees, rising nonlinearly to about 8 x 10-4 counts sec-1 pixel-1 at ZD of 25 degrees.
For f/48, these numbers should be multiplied by a factor of about four, reflecting the pixel-size difference.
7.6.4 Scattered Stray Light
Normally, the FOC background is dominated by the detector, by zodiacal light in the visible, and by geocoronal Lyman-alpha and diffuse galactic light in the far UV. However, stray light reaching the OTA focal plane due to scattering from the baffle system, the OTA tube, and dust on the mirror can dominate the background when a bright object such as the sun, moon, or the bright Earth limb is nearby. In-orbit calibrations of this stray light have been performed by P. Bely and D. Elkins using a solar spectrum combined with the Earth's and the moon's albedo. Only for observations where the limb angle is less than 50 degrees from either the moon or the Earth will stray light have an illumination brighter than 23 V magnitudes per arcsec2 at wavelengths greater than 3400 Å. More details on the determination of the stray light contribution and its wavelength dependence can be found in Section 6.5 of the FOC Instrument Handbook, version 7.0.