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7.6 Background

The FOC suffers from various types of background, the most important of which are thermal electrons, Cerenkov radiation from high energy particles, geocoronal emission lines, zodiacal light, and light scattered within HST from the bright Earth or Moon. Because the particle-induced background levels are essentially unpredictable, the FOC pipeline does not attempt to remove the background from a geometrically corrected and flatfielded image. In practice, most astronomical data analysis procedures derive the background locally as needed, so pipeline background removal is unnecessary.

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.

7.6.1 Detector Background

The detector background arises primarily from thermal electrons at the first photocathode and high energy particles. The dark current due to thermal electrons is rather lower than the particle-induced background, at approximately 2 x 10^-4 counts/sec/pixel. This background source is likely uniform over the field and temporally stable and does not show the reseau marks as dark holes. The particle-induced background is caused by high-energy electrons and protons which generate intense flashes of Cerenkov radiation as they pass through the photocathode window. The FOC's video processing unit (VPU) cannot distinguish the photons from these flashes from celestial photons, and so they appear as a background. The flux of these particles rises strongly over the South Atlantic Anomaly (SAA), but even well away from the SAA, they are the principal contributor to the background of most FOC images. For most of the useful orbit of HST, the particle-induced background is of the order of 7 x 10^-4 counts sec-1 pixel-1 on the f/96 side, and 1-3 x 10^-3 on the f/48 channel. Upward fluctuations of these values are sometimes recorded. Because the particle-induced background generates photons, its spatial distribution looks like a flatfield, except the shadows at the edges of the field caused by obstructions in the FOC beam between the aperture plate and the photocathode are not present. The reseau marks are between the photocathode faceplate where the Cerenkov radiation originates and the photocathode, so they will show up in exposures dominated by such backgrounds.

7.6.2 Geocoronal Emission Lines

The most important contributors to the background at ultraviolet wavelengths are geocoronal emission from Lyman-

(1216 Å) and the O I triplet at 1304 Å, which are relevant only during daytime observations. From on-orbit measurements using the f/96 camera, the former background has been found to vary with solar zenith distance (ZD); see Sections 6.4, 6.5, and 7.0 of the FOC Instrument Handbook, version 7.0, for more details. When the zenith angle is less than 160 degrees, the Lyman-

emission is zero.

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^-4counts 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.

7.6.3 Zodiacal Light and Diffuse Galactic Background

The contributions to the FOC background from zodiacal light and diffuse galactic background have not been measured with the telescope in orbit, so you should assume that the information in the FOC Instrument Handbook, version 7.0, is the best available. Typically, the particle-induced background dominates in an f/96 image under all but the most extreme conditions (e.g., on the ecliptic and pointing as close to the sun as constraints allow), when the zodiacal background and detector background become comparable. Similarly, the diffuse galactic background can be ignored for almost all situations.

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.

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