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Cycle 6 Information

GALEX Project at Caltech

GALEX data are recorded as photon event lists. Both local count rates (limited to prevent heating of and potential damage to the photocathodes and "gain sag" nonlinearity ) and global count rates (limited to control non-linearities due to finite electronics speed) must be kept below prescribed limits to avoid permanent damage to the detectors (See the Instrument Overview for an introduction to the detectors, or the Detector Guide for details).

For a given pointing, there are three different limits to worry about. First, the predicted total count rate in the Field of View (FOV, or “field”) is limited to 50,000 photons/sec in the NUV and 15,000/sec in the FUV. These rates are half of the count rates used as automatic shutdown limits by the onboard software. Second, there are local count rate limits on each bright star within the field (30,000/sec NUV, 5,000 for FUV, updated Cycle 3, unchanged Cycle 4). Finally, there are higher count rate limits for bright stars just outside the field. These brightness limits are a function of angular separation from the field:

The GALEX Brightness Checker uses estimates of the diffuse background and catalogues of UV stars to model the predicted total field flux for a specific observation. The predicted total field flux is the sum of three components, discussed in more detail below:

• The summed flux from individual bright stars in the field, taken from a bright star database developed for GALEX.
• Diffuse galactic emission (DGE), as predicted from the Schlegel dust maps.
• A standard, fixed zodiacal emission flux

Bright stars: The GALEX bright star database is built from three different catalogues:
TD-1 (ref and wavelength(s)), Tycho2 (ref and wavelengths), and SK2(ref and wavelengths) catalogs. The Brightness Checker tool searches all three catalogs for bright stars, removes duplicates, and includes only stars with predicted GALEX count rates over 100 counts/sec. When duplicate star entries are removed, a composite entry is created comprising the position of the closest entry and the highest count rates for each band. (Thus a star with duplicate catalog entries may have a position and NUV and FUV count rates from three different catalogs.) . The first part (bright star test) of the output shows the (“de-duplicated”) catalog entries for potential bright stars, noted with PASS or FAIL in the right-hand column. The report doesn't list all stars in the area--just those with a count rate above 5000 within 2 degrees. Also included are these stars’ distance from the field center (degrees) and the flux limit for that distance. The second part (background test) shows the number N of stars (>100 counts/sec) contributing to the total flux, and the zodiacal and DGE contributions, with totals and limits for the NUV and FUV, respectively.

Zodiacal Light emission: The Brightness Checker assumes a fixed photon flux from zodiacal light emission of 20,000 ph/sec in the NUV, and 30 ph/sec in the FUV for Cycle 1. This number is conservative, particularly for objects out of the ecliptic plane. A more accurate estimate may be obtained from the Zodiacal Light Calculator, which is based on Table 17 from Leinert et al. (1998), A&AS, 127, 1. In this model, the Zodiacal Light is assumed to follow a slightly-cooler-than solar spectrum (reddened), scaled using Leinert et al.’s values.

Diffuse Galactic Emission: The DGE is estimated from 100um dust maps (Schlegel et al. (1998), ApJ, 500, 525). The observed FIR emission is converted to a FUV flux using the method of Schiminovich et al. (2001), ApJ, 563, L161. In this method, the 100m emission is converted to a UV flux by I (174nm) = 200. + 100 * I (100µ). This is then scaled by a B3-type stellar spectrum (fairly typical of the UV DGE) and integrated over the GALEX bandpasses. Please note this method does not include scattering from discrete nearby bright UV sources.

Because the continued successful operation of the GALEX instrument is a very high priority, the Brightness Checker is designed to produce “cautious” results and to identify potentially dangerous observations. In some cases, information from the Brightness Checker may be helpful in devising workarounds for troublesome fields.

The only input to the Brightness Checker is the pointing center (the center of the desired GALEX FOV). If there are no potential dangers, two “PASS” ratings are given. If there are potential problems with the field, two tables are returned, with problem cases labeled “FAIL”.

• The first table is a list of all bright stars in or near the field, with their positions, separations from the field center, required separations, and for each band the expected count rate and relevant limit. The report doesn't list all stars known to be in or near the FOV, just those with a count rate above 4500 c/sec within 2 degrees.
• The second table is the total flux test. It gives the number N of stars that are brighter than 100 counts/sec in the field; then it lists the total counts expected from these stars, the (fixes) zodiacal and DGE contributions, and total count rates expected and limits for both the NUV and FUV.

Star Catalogue References:

TD1: Catalogue of stellar ultraviolet fluxes (TD1): A compilation of absolute stellar fluxes measured by the Sky Survey Telescope (S2/68) aboard the ESRO satellite TD-1, Thompson G.I., Nandy K., Jamar C., Monfils A. Houziaux L., Carnochan D.J., Wilson R., The Science Research Council, U.K. (1978) http://adsabs.harvard.edu/cgi-bin/bib_query?1982A&AS...49..427W

Tycho-2: The Tycho-2 Catalogue of the 2.5 Million Brightest Stars, Hog E., Fabricius C., Makarov V.V., Urban S., Corbin T., Wycoff G., Bastian U., Schwekendiek P., Wicenec A. ,Astron. Astrophys. 355, L27 (2000) http://www.astro.ku.dk/~erik/Tycho-2/

Sk2: SKY2000 Master Catalog, Version 4: Myers et al (2002) Goddard Space Flight Center, Flight Dynamics Division:http://tdc-www.harvard.edu/software/catalogs/sky2k.html

Responsible NASA Official: Susan G. Neff
Curator: J.D. Myers

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