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Galaxy Evolution Explorer

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Instrument & Calibration



Cycle 6 Information

GALEX Project at Caltech


For Cycles 1 and 2 the brightness limits were conservative. The brightness checker has a safety margin, its limits may be stretched or waived with extremely strong scientific justification. Brightness limits were raised for Cycle 3 and following.

The total field estimated flux that will be considered for observing is 80,000 counts/sec in NUV and 15,000 counts/sec in FUV

Fields in the Magellanic Clouds and in significant parts of the Galactic Plane are very likely to "FAIL" the brightness checker. Central regions of the Clouds are specifically excluded; LMC within 3.75 degrees of 80d, -68.5d (5h20m, 68d30m), and SMC within 2.00 degrees of 15.0d, -73.2d (1h00m. -73d12m). Examine the output carefully to determine why (most common reasons are given below). If the field you wish to observe has been rated "FAIL" but you believe it is still safe to observe, you will need to address this carefully in the "Safety and Feasibility" section of the proposal, and mention it briefly in the "Comments" section of the Observation form.

Some reasons your field might "FAIL" include:

1) Bright star(s) counted more than once: Sometimes the duplications between star catalogues are not removed properly, and a star is counted more than once. If removing the duplicates brings the total expected counts below the limit, the field is probably safe. Note this in the "Feasibility" part of your proposal and the "Comments" field of your Observation Form..

2) Zodiacal light background overestimated: For this checker, we assume a constant value for the zodiacal light of 20,000 ph/sec NUV and 30 ph/sec FUV. This is a reasonable approximation for objects in the ecliptic, but is probably overly conservative for objects near the ecliptic poles, particularly in the NUV. You can check the zodiacal emission for your object with the Zodiacal Light Estimator. If the more precise value of zodiacal emission calculated by this tool lowers the total field brightness to the safe range, note this in the "Feasibility" part of your proposal and the "Comments" field of your observing form.

3) Bright star too close: You may be able to find a safe pointing by not centering your object(s) of interest in the field. The stars that turn up in the Bright Star Check will indicate the stars from which you need to be further away. Note that there may be up to a 10' offset for spectra from their zero order images. Discuss this in the "Safety and Feasibility" of your proposal and mention it in "Comments" on the Observation Form.

4) In or near Magellanic Clouds: GALEX has exclusion zones around both Magellanic Clouds. Repeated attempts to observe in these regions have resulted in multiple and protracted detector self-shutdowns.

Large Magellanic Cloud: Here are FITS images for FUV and NUV images of the LMC [Smith, Cornett and Hill (ApJ 320,609 [1987] & ApJ 355, 746 [1990]), which may be useful in planning observations to avoid bright objects in the LMC. More details on these images can be found here.

Small Magellanic Cloud: Here is an FUV mosaic of Ultraviolet Imaging Telescope (UIT) images of the SMC [Cornett, et al, 1997, AJ 133, 1011], which may be usefil in planning observations to avoid bright objects in the SMC. A FITS download is available here. More details on the SMC fields are available here.

5) Any other reason: If you have any other reason to believe the Brightness Checker is incorrect in your case, you are free to go ahead and propose with a full explanation in the "Safety and Feasibility " section of the proposal and in the "Comments" field of the Observing Form. All proposals will be subject to a technical review, which includes instrument safety. Note that proposals that you believe (and argue) are safe may still be rejected during technical review if they do not pass the brightness checker and they are deemed dangerous to the instrument.


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

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