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GALEX Diffuse UV Background

10. Instrumental Scattered Starlight

 10.01
First, here is the Dis.05 NUV image (to shock you!)
 10.02
The horrible bar across the image is NOT present in the Dis.05 FUV image. The bar is due to a single star being in just exactly the wrong place with respect to the camera pointing. The result is very bad instrumentally-scattered starlight. The GALEX images ALL have a certain amount of this around the outer rim; as a result, we (and everyone else) do not use the outermost parts of any GALEX images: just the central one degree.
 10.03
So this raises the question, could we possibly simply be seeing instrumentally-scattered starlight, and mistaking it for diffuse UV background? That the answer to this question is "NO" is easy to establish!
 10.04
First, lets look at the diffuse background (pipeline) for our Target One FUV image:

      average = 0.00018444 = 372.38 photon units (pu)

      smallest = 0.00016580 = 334.75 pu = 0.90 X average

      largest = 0.00030840 = 622.66 pu = 1.67 X average

total = 1156.8 <= total cps from the pipeline diffuse background

Next, the contribution of Starlight to our Target One FUV image:
      we now assess the whole image, including stars

      average = 0.00025340 c/s = 511.62 pu, treating everything as "diffuse background"

      smallest = 0.00007452 c/s = 150.45 pu = 0.29 X average

      largest = 0.61276805 c/s = 1,237,178.75 pu = 2418.18 X average

      total = 1388.8 <= total cps
from diffuse background + 1,412,697 stars (pixels above highest background),
                                                          of which 5,940 are very bright

            = 1.20 times the pipeline background, which was

      sky = 1156.8     <= sky total

Δ = 232.0 above highest background

total # of pixels = 6,272,329 of which:

      1,412,697 = 22.5 % > 1.0 times sky, Δ = 77.5 %
      1,074,483 = 17.1 % > 1.1 times sky
        881,160 = 14.0 % > 1.2 times sky
        661,716 = 10.5 % > 1.3 times sky
        526,916 = 8.4 % > 1.4 times sky
        422,001 = 6.7 % > 1.5 times sky
        212,463 = 3.4 % > 1.8 times sky
        144,887 = 2.3 % > 2.0 times sky
          47,735 = 0.8 % > 3.0 times sky
            5,940 = 0.1 % > 10.0 times sky

Number of Zero pixels in core = 791,508

 10.05
So what is the bottom line? The whole image contains only 20% more counts than the background, so the notion that the background is, say, wide wings of stellar images, cannot be credibly suggested.

Next Section ... demonstrates that the Pipeline background is of high quality.

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