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GALEX Diffuse UV Background

6. Testing Target One for Geophysical/Orbital Effects

We can do this, by comparing what we get on separate visits to the same target (our Target One).
 6.01
Our measurement of Target One involved eleven visits (eleven orbits). We have constructed 11-frame movies for both the FUV and NUV images, and here we show them, together with the Co-Add of the eleven images.
 6.02
While it is very impressive how greatly the signal-to-noise improves with the Co-Adding of the eleven frames, it also is true that the basic structure of the target does appear in every frame, in both FUV and NUV. The conclusion is that we are not being fooled by some kind of orbit-dependendent geophysical effect.
 6.03
We have already seen how improved the image is, when eleven visits are co-added. Now let's get quantitative: what fraction of what we thought (on a single visit) was diffuse, is in fact removed as fainter point sources, when we co-add eleven?
      Well, here is the result for Target One:
            FUV 6.3 % reduction
            NUV 4.1 % reduction
Now, it still could be true that all the remaining "diffuse background" is actually point sources! Our result suggests, however, that we really have removed essentially all the point sources, and that the remaining radiation is diffuse.
 6.04
A key resource in understanding GALEX data is the important paper The On-Orbit Performance of the Galaxy Evolution Explorer by Patrick Morrissey et al.
 6.05
Patrick kindly provided us with data indicating an "airglow" contribution to both FUV and NUV.
 6.06
Here are some examples of our own efforts to understand the amount of the "airglow" contribution:
                        dis6fuv37-51.pdf                              dis6nuv37-51.pdf
                        dis6fuv53-60.pdf                              dis6nuv53-60.pdf
                        dis6fuv63-70.pdf                              dis6nuv63-71.pdf
                        dis6fuv71-75.pdf                              dis6nuv72-75.pdf
Note that in some cases the curve is flat-bottomed, indicating that the measured amount is all there is. Units are "continuum units," photons cm-2 s-1 sr -1 Å-1

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