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Best of the data -- averaged frames with 100 good slices

Observing logPosted by Chris Flynn Dec 14, 2012 05:58AM
I wrote a program called moonraker in fortran, which processes all our data, seeking out the best for further reduction. Peter has written something similar in IDL.

It goes through all the images and throws out ones with

1) saturated BS (> 50,000 counts in the peak of the BS)

2) underexposed BS (<10,000 counts)

3) smeared images (heavy bleeding of photons in the Y-direction on the chip, reaching the bottom 20 pixel wide strips of the frame). A mean count level of more than 50 counts in this strip triggers a flag and the frame is rejected.

4) rough check if the moon is too close to the edges of the frame. This is not yet optimal, but throws out the worst cases.

5) checks that the temperature of the CCD is OK (an entry in the fits file header tells us if the temperature of the CCD has stabilised or not)

If all 100 slices in multiple exposure frames are OK, according to the above, they are bias subtracted (using the biases on either side of the exposure, averaged appropriately for each slice, and scaled to the superbias created by Henriette) and averaged, and written to disk.

6) The magnitude of the total light in the frame (i.e. apparent magnitude) is then compared to the expected V magnitude (using the JPL model, with corrections for the east and western sides of the moon, depending on which side is illuminated by BS), taking into account the airmass of the observation and the extinction for the particular filter. This isolates for removal exposures which have been taken with an incorrectly reported filter. Finally, after extinction correction, only those frames which fall within ~ 0.2 magnitudes of the correct magnitude using the JPL model are retained.

The list of the final set of good exposures using this method is attached (all100.doc -- actually an ascii text file):


There are 534 good frames, spread from nights JD2455938 to JD2456104.

A summary is as follows:

Night, number of good frames in V, B, VE1, VE2, IRCUT, comments

JD2455938 2 2 0 5 0 fullish moon, some foggy frames
JD2455940 3 1 0 2 0 fullish moon
JD2455943 3 3 0 3 1 fullish moon
JD2455944 3 1 2 1 3
JD2455945 1 1 1 2 0
JD2456000 3 0 0 1 1
JD2456002 7 0 0 0 0 V band only
JD2456003 9 0 0 0 0 V band only
JD2456004 1 0 0 1 0
JD2456005 6 0 0 0 0 V band only
JD2456006 4 0 0 0 0 V band only
JD2456014 3 0 0 0 0 V band only
JD2456015 7 10 4 6 4 1/3 moon, lots of frames
JD2456016 11 10 9 11 11 "
JD2456017 7 11 0 11 2 "
JD2456028 5 5 0 9 0
JD2456029 0 0 0 6 2 2/3 moon, lots of scattered light
JD2456030 1 0 0 3 2
JD2456032 1 0 0 4 1
JD2456033 4 1 4 2 4
JD2456035 0 0 0 1 0
JD2456045 5 5 4 4 4
JD2456046 4 8 10 11 10
JD2456047 6 5 8 6 7 stars visible in some frames, useful for PSF
JD2456061 2 4 3 6 4
JD2456062 0 0 0 2 0
JD2456063 0 0 0 5 0
JD2456064 0 0 0 3 0
JD2456073 4 7 7 10 8 some eclipsing of moon - dome issues?
JD2456074 4 2 3 7 5 scattered light haze or fog?
JD2456075 8 7 8 7 8
JD2456076 9 1 0 2 0
JD2456089 3 2 1 1 0
JD2456091 2 1 5 0 4
JD2456092 0 0 3 0 5
JD2456093 0 0 1 0 5
JD2456104 5 7 5 1 5 some foggy frames, moon a bit close to bottom corner

I've inspected all of these by eye and they look mostly pretty good! There are still a few bad frames here because the scattering of the halo is not selected for yet -- but the number of frames with low alpha (i.e. haze or fog) is just a handful.

Some of these nights will be pretty useful for looking at colour changes in the ES over time. I am doing that next.



  • Comments(2)

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Posted by Chris Flynn Dec 18, 2012 03:51AM

Agreed -- I stuck to only the very best stuff at this stage, as I was interested in colour maps and the Laplacian method -- this can certainly be made less rigorous: I noticed that there were quite a few frames with plenty of good exposures (>50) but not exactly 100 good ones. These we can fold in later. The fitting for the apprent magnitude of the moon used all good slices of all frames, even if there was only one slice -- so that was the other extreme!

Posted by Peter Dec 14, 2012 08:24AM

Only 500 good frames? With 100 images in each, that is. We could also align and coadd some of the 'single-shot' nights: But that requires better alignment techniques, I think.You cut at 50,000 counts. I cut at 53,000. The 'real' value is probably lower - muich lower. Somewhere on this blog we did an analysis of the linearity question and found scary things.Flatfields: I think I see the FF structure in some of your B-V images, so we should start looking at that question. I have some FFs that Henriette made - not all filters have believable FFs, though. Perhaps yet another step of scaling the FFs until FF structure is no longer apparent is needed. I will forward Henriette's flats to you.