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Old 15-08-2012, 07:52 PM
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pvelez (Pete)
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Join Date: Aug 2006
Location: Sydney
Posts: 1,250
How flat is your lightbox?

I've posted recently about my travails with flats. I had recurrent blue background around the edges of my images. I had multiple diagnoses - dodgy filters, light pollution, poor calibration routines etc etc.

In the end, I concluded the issue was low SNR for my blue filter resulted in excessive stretching in the R and G which meant I was actually highlighting the noise in the background.

Still need to test this - I'm waiting for the Sydney weather to play nicely.

Anyway, just for fun, I thought I'd test my filters by taking some images with my spectroscope. I have a LISA (a great bit of kit) so I decided to take some images of the sun as it came through the skylight in my study using R, G and B filters. This was just a test really.

I then took images through the same filters using my lightbox as my source.

The transmission curves should be broadly similar as for the sunlight - ignoring absorption lines in the sun's atmosphere and the earth's atmosphere. I expected 3 inverted Us crossing over midway up the curve so that the combined signal of overlapping signals at any point in the curve summed to the peak intensity at the top of the 3 Us.

What I found was that in B, my lightbox has a relatively sharp cutoff in the range 4600 - 5050 Angstroms.

The first image illustrates this. I've superimposed the lightbox curve on the curve for the diffuse sunlight.

The second image shows the same pair as well as the curve for the lightbox for G and R - R is to the extreme right.

The last shows the curve for images of the lightbox taken through B, G and R filters (left to right).

I haven't corrected these with flats or darks nor taken account of the response of the CCD so I know the data isn't necessarily accurate in absolute terms. But it does show the difference between the lightbox and solar illumination.

I don't think this really impacts on the performance of the lightbox - though I'm happy to stand corrected. I guess its a function of the type of diodes used in the lightbox.

Pete
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