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Earthshine blog

"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Two-way smear from overexposure

Data reduction issuesPosted by Peter Thejll Apr 27, 2015 10:41AM
Here is an example of a two-way smear. The red area is the overexposed region, and it has bloomed along vertical columns in the image. To the right of this bloom is a smear extending hundreds of pixels. What is that?

Read-out direction is top-down.

What is causing this smear to the right? Is it related to the 'depression of the sky in a broad band left-right' that we see in some images?

The intensity of the smear is at the 500 count level - i.e. it acts just like part of the halo from the BS. This has, surely some consequences for data reductions? Do images that have not bloomed have this halo? Is this why the bright edge of the BS-halo is always hard to model?

Can we see if the fitting of the BS halo is easier in images that have been modestly exposed, compared to those exposed right up to 50.000 or 55.000 counts?

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Posted by Peter Thejll Apr 27, 2015 10:50AM

Actually, come to think of it - Chris and I saw this sort of 'smear to the right' in strong point sources - i.e. the 'spur' that we spoke of on this blog!

The PSF is therefore not rotationally symmetric - but is so at a low level few hundred counts out of 50000.

The CCD is read out with shifts down and to the right ... shift whole image one row, then read out last line one column at the time. So readout is to the left? All readout in the direction of decreasing axis - down and left?