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

How bad is the Drag?

Data reduction issuesPosted by Peter Thejll Sep 22, 2013 11:04AM
During data-reduction we found the many 'dragged images'. These are images where the shutter probably did not close before readout started. They look like this one (histogram equalized):
The 'drag' is from the BS but is probably present everywhere, but not visible from the DS. The BS has a maximum near the right hand limb where the counts are near 10000. The 'drag' is near 240 at most. There is no gradient in the drag along the direction of the drag:

The absence of a gradient in the drag helps us understand its origins - it is the effect of constant-speed readout. The readout speed is supposedly 1e-6 s per pixel (in the image header: this may be read from the camera but could be a fixed number entered at setup ....). Reading 512x512 pixels should take 0.26 seconds. The exposure time for this image was set to 0.0274s. Being a 'dragged image' we have no idea if this is right.

I do not quite understand the ratio of maximum drag value to maximum BS value. If the drag/BS ratio is 240/10000 and the BS exposure time is 0.0274 s the drag must have lasted just 240/10000*0.0274 = 0.00066 s. The camera is a frame transfer system - is frame transfer that fast? Perhaps the 0.26 s readout time is a slower line-by-line operation? This page mentions 'a few msec' for the frame transfer: . We have 0.66 msec, so - close.

If the shutter got into the act in mid readout the amplitude of the drag is affected by this. I think many dragged images we have seen are like the above one - dragging is seldom much worse than this.

Let us check other dragged images with higher exposures and see if the drag/BS ratio is constant or not.

The above was prompted by pondering whether we could have a 'shutter-less' camera. It would appear not - at least not if the drag is due to open shutter during readout. The readout would have to be 24 times faster than it is and I am not sure CCD cameras come with such fast readout. 24 MHz readout?

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Posted by Daddy-o Sep 23, 2013 09:34AM

I had forgotten about CMOSs' ability to read out 'at once'! This does seem like we might be able to avoid a major mechanical part, then! No FWs - no shutter!
Hang timing accuracy issues, as we are looking at 'coadd mode' imaging here!

Now, how do we intelligently also get rid of the mount and its gears and grinds?!

But: we need to understand bias-frame and flat-field issues with CMOS technology - perhaps there are papers out there? I see 'DSLR' in quite a few preprinmt titles these days.

Posted by Chris Flynn Sep 23, 2013 02:33AM

Really nice work!

For the newer CMOS CCDs, each pixel is mapped into memory, so they are all "read out" at the same rate -- rather than being dragged across the CCD to get to the edges and out into memory that way. This changes the game completely! For the recent ANDOR astronomy grade CMOS CCD, read rate is up to 100 Hz, i.e. 10 ms frames can be captured sequentially. I don't know what the timing accuracy is for the 10ms though, but it could be very high. I'll look into this!