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Is Rotation all that bad?


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I'm thinking .. imagine a Alt-Az mount with one or more scopes with cameras.

On a 20 minute image, the sky will rotate and leave arcs on the camera. However if the image is de-rotated in software using a 100:1 sub grid - what problems could you get?

The arcs would vary in size however given you know the time and the shift, it should be possible to revert the image by transform stacking. The only issue is over saturated stars arching and obliterating the signal over the entire arc - however if you've got to that point then there's a bigger issue of exposure control The reverse stacking could also be used to reduce noise further.

The main issue I can see is that as the atmosphere wobbles the arc will change - a bit like the thickness of a pen changing and the hardness of the nib being pushed down.

You could even take a sub shot between the long exposures although this would warm up the camera it would allow registration if the algorithm isn't very advanced.

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Unless I'm missing the point, this is a common problem faced by us Alt-Az imagers. DSS and others will align multiple frames both in translation and rotation. It does of course limit the sub duration commensurate with altitude and direction, but generally 20s to a minute is possible.

If you haven't spotted it, have a look at the "No EQ Challenge" thread.


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  • 4 months later...

My thinking here is - if the image rate of rotation is known, it can be transformed by sampling.

So you take an image Isub, that is captured during a period of Tsub when a rotation of Rarcsec rate occurs.

The output image Isub has trails, however by applying sampling using grid that rotates and takes a weight based on Tsub and Rarcsec then the output is stacked into a final frame.. after derotation, the result would be a normal image.

Note this is simplified in that it's a straight rotation rather than a sweep..

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I'm sure you could improve the look of a rotated image, but instinct tells me that blurring an image in any way loses information that cannot be recovered. Indeed, a quick bit of research seems to confirm this but I'd have to do a lot more to be 100% certain!

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De-rotation software exists in windupos..but it only works because certain planets,ie jupiter/Saturn  have been mapped so it knows where certain points of reference are...on hugely multiple reference points it might be just too much..

There's only so much software can do..maybe in the future but for now we need to put some work/effort in...and that's the part we all enjoy..in minus whatever temps

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