Field derotation

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GSBass
Posts: 59
Joined: Tue Aug 11, 2020 2:45 pm

Field derotation

#1

Post by GSBass »

Still loving Sharpcap 4.0, you have done such a great job, I’ve been pretty successful imaging DSO but every now and then the recommended exposures will elongate my stars a little depending on where the object is located, the derotation feature works fine, it’s just the exposure on the Az mount before the derotation kicks in that is the culprit…… so here is my feature suggestion, since it is a known how long an exposure can be at a certain area of the sky, maybe we could tell sharpcap we are using an Az mount what object we are imaging and have that info incorporated in to the brain function, that way it will know maximum exposure we can tolerate and recommend a new gain to compensate.. it would help keep me from guessing and doing pixel peeping trying to keep my stars nice and round
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Re: Field derotation

#2

Post by admin »

Hi,

I guess in theory you must be able to work out the field rotation rate from the fact that you have an Alt-Az mount and the co-ordinates. Never actually looked into the math for it, but I expect it is out there somewhere :) This would be an interesting thing to include in the brain (I have plans to include other things like limits due to the brightness of stars - too short may not pick up enough stars for stacking).

In the shorter term though, I had a slightly different plan for this...

In the live stacking, SharpCap knows how much the frame rotates and how far it shifts between exposures - that gets worked out as part of the stacking calculations. All that needs to happen is to further work out if the shift/rotation adds up to more than some limit (1 pixel? 2 pixels) at any point in the image and give a warning : 'rotation rate is leading to elongated stars in the corners of the field - consider using shorter exposures'. The advantage of this approach is that it works in all sorts of situations - no connected mount... no problem, field rotation due to poor polar alignment... no problem, etc.

cheers,

Robin
GSBass
Posts: 59
Joined: Tue Aug 11, 2020 2:45 pm

Re: Field derotation

#3

Post by GSBass »

Thanks for the reply, since all my cameras utilize fairly small chips I have not seen any edge of field issues, stars look pretty good across the field as long as the exposure is not to long in the subs…. On the bright side I can zoom in after the first few frames come in and see if I have issues, only way to correct is to start over and select a shorter exposure and raise the gain, not ideal mostly because it’s a guess instead of the more scientific method the brain provides. But I think quite a few of us are using Az mounts with sharpcap so keep us in mind. The only other issue us Az users have with field rotation is because the cams are a 16x9 format, you can actually lose almost 50% of your image in a long exposure, it simply rotates out of frame and so you have to be very careful to have your object perfectly centered. Not sure if anything could be done about that. We just crop out the field rotation in post processing for the other areas that didn’t get pushed out of frame.
GSBass
Posts: 59
Joined: Tue Aug 11, 2020 2:45 pm

Re: Field derotation

#4

Post by GSBass »

This is probably my best effort so far, m8, the resize kinda messes with the stars but they look nice in the original
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GSBass
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Re: Field derotation

#5

Post by GSBass »

Btw… I feel like your work has revolutionized the industry, there is zero doubt sharpcap has inspired the robotic companies to develop their systems, I’ll be trying out a Vaonis Vespera in December , these companies exist because you showed them what was possible with short exposure astrophotography. I just thought you should know how much your appreciated. I’ve thought several times over the last year that one of the big companies should contract with you to use the sharpcap name as they inevitably will develop their own imagers
GSBass
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Joined: Tue Aug 11, 2020 2:45 pm

Re: Field derotation

#6

Post by GSBass »

Btw, I thought my comment about losing large portions of the photo due to the 16x9 format rotating out of the frame might have been confusing so I am just posting a photo from a couple months back to demonstrate what I meant, as you can see, by the time you straighten and crop the good parts of the image you sacrifice a lot. It’s fine, just part of the challenge of imaging with az
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Re: Field derotation

#7

Post by admin »

Hi,

I can see that the rotation (plus a certain amount of drift by the look of things) has had a big effect there.

Actually the rotation amount is quite large in this case - that either means that you spent a long time taking the image, or that you were imaging near the zenith, where the field rotation rate for an AltAz mount is at its highest. If you image due east or due west then the rotation rate will be very small.

cheers,

Robin
GSBass
Posts: 59
Joined: Tue Aug 11, 2020 2:45 pm

Re: Field derotation

#8

Post by GSBass »

Yes it was a double whammy, near zenith and long exposure, extreme example but just wanted to clarify what I was referring to, but I’m happy, I’ve taken some pretty shots during the learning process
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