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Uneven vignetting because of bad collimation?


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Hello all, here to ask about uneven field vignetting and whether it is caused by collimation or not.  Attached are pictures of a single raw flat, then the same flat applied with contrast adjustment, telescope's collimation, lagoon nebula stacked of 37x1 minute subs consisting of just light frames. I found that the coma in the field has been getting worse that they started showing so early, close to the center of the image. I was wondering whether this is normal or just because of my bad collimation, but collimation looks fine in the attached picture with everything centered. And the flat as well, it looks darker on the left side, I think this might be due to camera or adapters tilting a bit, or even collimation again? I thought about light leak in the scope, but it does not seem be the problem. But still, I did some light leak test because of the unending, annoying gradient in the picture, by shining my phone's flashlight on the primary mirror, and there's light leak! So in the next imaging session I will try to cover the mirror and also my DSLR's viewfinder as I am imaging near a streetlight.

I have also heard that without bias frames, flat frames wouldn't work well when calibrated to the lights. 

Thanks in advance and clear skies.

DSC_1108.jpg

Flat.jpg

IMG20200512165154.jpg

LagoonStacked.jpg

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I too experience this on my 130pds. Never thought that light could leak in through the camera's viewfinder. Thanks for sharing.

Also, how about the coma sneaking towards the center of the image ? Was that due to camera tilting ?

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Hi

Looks good. Why not stack with the flat frames too? Then you'd have even illumination. 

If you want to lose the coma, you'll have to use a coma corrector.

Is this the 150/750? If so, the collimation looks correctly offset. 

Make a master bias from say 50 or so frames. Subtract this from each of the flat and light frames before stacking. Most apps which do stacking will do this calibration for you. 

Use a black shower cap over the primary mirror end of the tube and tape over the camera viewfinder.

HTH

Edited by alacant
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1 hour ago, alacant said:

Hi

Looks good. Why not stack with the flat frames too? Then you'd have even illumination. 

If you want to lose the coma, you'll have to use a coma corrector.

Is this the 150/750? If so, the collimation looks correctly offset. 

Make a master bias from say 50 or so frames. Subtract this from each of the flat and light frames before stacking. Most apps which do stacking will do this calibration for you. 

Use a black shower cap over the primary mirror end of the tube and tape over the camera viewfinder.

HTH

Alright, took 50 bias frames just now and got this. I just won't learn from my mistakes lol, I took the flats with a slightly different camera orientation as I removed it before the next session... This is probably why the image has a linear gradient across I think.  The overall gradient looks slightly better compared to the ones uncalibrated and with flats only, in my opinion.

Yep, this is a Skywatcher 150p.

I just loaded all the bias and flat frames into DSS and let it do its job. I think that's ok too?

Great idea! I will get a shower cap then. Thanks a lot.

biasflatm6.jpg

biasflatm6.TIF

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6 hours ago, ZiHao said:

DSS and let it do its job

I think most processing software will know what to do. 

6 hours ago, ZiHao said:

different camera orientation

Yeah, it's quite critical. Best to take the flat frames either immediately before or after the imaging session. That way, you don't forget. Certainly, before you've touched the camera.

The lagoon must be nice and high from your location. Some superb targets in that region of the sky. Nice:)

 

 

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