Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Issue with asymmetry

  • Please log in to reply
13 replies to this topic

#1 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 14 October 2020 - 04:59 AM

HI all! I hope you are well. 
I am trying to get to the bottom of an issue I am having with some new equipment I have (Modded Canon 6D, Askar ACL 200 lens). 
I've been doing some tests and from what I can tell, there appears to be some misalignment - perhaps with the adaptor (m48), the lens itself or the newly modded 6d itself. Difficult to isolate so to try and mitigate/reduce any tracking errors I used only short exposures (8 seconds). I shoot the lens wide open at f/4. I am using an iOptron SkyGuider Pro, no guiding. 

Anyway, the problem itself - it appears to me that I cannot get the top right corner and bottom left corner in focus at the same focal plane. If I favour the top right for sharpness vs out of focus/coma effects - the lower left suffers - and vice versa. I can try to 'balance' the focus by sacrificing each corner a bit for better stars across the frame but minor deviations in focus throw the whole frame out of symmetry. 

 

I should add, that I am VERY new to this hobby so I am not 100% sure what my expectations should be of the equipment I have. I am a very demanding landscape photographer, shoot zeiss lenses and focus stack - so pixel peeping is a sin that I am guilty of - I confess! When I buy new gear for my landscape rig, I usually perform quite rigorous lens tests (particularly for skew/tilt/asymmetry). What I usually do is focus on a planar subject (a distant cliff face for eg) where everything should be on the same focal plane. A star field is good for this too. This is a very demanding subject for any lens so it reveals flaws in the lens (as well as the user - focus issues, for eg). After I think I have critical focus, I shoot, then I refocus the focus ring as small as possible clockwise, reshoot. I do this many times, I then do the same anticlockwise (twice as far, usually). The results can be very revealing of lens behaviour - skew/field curvature etc. Lens MTF charts are great eye candy but real lenses are built, and there are tolerances (Sigma, rokinon in my experience are not great at building lenses to the stated specifications. But I've also had Zeiss lenses with skew so no brand is immune). IMHO, real world tests are more relevant than design specifications. Plus, tiny adjustments in focus can yield very different results with the same lens - this is why I don't think it's realistic to compare lenses. You have to compare a lens to itself first! Anyway.... Here are some images I took (performing the same test of rotating the focus ring as little as possible per frame).

 

https://www.dropbox....G_5827.JPG?dl=0

https://www.dropbox....G_5832.JPG?dl=0

https://www.dropbox....G_5836.JPG?dl=0

 

They don't look too bad but the asymmetry is evident. 
Now, look at this one:

https://www.dropbox....G_5838.JPG?dl=0

The centre and lower left are clearly out of focus, but other parts of the frame are better (top right is ok, not perfect, I confess). 

IMG_5827 shows the opposite, the lower left is better, at the detriment of the top right, and top left - to a lesser degree. 

 

This one might perhaps be the best in the sequence, there are many in this sequence - it was quite a though test. I can show more images if needed.

https://www.dropbox....G_5831.JPG?dl=0

 

The key to examining these is to view at 100% and toggle between them, locking the view position. Lightroom is good for this. I am aware that stacking might help some of these issues but I believe it's best to get the best baseline data before working on it. 

 

Please excuse the poor white balance, I live in bortle 8 and due to the covid situation there are fines for leaving my geographic area (which I am not happy about, I am an avid hiker and I have lovely bortle 2/3 skies only 1 hour from me!).

Thanks, and let me know if my expectations are unrealistic, or if there is something silly I am doing (I am new to this) or if there is a clear problem here.

P.S. I should add that I already tried the 180° camera rotation method (thanks Mark) but when I tightened the M48 adaptor, the focus was thrown out totally (I did not touch the lens or focus ring). Curious! 


Edited by LooseFur1, 14 October 2020 - 05:04 AM.


#2 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 08:35 AM

I know it's a tricky question but nobody has ideas on this ?



#3 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,226
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 15 October 2020 - 08:47 AM

It certainly does look like a little bit of tilt in there. Did you mod the camera yourself or did someone else do it? I had an issue with a camera where the sensor was reinstalled tilted, which caused similar issues with an f/2 lens. Do you have a stock camera you could use to test the lens to see if you get similar aberrations? Likewise, do you have a different lens you could test the stock camera to see if you see the same things with a different lens? This might help identify if there's a problem with the lens or camera.

 

Full frame is tough, though.



#4 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 08:58 AM

It certainly does look like a little bit of tilt in there. Did you mod the camera yourself or did someone else do it? I had an issue with a camera where the sensor was reinstalled tilted, which caused similar issues with an f/2 lens. Do you have a stock camera you could use to test the lens to see if you get similar aberrations? Likewise, do you have a different lens you could test the stock camera to see if you see the same things with a different lens? This might help identify if there's a problem with the lens or camera.

 

Full frame is tough, though.

Hi, and thanks for the reply - I do have a canon 500D (APS-C) which doesn't show the issues with the same lens. But, of course - it's not full frame. I did not do the mod myself. 
I have a canon 100mm f/2.8 USM (the non L version) but even on the 550D that needs about f/5.6 - f/8 for reasonable stars edge to edge (it seems to need about the same on the 6D, actually).


Edited by LooseFur1, 15 October 2020 - 08:58 AM.


#5 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,226
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 15 October 2020 - 09:18 AM

You could also try to take some shots through a bahtinov mask, which might show if the corner is inside or outside of focus. For the cost of that lens, I'd think you would want better stars than that. Unless the sensor itself is tilted.



#6 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 09:20 AM

You could also try to take some shots through a bahtinov mask, which might show if the corner is inside or outside of focus. For the cost of that lens, I'd think you would want better stars than that. Unless the sensor itself is tilted.

Thanks, yes I am waiting on delivery for a bahtinov mask. Maybe a week or so. Just to be clear, it's the 100mm canon that needs about f/8 - the askar lens I've been trying pretty much only at f/4 (wide open)


Edited by LooseFur1, 15 October 2020 - 09:22 AM.


#7 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,226
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 15 October 2020 - 09:39 AM

In that case, I think I'd withhold judgment until you get the b-mask and can test it again getting focus as close as possible. What I thought were elongated stars in the corners look like tracking errors, in that you can see them in the middle of the frame as well. Sure, the lower left corner might be a little wonky, but you really need to zoom in to see it. The one where the lower left is clearly out of focus, the center is also well out of focus. So there might be a little tilt between the bottom left and top right corners, and being slightly out of focus may make it worse.

 

I'm surprised that the stars are slightly oval in an 8-second exposure. This is probably making the top right look worse, in that if there's a bit of aberration in the corners, star trailing will smear it out in the "wide" direction. If you can get a little bit better polar alignment, that should help. Not sure why else you would be getting elongated stars - 8 seconds at 200mm should be well within what you can do unguided.

 

Nobody else can tell you whether you should be satisfied with the performance with a particular lens. If you can sharpen your focus just a bit with the b-mask and take care of the tracking issue, I think I'd probably be happy with the performance of that lens and camera. 



#8 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 09:46 AM

Yeah thanks, I was wondering about tracking issues myself actually. My PA was pretty good, I thought! But maybe not. Like I said, I'm very new to this hobby! The thing that was confusing me was that the elongation seemed more obvious in the lower left and central area than the top right area. I'm probably pushing the ioptron skyguider pro a bit

Edited by LooseFur1, 15 October 2020 - 09:48 AM.


#9 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,226
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 15 October 2020 - 10:06 AM

I don't think you're pushing it too much. I could do up to 60-second unguided subs at 360mm with my refractor with decent stars, so 8 seconds at 200mm should be well within its capability. Something could be slipping or flexing, though. Also, the polar scope can be slightly misaligned from the factory.

 

Hopefully someone else will chime in with some suggestions as well. 


  • LooseFur1 likes this

#10 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 10:09 AM

Thanks, yeah I was able to get many minutes using my nikon and 100mm zeiss

#11 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 10:45 AM

It could be down to balance, I have to use 2 counterweights on the SkyGuider because the Askar itself is 1.8kg. I try to do it a bit east heavy (camera side) as someone else suggested on another thread. I am not sure how it can be balanced in Dec though. I do not use a ballhead, just the vixen plate that came with the mount. 
I'd rather hear that it's my fault - I can improve me, I can't improve my equipment (I am not rich lol).


Edited by LooseFur1, 15 October 2020 - 10:46 AM.


#12 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,226
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 15 October 2020 - 10:58 AM

It's quite possible that balance issues can cause trailing, especially with a heavy lens. Balancing in the second axis isn't too bad, but it might take another piece or two to make it happen. Here's a picture of my setup (minus the neck strap), which includes a panoramic head on the DEC bracket and longer arca-swiss plate on the bottom of the scope. This lets me adjust the scope forwards/backwards on the pano head until it is balanced. I'll take off the counterweight, point the scope at 0-degrees declination so it's sideways on the SGP, and then move it in the clamp until it is balanced in this direction. Replace the counterweight and then balance it with the scope pointed towards zenith. This way it's more or less always in balance when you rotate it in declination.

Annotation 2020-09-22 140413.jpg


  • LooseFur1 likes this

#13 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 15 October 2020 - 11:30 AM

Cool thanks.

I just checked the polar scope actually, using a distant chimney in the centre and as I rotated the RA axis, it moves only very slightly out of the centre. Maybe I need to adjust it. It's only a small movement from centre but it might be misaligned enough. I dislike opening stuff up though (as per http://www.ioptron.u...ScopeAlign.pdf)



#14 LooseFur1

LooseFur1

    Vostok 1

  • -----
  • topic starter
  • Posts: 138
  • Joined: 08 Sep 2020

Posted 18 October 2020 - 04:54 PM

It's quite possible that balance issues can cause trailing, especially with a heavy lens. Balancing in the second axis isn't too bad, but it might take another piece or two to make it happen. Here's a picture of my setup (minus the neck strap), which includes a panoramic head on the DEC bracket and longer arca-swiss plate on the bottom of the scope. This lets me adjust the scope forwards/backwards on the pano head until it is balanced. I'll take off the counterweight, point the scope at 0-degrees declination so it's sideways on the SGP, and then move it in the clamp until it is balanced in this direction. Replace the counterweight and then balance it with the scope pointed towards zenith. This way it's more or less always in balance when you rotate it in declination.

attachicon.gifAnnotation 2020-09-22 140413.jpg

I am curious as to how you managed to get away with the one counterweight there.... How much does your scope weigh? For my 1.8kg Askar I need two c/w's. They are near the top of the shaft, mind. But I couldn't achieve balance with the one c/w - even at the limit of the shaft.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics