Image quality at f/2 - Forums

Topic starter

Nov 24, 2021

Inspired by @Mathew Ludgate 's work, I recently purchased a second-hand Nikon 200mm f/2 lens, which has the benefit of working pretty well wide open - at least based on the images I have seen here. However, I am having some problems with image quality at the lens at f/2.

While focus, eccentricity and tilt (measured variously by PI's FWHMEccentricity script and CCD inspector) indicate everything's OK (1.85 - 2.05 FWHM/ e=0.4-0.5 across field and tilt less than a few degrees) but on detailed inspection the images show faint coma like tails across the field, and always in the same direction. It would appear that the quality programs are measuring the head"of the coma-comet, but not the tail. Now I do have to stretch the images a fair bit - and indulge in a bit of pixel peeping, as they only show up clearly at 2:1 or 3:1 image scale.

Out of focus images show radially symmetric doughnuts, so I am at a bit of a loss to understand what is going on. I attach an image taken last night of the Witch Head nebula comprising of 16 x 300 sec exposures with the Nikon 200mm at f2 and I would be grateful for any thoughts/advice.

1) Am I worrying too much about image quality, based on this image? [I can see the coma tails in this image, but then again I know what I am looking for]
2) Alternatively, can I do better at f/2?
3) Should I look to stop down a little? This would be a pity as the speed is pretty impressive. Also somewhat problematic as the lens diaphragm is currently wedged open by part of a plastic milk carton, the ZWO connector doesn't have a facility to manually adjust the manual aperture ring on the Nikon
4) Is my second-hand lens not performing as it should?

Finally, I would note that the problem is even worse when I use my ASI6200MM camera (with filter wheel), In this case, I can convince myself it is tilt, since there is some flex (well.. fraction of a millimetre) at the lens/camera connector caused by the weight of the camera/filter system. I have tried supporting it, but with no luck. I though I would first track down the problem with the OSC and then see if the solution could also fix the problem with the mono camera.

There are some great photographers out there working at f/2, and I am hoping that someone may have come across something like this before.

Many thanks in advance for any thoughts/advice.

Regards

Brian

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Michael Feigenbaum

Nov 24, 2021

Looks like a really excellent lens Brian and an impressive image to be sure. I do see the star "coma" or whatever that might be and I would suspect a tilt of "a few degrees" is likely the issue.

I am by no means any kind of expert but when I use a long lens, I like to keep it wide open to take advantage of the speed, as you mentioned, and I do not like the diffraction spikes caused by stopping the lens down. Conversely, in my reading and listening to others well versed, stopping down one stop is a very good idea generally.

Of course with the caveat that every lens model is different, I have a Samyang 135 that I use wide open and I generally like the results.

Hope that helps a little!

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Brian Boyle

Topic starter

Nov 24, 2021

Thanks for the encouragement, as ever, Mike. Yes to stop down or not to stop down. That is the question. Whether to put up with the coma and distortion of a wide open lens, or take steps against them by reducing the aperture.

After you mentioned tilt, I went back and checked a few more frames. The 3D plots in CCDinspector looked spherically symettric with 1px and 0px shift in X and Y across the sensor. But then I realised that I had been misinterpreting the colour scheme. The image is actually telling me that my worst focus is in the middle of the field - getting better toward the edges. Perhaps it is just out-of-focus and I am not familiar with the sensitivity of focus at f2. Perhaps a Bahtinov mask simply doesn't cut it.

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Michael Feigenbaum

Nov 24, 2021

Well, if you stop down once, you’re still at f/2.8 which is still pretty darned fast, right ?

Brian Boyle

Topic starter

Nov 24, 2021 · Edited: Nov 24, 2021

With my milk carton wedge on the aperture spring, I have infinite flexibility on my aperture. F/2.4, F/2.5 even F/e - a transcendental aperture stop. And yes, all darned fast.

andrea tasselli

Frequent contributor

Nov 24, 2021

For the image scale I can see it's pretty normal for a Nikkor AFS 200 f/2. I mean, I could ride a wagon across sideways and still leave plenty of room given the size of the stars. IOW if you want comments of lens performance you should present the image in a way we can actually see what is going on.

Tom Gray

Nov 24, 2021 · Edited: Nov 24, 2021

Brian, this is an impressive lens/camera combo. Stopping down a little does help reduce coma and 'softness' in the image, and you could do this with an aperture mask rather than the iris to avoid unsightly diffraction spikes. I have tried imaging with my old DSI Pro III 1.4 MPx (6.45 micron pixels) using a 200 mm f2.8 SLR lens this gives ~9 "/px which is OK (even at f4 in a 28mm SLR lens this gives an imaging scale of 45"/px and just about looks OK - I'm not a perfectionist!)

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Brian Boyle

Topic starter

Nov 24, 2021

Tom, what a good idea to use an aperture mask. I must experiment and report back.
CS Brian

Andy Wray

Nov 25, 2021

Impressive image! To be honest, I would not have noticed anything unless you'd mentioned it. Even pixel-peeping, I struggled to find what you were talking about until I looked at the bottom right-hand corner at 200% scale (not the other 3 corners). So, for me, that might be a very slight tilt issue, however I am no expert.

I am struggling with something similar at the moment with a very different setup (8" F4.5 newtonian with coma corrector); right now I'm putting mine down to a tilt issue at the focus tube end, but need to do more testing to be sure.

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Stargazers SA - Andrew

Nov 25, 2021

We work with an old 200 F1.8 Canon and wide open we have noticed a very slight "flare" on med to brighter stars (same as yours) but not too objectionable. Your image is pretty good in my opinion but looks like a slight tilt issue in the bot RH corner (could be connectors or even internal alignment at play) Definitely worth trying at F2.8 to see if it sharpens things up a touch but the tilt will be hard to get rid of as the shimming will be in the order of .01mm at F2. You also could be very slightly out of focus, always difficult to get perfect on a lens system. I did a comparison between our shots and yours and the smallest stars look pretty well the same so you cant be far out. Have included a Rho Oph image for comparison done with a Z6 Nikon mated to the Canon. (check my page for higher res version. No lens is perfect and you can see faults in every one if you look hard enough. Astro images are incredibly demanding on optics, especially fast one.

Cheers Andrew

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Tom Gray

Nov 25, 2021

I am struggling with something similar at the moment with a very different setup (8" F4.5 newtonian with coma corrector); right now I'm putting mine down to a tilt issue at the focus tube end, but need to do more testing to be sure.

Andy, try Han Klein's free CCD Inspector in his excellent ASTAP astrometric solving software. Hope it helps.

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Andy Wray

Nov 25, 2021

Tom Gray:
Andy, try Han Klein's free CCD Inspector in his excellent ASTAP astrometric solving software. Hope it helps.

Thanks Tom! I tried it and it does show that I have a 6 to 7% tilt which it calls "almost none". That does give me something to work with though.

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Brian Boyle

Topic starter

Nov 27, 2021

Andrew
Thanks for the reassurance than I am not too far away. Your Rho Oph wide field is amazing….
Brian

Brian Boyle

Topic starter

Nov 27, 2021

andrea tasselli:
For the image scale I can see it's pretty normal for a Nikkor AFS 200 f/2. I mean, I could ride a wagon across sideways and still leave plenty of room given the size of the stars. IOW if you want comments of lens performance you should present the image in a way we can actually see what is going on.

Hi Andrea, Thanks for your help. It was less detailed comments on the lens performance (in which case I would have posted detailed images) and more the general expectations of a system of this kind. I have certainly received the latter, and so I can go back out imaging knowing that, while I could probably do a little better, I am at least in the ballpark.

Ince again, the good folks of Astrobin come to the rescue with useful, generous and kind advice.

Brian

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watcher

Nov 27, 2021

As a photographer, "earthly", the rule was always stop down one stop from full open. But then I am old and maybe the rule is only good for old lenses but then again my 200mm f/2 is an old lens and I stop down one.

Chris Bailey

Dec 3, 2021

Cheap plastic step down rings from Amazon. Taking my Samyang 135 f2 down to f2.4 ish made a big difference to star shape in the corners. I also supported the front of the lens with a large guide ring to avoid ‘droop’ on the rather flimsy lens adapter.