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Old 09-05-2017, 01:01 PM
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Samyang 500mm F6.3 coma, question

The attached image shows coma produced by this lens in the corner of the frame... apart from this, the centre of the frame is sharp enough.
Is there a (low cost) way to reduce the problem?
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Last edited by bojan; 09-05-2017 at 01:44 PM.
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Old 09-05-2017, 02:18 PM
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Easiest may be to buy StarTools and use one of the star shape correction algorithms to tidy up the image in the corners. Your optics are telling you where the stars are, how bright they are and what colour - you only need a bit of cosmetic manipulation to make them look pleasingly round and StarTools can probably do that for most of them.
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Old 09-05-2017, 02:31 PM
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That is some serious coma there. You could fix it in Photoshop too, but you need to understand masking. If you can just run a routine in Startools that sounds like the best option.
Or you could just crop it out, no cost, minimal time.
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Old 10-05-2017, 09:14 AM
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can you try stopping down the aperture? if you're taking long tracked exposures, just crop. But I take hundreds of short untracked exposures and stopping down eg f2.8 to f4 or f5.6 gives more consistent star shape. Plus I stack and with field rotation the worst get cropped out anyway.
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Old 10-05-2017, 09:31 AM
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I was doing cropping, however I am reluctant to stop down, because of relatively large central obstruction I will loose too much light for very moderate (if any) improvement...
This lens is very light and handy to carry around, that is why I would like to somehow improve coma without compromising exposure time and/or FOV.

Last edited by bojan; 12-05-2017 at 11:54 AM.
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Old 10-05-2017, 04:34 PM
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Atmos (Colin)
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If you wanted you could get creative and put a lens element over the front and do what the Meade ACF series have done
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Old 12-05-2017, 10:41 AM
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Quote:
Originally Posted by bojan View Post
I was doing cropping, however I am reluctant to stop down, because of relatively large central obstruction I will loose too much light for very moderate (if any) improvement...
This lens is very light and handy to carry around, that is why I would like to somehow improve coma without exposure time and/or FOV.
Still I would try some test shots and just see, you might get an improvement for 1/3EV loss only,. I know its not nothing but any hardware fix will cause some loss. Another possibility that will cost you time, maybe a little money if you dont have it already. Is software to correct lens distortion/ chromatic aberations, maybe something you can profile your lens/camera combination yourself then turn all your RAWs into 16bit TIFFs then use those for your AP workflow.

Otherwise do what I do, rely on reframing and registration/integration to even out all the stars around my target. maybe if you can shoot tracking half subs with camera upside down or tilted 90 degrees? I find corner distortions often asymmetrical so turning the camera they all get more rounded after stacking.

Something I'd forgotten is some camera bodies allow front/back focus adjustments per lens. Maybe a tiny adjustment each way and take test shots. Hell I'd be making sure everything fits tight and firm in case tiny flex in the system is causing excess distortion. Anything I can think og and just take test shots, figure out the best of the bad. The coma is there in the optics but just in case another factor is causing a bit more than it should you might get a tiny improvement at no cost. Always worth testing rather than just assuming it wont make any difference.
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Old 12-05-2017, 11:57 AM
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Quote:
Originally Posted by sil View Post
... front/back focus adjustments per lens.... Maybe a tiny adjustment each way and take test shots....
That sounds like a good idea.
The coma looks very symmetrical BTW.

Last edited by bojan; 26-05-2017 at 11:47 AM.
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Old 17-05-2017, 01:20 PM
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I'll probably be in your boat when the Tiny1 camera ships, I've got an old soviet 500mm mirror lens I'd like to mount it to for fun
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Old 17-05-2017, 01:26 PM
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Quote:
Originally Posted by sil View Post
I'll probably be in your boat when the Tiny1 camera ships, I've got an old soviet 500mm mirror lens I'd like to mount it to for fun
No, you very probably won't ...

I have Rubinar 500, Rubinar 10_1000 and MTO1100-A.. All of them are very good in terms of coma and CA, however they are sometimes somewhat problematic in terms of astigmatism (main mirror is sometimes tightened too much and is a bit stressed/deformed, it needs to be relaxed).
Relaxation helped a great deal with 1000mm lenses, but not with Rubinar 500... so I will probably use it one day for guiding only.
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Old 17-05-2017, 01:37 PM
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Mine looks like that mto1100, heavy and solid to be used as a tank round.
Its 500mm f/8 and a 2.0x teleconverter. Has that "haziness" older non-coated lens have but got my Sigma before I got into AP so never really tried to for that.
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Old 17-05-2017, 01:47 PM
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It would be interesting to know how it performs...
500mm version (MTO 3M-5CA 8/500) is very valued, it is better (lines/mm) than most other Maksutov Cassegrains, see here.
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Old 18-05-2017, 01:34 PM
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I'll have to try to rig something. To mount on my nikons it needs a mount adapter with a lens element to achieve focus. Its this lens element that is always poor quality and adds a lot of haze and CA to shots I used to take. I've never found a good quality adapter to try, the expensive ones I've seen are just overpriced cheapies which I have. I'm getting all the accessories with the Tiny1 so I will be able to mount imediately with my adapter . Wonder if I'd be able to replace the M42 mount back off the lens and turn it back into a Mak again? future project #2545?
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Old 18-05-2017, 01:40 PM
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To get focus, you only need cheap M42-Nikon adapter from ebay and you need to remove the focus stop that prevents focus ring to go "beyond" infinity.. no additional lens is needed.
Have a look here, near the bottom of the page:
http://www.strickling.net/russentonne.htm
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Old 23-05-2017, 08:36 AM
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tackling the screws is a long term project in itself for me with one arm and no bench vice. I tried years ago and couldn't get any to budge, dont want to drill them or risk damaging the optics. Would love to find a good quality adapter with a great optical element. Canon owners are lucky they dont have this problem.
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Old 23-05-2017, 08:44 AM
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Is this adapter you need for your nikon?
Another one...

I can try to do the mod on your lens for you.... if you send it over here.

Last edited by bojan; 23-05-2017 at 09:11 AM.
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Old 23-05-2017, 09:24 AM
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no thanks mate. I'll have to devote some time and space to set up and go over my lens and that article and give it a shot, I can smash the glass element from one of my cheap adapters. not a priority for me but will keeps in mind, thanks though.
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Old 25-05-2017, 06:05 PM
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dug out the lens, its MC 3M-5CA located those three screws, hard to see even with magnifying glass. apprehensive about trying to open, the lens moves smooth and tight, worried it'll be ruined when reassembled. I'll do a bit more research first see if i can find a good dismantle guide/video for this lens and make sure there's no tricky pins/springs or anything. got caught out dismantling a soviet leica fake camera using more general instructions. I keep forgetting it always looks bigger in photos than in person.
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Old 26-05-2017, 11:07 AM
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just for reference I found this elsewhere, so basically remove the three screws and unscrew front half from rear CAREFULLY and mark both halves at the point they disengage so when assembling the triple thread aligns properly for focus markers. I wonder if deliberate misalignment can help with getting infinity focus on its own? I've got a nice 70-220 m42 lens too, might have to make this a refurb project and give both lenses a clean (degrease and relube, optics with my scope optics cleaner and blow out the dust). the 500mm looks optically good some loose dust inside, the 220 has more, still, would be good to get to use them on nikon with a non-optical adapter or more likely the Tiny1 when it arrives. Dont really need them, have better nikon lenses but I like these lenses and dont want them to go to waste. Would be simpler to save for a second hand canon I know.
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Old 26-05-2017, 11:52 AM
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MTO has a screw that protrudes inside and serves as focus stop, I only needed to remove it (and replace it with short one so dust doesn't go inside.

It is similar with yours....
http://www.strickling.net/russentonne.htm

BTW, when you dismantle (unscrew) the back end, do it carefully and slowly.... the lens will behave like expanding bellows, and it will "suck in" a lot of air and dust with it.
I think you only have to remove the screw (indicated on attached image I borrowed from webpage mentioned earlier).

If you do not plan to remove the main mirror, there are no "traps"... except those small screws (careful - they like to split where the slot is) but if I were you I would try to relax the mirror. It may not help a lot (depending on mirror shape precision and stress), but it will not hurt either.
Have a look at my thread below:
http://www.iceinspace.com.au/forum/s...&highlight=MTO
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Last edited by bojan; 26-05-2017 at 02:32 PM.
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