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I purchased a Rokinon 135 mm f2 earlier this year for use in wide field work with APSC and full frame cameras both digital and CMOS. I've tried numerous setups with this lens direct camera mounted, multiple different set ups with my ASI 2600 MC Pro and a range of f ratios from f2 to f4. In all cases I get very bad aberration at the edges of field even with an APSC chip. I've posted a typical aberration inspection at the end of the post. Anyway based on all my various combinations I'm pretty certain this isn't tilt (at least on the camera end) given that camera and f ratio don't improve things. I also get perfectly good images with the same set ups and my Askar 200 mm f4. I sent the lens in for service (along with the aberration chart below) and was told it was functioning as designed but I should be aware that it wasn't designed for astrophotography and as such I shouldn't be surprised by my results. I've read rave reviews of this lens on Astrobin and other places so I just wanted to check if others were getting better performance than this with larger chips (I can see it works fine with something like the 183). Rokinon have offered to switch the lens out for me for a nominal fee but did suggest I should not expect any better performance. Anyway interested to see what others think. Many Thanks John  |
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The right side of your chip appears to be inside of the focal plane (too close). I would try to put a shim on the right side and test it. Also, post your aberration plot for the askar. For context this is my grid on the 6200MC at f/2.8 after tip-tilt correction  |
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In Rokinon's defence, they are right. This lens was designed for terrestrial photography at a time when the pixels on cameras were bigger. The lens could meet their specs but still fail for astro. Astrophotography is truly the acid test for any optical system because of all the point sources of light, not to mention how today we have much smaller pixels. That being said, it looks the stars in the middle of your frame do look decent, so you have that going for you. There is certainly a lot of tilt though. At F/2 the system is extremely sensitive to any kind of minuscule tilt. Its possible that the tilt is in the lens but either way you will have to find a way to correct for it. I had to on my Rokinon. |
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Thanks Bray that looks pretty good to me. I've attached the Askar aberration plot below - not perfect but infinitely better. What has me worried on the lens is that the aberration is the same whether it's on a ASI camera or direct mounted to a canon body (I had assumed tilt would not be an issue with the canon body - perhaps wrongly). |
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I have the Samyang 135, which I believe is the same lens. I am learning to correct the tilt on mine. It’s a process - that’s for sure 🤣🤣🤣 In my case, I ended up buying an Octopi to handle the tilt and backspacing. That made everything much more approachable - and I’m slowly homing in on perfect tilt and backfocus. Arm yourself with some good old fashioned patience 😉 |
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Thanks Christian! Patience I have but time and clear skies I don't At least I now know this isn't the out of the box solution I had hoped for - nor perhaps should I have expected it at the price point! I was hoping I just had a bad lens but it seems that's not the case from the replies thus far. Thanks again John |
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If you have a DSLR with an APS-C sized chip, put that on and see how the stars look, especially on the right side where you're currently seeing aberrations. This is really the best way to test a lens. Even better if it has 3.76 micron pixels. If the aberrations appear with the DSLR, it's likely you have a bad lens. If not, you just have tilt somewhere else, which is relatively easy to fix. |
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Thanks the aberration plot is from a picture taken with a canon camera body, I get pretty much the same results with the ASI 2600 MC Pro and the Canon lens mount. John |
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That's my results with a Canon 7Dii (4.1 um pixels) so about 9% larger than a ASI2600's pixel: https://www.astrobin.com/full/ijq903/0/ Slight tilt in the left lower corrner but tweaking focusing (that is trading the upper end for the lower end sharpness) should do the trick. Shot at f/2, obviously. |
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I don't know what could be wrong with your setup, but I think you may be able to correct the tilt on the focal plane (which is quite sensitive at f/2) with the tip/tilt screws on the 2600MC. This is the AberrationInspector of my Samyang 135mm and ASI533MC Pro at f/2.8. It was shot on a full moon night so there are some gradients. There is definitely some tilt on the right but the ASI533 not having the tip/tilt corrector plate doesn't help. Single sub frame:  Master file drizzled 1x, 45x1min:  |
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Ok to draw this to conclusion. I sent the lens off to Samyang as noted above and while they said it was in spec they did offer to replace it for a $40 restocking fee plus shipping which for me was $15. I figured I might as well try it. Finally got the lens back and a clear night. So armed with a GerdNeuman Tilt plate (CTU) I was all set for a night of tweaking. What do you know near perfect stars to the edge of my APSC field at both f2.8 and then wide open at f2. I switched out the tilt plate and went back to my ZWO to EOS filter drawer and got the same results. I'm feeling good about this, at full frame may well have needed the CTU but seems I'm ok for my ASI 2600MC Pro. It's not often that things work out in this hobby on a clear night and when its well below freezing! Thanks for all the tips and also thanks to Samyang/Rokinon who were actually very helpful and easy to deal with. John |
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This is taken last night. I bought the bracket and put on my zwo294mc with an L-pro filter on my Rokinon 135 f2 that I bought years ago as a Flower closeup lens. Second time I have used it, I must have a good Rokinon because the stars are sharp edge to edge. I use just the filter drawer adapter for Nikon lens Gradients are my problem shooting in bortle 8. This is taken at f2.8. Manual focus with Bat mask. NGC1499 wide field.  |
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Dan Kearl: Thanks for posting this Dan and happy that everything worked out well. I bought the Samyang version of this lens about a month ago to kind of renew my interest in photography with plato use it for astro sometime this year. So far using the lens, it is definitely re-sparked my interest in photography. The lens is super sharp and well balanced. i’ve been reading this thread, and preparing myself for backspacing issues, and hopefully not too many issues with tilt. It’s refreshing to see a post where you had neither of these issues and the lens did well for you without a lot of hassle. hoping very much for a similar experience!Looking forward to seeing more images! Jim |
