0.90
6/27/2022
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Hello,   I bought a new Sigma ART 135mm f/1.8 and testet last night with ASI6200MC-PRO, 80 x 60s, IDAS NB-1 filter. I used the lnes fully open and I am not satisfied with the result. So I ask if this quality is normal. Do I have to stop dow at f/2.8 or so ? I attach the whole image and a crop of the 4 corners in 100% Best wishes Reinhold |
11.96
6/27/2022
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I used the abberation inspector script in PI and I would say that the answer to your question is yes.  |
5.93
6/27/2022
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@Reinhold, I own a Sigma Art 135mm lens and I mounted it to my QHY600 (same sensor as your ASI). I have to stop the lens down to at least 5.6 to get star shapes in the corners that I (and the mosaic stitching sofware) can live with. Actually, I take the DSO at f/3.5 and remove the sarts in Starnet. These stars are then replaced by separate exposures taken at f/5.6. One of my major projects created this way was a 6-panel mosaic of the Taurus-Auriga dark clouds: https://www.astrobin.com/tkoag2/ The small picel size of this sensor is just unforgiving....! If you are planning to use narrowband filters with this combination, you have to stop the lens down anyway, because of filter bandpass shift. Best regards! Chris |
8.85
6/27/2022
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I don't know of any lens that can be used at f/1.8 without any issue at full frame. Or even APS-C.
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0.90
6/27/2022
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Thanks Andy, Chris and Andrea, so I know that I will keep this lens. I will try the trick with the stars at f/5.6 Best wishes Reinhold |
7.80
6/27/2022
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I wonder a bit whether we push perfectionism too far if we pixel-peep on an exceptionally nice image with an image scale of 5 arcseconds. I can't find the stars disturbing in the full size view. Why buy an f1.8 lens if you then stop it down to f5.6 just to get round stars. You will end up investing eight times the integration time to end up with a very modest improvement. In your shoes I would check with f 2.4 and 2.8. Based on my own experience with a Samyang 135mm f2 - which obviously is not as good as your Sigma but hard to beat on a "worth the money" scale - I would be willing to bet that stars will be pretty decent at f 2.8 already, not perfect of course. F2.8 may still provide the best compromise between time you need to invest and quality of the end result. Have fun and clear skies Wolfgang |
9.03
6/27/2022
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Reinhold, I think your copy belongs to the better ones! I tried one a while back on APS-C sensor and it was much worse at the corner. Your test is on the FF!
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0.90
6/28/2022
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Imo the lower right is more round than the other 3 corners. I would do some tilt adjustment to see if you can balance out the symmetry. Shooting this lens at slower speed defeats the point of the sigma being f1.8
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0.90
6/28/2022
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Thanks Wolfgang, Min Xie and Wes, I have learned a lot from all of you. So I decided to keep the lens, it seems to be a better one. I removed the stars from my image with PixInsight. When the weather is clear I will shoot the stars with f/5.6 and combine. For me, this seems to be the best way to take full advantage of f/1.8 Best wishes Reinhold |
0.90
6/28/2022
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Hi, I join to the discussion about photographic lens because i tried to use my Canon EF 100-400 with ASI294MC Pro, but i had some issues. The focus operations are difficult because the the focus ring requires micrometric shifts and is therefore difficult to find the best focus position, and during the shooting session, while the object rise in the sky, the frames are increasingly out of focus. From your experience, i was wrong to consider some thing or this behavior is normal? |
11.34
6/28/2022
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Fabio Guerceri: Hi, This is normal, and that's the reason why it is preferable to use prime lenses with good mechanical properties. Fully manual prime lenses tend to be the best: Zoom creep is in-existent by definition and focus creep is far, far less probable. |
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0.90
6/28/2022
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If you have a autofocus lens with Canon EF mount you can use this adapter for perfect focus: https://www.astromechanics.org/ascom.html I bought it in Finland: https://www.astroart-store.com Best wishes Reinhold |
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11.34
6/28/2022
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Fast lenses typically suffer from strong vignetting, which means less S/N ratio towards the edges of the frame. And no, flat-field correction will not correct for that loss of S/N ratio. The Sigma lens has more than one stop of vignetting at f/1.8. Stopping the lens by one stop (or a tad more) is an acceptable compromise since it will improve the star quality and still the entire frame will have a similar S/N ratio as the corners of an f/1.8 integration for the same integration time.
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0.90
6/28/2022
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Taking images of the stars with the Sigma 135mm f/1.8 stopped down to f/5.6 seems easy. I think this would help with the RASA too. But how can I stop down a RASA? Has anyone done this? Best wishes Reinhold |
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7.80
6/28/2022
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Reinhold Wittich: I stop down my Canon EOS 200mm f2.8 lens without aperture control by adding a step down ring which basically reduces the diameter of the front element. I assume the same method should work with a RASA. Must likely you have to construct this by yourself but in times of 3D printing it shouldn't be an issue. I'm looking forward to your end result with exchanged stars. I don't have any experience with exchanging stars yet but it sounds like a clever approach - although in the Cygnus region there are really plenty of stars to exchange. Certainly a tough test for the software to prove what it can achieve. With respect to tilt: I have the same issues with my Samyang. I address it by holding the lens with a tube clamp and adjusting and checking the stars in the edges at the start of each imaging session. The worst thing about itis that during the imaging session due to the rotation of the ra axis the tilt may shift. Actually I think it's more precise to call it flexure because that's what it really is. Under the influence of gravity the lens doesn't sit perfectly orthogonal in the lens mount.  This is not going to win you a beauty prize but it works. The second clamp is for smooth focusing without wobbling too much in the finder. Clear skies Wolfgang |
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8.85
6/28/2022
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Except for the lens stop isn't at the lens front but further inside. A RASA would have the inlet pupil right at the front corrector.
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7.80
6/28/2022
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andrea tasselli: As usual you're 100% right Andrea and calling it "stop down" was wrong. What the step down ring does is it limits the angle of the incoming light rays which helps to improve the corners somewhat. Of course it's a crook but at least for my lens it seems to work. Clear skies Wolfgang |
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0.90
6/30/2022
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Yesterday the sky was ok and so I imaged the nebula again with the Sigma 135mm but now stopped down to f/8 to get better stars. As you can see this workes. Not perfect but you get much more better stars. I combined the starless image with f/1.8 with the image with f/8.  |
1.51
6/30/2022
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I’ll buy it from you to use with my APS-C! 😅
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7.80
6/30/2022
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Thank you Reinhold, I'm going to include this in my own workflow. Clear skies Wolfgang |
