Sigma ART 135mm f/1.8

@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

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

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!

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

Fabio Guerceri:
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?

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.

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.

Reinhold Wittich:
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

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

andrea tasselli:
Except for the lens stop isn't at the lens front but further inside. An RASA would the inlet pupil right at the front corrector.

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

I’ll buy it from you to use with my APS-C! 😅