First light new lens - triangular stars!?

[Space_Plane]

Hi,

Recently got a new Samyang 85mm 1.4, and as luck would have it, had a clear night to experiment with it on my asi178, attached via geoptik adapter. 

Did a quick smash and grab on andromeda after taking dozens of different exposures testing focus and how far to step the lens down.  I'm relatively confident my PA & tracking etc weren't "too" bad. 

The attached png of the stack is 20x1min at f2.8, with nothing more than a crop at the edges and 1 set of levels to make the issue more obvious. Best focus I could get was just shy of infinity. 

All the stars are triangles! 🤣

Any ideas? 😀

Chris

[RolandKol]

Omg...
I would expect something similar at wide open... But not from Samyang...
Is this one from the latest release?

My apologies, - sadly I have no advise... but was keeping an eye on it as a next possible lens...

[Space_Plane]

Thanks Roland, yes, it was bought new only a few days ago, 85mm F/1.4 AS IF UMC (if any of that helps!) Manual Focus.

I was quite disappointed lol, I'm hoping its just something I'm doing wrong, but I've used multiple other (and much heavier) lenses with exactly the same setup and haven't had any issues like this 🤷‍♂️

 

[michael8554]

Need to see just one of your exposures, stretched, to make judgments on quality. 

Michael

[Space_Plane]

Many thanks @michael8554, I've attached one of the preview subs from that session, auto-stretched straight from the ASIair.  Its a bit lower resolution but its particularly noticeable in certain areas.  I can upload some of the .fit files later if you think that might help?

Thanks

Chris

[michael8554]

I see the effect on the sub, so not a stacking artefact.

I suppose you could eliminate guiding errors from the causes on 1 minute exposures. 

I'm not a user of lenses for imaging, but possible (obscure) causes are iris effects, since you're stopped down a bit, or pinched optics.

Michael 

 

[Space_Plane]

Certainly a strange one! From memory it was occurring no matter the exposure, 10, 30 or 60 seconds, and I wasn't guiding either.

I see some minor diffraction spikes on the bigger stars from being stopped down a bit and oddly they don't seem obviously triangular.

I'm out of ideas anyway, no clear nights for a while for any more tests, and I don't assume I can fix pinched optics in a lens 😐

Thanks again for your input. 

[RolandKol]

If you still have occasion, maybe try few subs at F4 and F6 and check how it performs....

It would be really painful to use such a lens even at F4, - but I would test it just out of the curiosity....

and would send the lens back anyways.... If you bought it online, you have a month to change the mind