11 replies to this topic

[JackSnyder]

Hello all! So I have a question about something I've been seeing with my stars when I use my Sony A7C and Samyang 135. I have included a picture to show what I am talking about. 

 

 

If you look at the large stars, they have this odd almost diffraction effect. At first I thought it may have been a tracking issue as I had the same problem around a month ago during a shoot where I had tracking issues. However, the smaller stars in this image look sharp and round. Furthermore, I used my Z61 that same night on the same mount with no issue. I am wondering if this is an optical flaw in the lens or perhaps something else. This also seems to be a new issue, I have used the 135 in the past and have never seen the stars look like this. 

 

Any ideas what this might be? Thanks!

[rollomonk]

This effect is known as aperture vignetting. This is not something that I understand very well but I have had the problem and learned about it here at Cloudy Nights at this thread:

 

https://www.cloudyni...tars/?p=7388127

Edited by rollomonk, 22 June 2025 - 09:26 PM.

[John Berger]

I have a little bit of that with my refractor, you can see it a little in the rays of the brightest star up top in this image of mine https://www.astrobin...ction&nce=32422

 

not much of a problem, it's not very noticeable, but wondered why this happens (is it the same phenomenon? mine seem to be a slimmer hourglass shape)

Edited by John Berger, 22 June 2025 - 08:11 PM.

[TOMDEY]

This effect is known an aperture vignetting. This is not something that I understand very well but I have had the problem and learned about it here at Cloudy Nights at this thread:

 

https://www.cloudyni...tars/?p=7388127

Yes, that completely explains it. It's the simple Fourier Transform of the otherwise good pupil. It's actually a very good way to determine whether your system's pupil is suffering traditional vignetting (or not).    Tom

[Kevin_A]

That is typical for the Rokinon 135 when the aperture is set between f2.0 to f2.4. When imaging at f2.8 it completely disappears. F2.8 is the sweet-spot and at f4 the lens creates stars that are spikey and just as bad as the iron cross/ bowtie big star shapes.

[Zambiadarkskies]

Yes, that completely explains it. It's the simple Fourier Transform of the otherwise good pupil. It's actually a very good way to determine whether your system's pupil is suffering traditional vignetting (or not).    Tom

Yup.  The dreaded lighthouse effect.  My Nikkor 180mm does it at f2.8.  At f4 there are starbursts which for me (opposite of Kevin) I find less offensive than the light house effect.  It also seems to be a feature of certain models of very expensive Takahashi 106 scopes.  Which would ruin my day completely....  You can experiment with step down rings as well to mitigate it, but I find it too much faffing around and then leads to flat frame calibration headaches.  I am lazy so would rather just deal with the starbursts at F4 with my Nikkor.  

[TOMDEY]

Yup.  The dreaded lighthouse effect.  My Nikkor 180mm does it at f2.8.  At f4 there are starbursts which for me (opposite of Kevin) I find less offensive than the light house effect.  It also seems to be a feature of certain models of very expensive Takahashi 106 scopes.  Which would ruin my day completely....  You can experiment with step down rings as well to mitigate it, but I find it too much faffing around and then leads to flat frame calibration headaches.  I am lazy so would rather just deal with the starbursts at F4 with my Nikkor.  

Yep --- the lighthouse (disposed around the edge of the field) is caused by traditional vignetting and the starburst (present at all fields) is caused by the iris leaves. I actually don't mind these, provided they are understandable and actually intended/accepted by the designer and user.    Tom

[Robert7980]

Looks like you have the answer above… 

 

It’s not a bad image… some of the optical quirks we face add character, so it’s not necessarily a problem, just how physics play into this stuff… It can be enjoyable trying to reduce the effects of unwanted effects, if you manage expectations and take your time you’ll get it working better. I’ve spent quite a lot of time tweaking and tuning my Sammy 135, it’ll never be absolutely perfect, but there’s more performance in there than you’d think… 

Here’s where I was at toying with wide open f/2 before taking a break due to hurricane Helene… Stopped down just a little should tame all the most aggressive stars. My game plan is to stop down the aperture with filter adapters and not use the internal iris and that causes other diffraction issues… Adjusting backfocus and tilt are also worthwhile tuning methods, these things are very sensitive to anything that’s misaligned. Just take your time and enjoy the process, it’s a nice lens with some fantastic possibilities, it takes a little work though… 

 

[joshman]

Yes, that completely explains it. It's the simple Fourier Transform of the otherwise good pupil. It's actually a very good way to determine whether your system's pupil is suffering traditional vignetting (or not).    Tom

Is there a reason that could cause this in a traditional refractor? I've noticed that my latest image on my Takahashi has demonstrated this in the extreme edge of a full frame field.

 

[TOMDEY]

Is there a reason that could cause this in a traditional refractor? I've noticed that my latest image on my Takahashi has demonstrated this in the extreme edge of a full frame field.

 

I don't think so --- that looks like something else.    Tom

[Spaceman 56]

I have experienced this, and also found a solution.

 

apparently it is caused when the outside edge of the sensor, does not get a clear view of the outside edge of the optics.

 

this can happen when a large diameter focuser tube assembly and front objective is coupled to a smaller diameter camera spacer assembly, sitting between the objective/focuser and the Camera

 

in my case I had 2 x 42mm extension spacers between the camera and the focuser. due to the restricted diameter (42mm) they were restricting the light path. when I removed these 42mm spacers, and changed the spacers out for 48mm spacers the Vignetting completely disappeared.

 

this was with an ASPC 2600MC size sensor, and the effect was only seen near the outside of the image.

 

if you use a smaller 533 size sensor most likely the issue would also go away, even with M42 spacers.

[JackSnyder]

That is typical for the Rokinon 135 when the aperture is set between f2.0 to f2.4. When imaging at f2.8 it completely disappears. F2.8 is the sweet-spot and at f4 the lens creates stars that are spikey and just as bad as the iron cross/ bowtie big star shapes.

Ah okay I see. Thank you very much, I'll stop it down going forward. Glad it was a simple solution!