Artifact on Samyang 135mm

[dieg00]

My current setup is a Samyang 135mm f2 and Canon 60D. I have been doing untracked astrophotography for a few months, and two days ago I using a iOptron SkyGuider Pro from a friend. After reviewing the lights, I have found an "inverse" diffraction pattern in the brightest stars. Some info:

- The diffraction pattern appears to have a different direction on different stars, although the angle between the "antispikes" remains constant (see, for example, the pattern on the different stars of the Orion belt).

- I have checked lights from previous imaging sessions, and I cannot find it. However, it should be noted that all previous sessions were untracked, and therefore the exposure time is widely different: 30 seconds vs 1 second.

-  Similar to the previous point, all my previous sessions were done in a much warmer climate, and this is the first time I am imaging at near 0 celsius. Also, because I had some issues polar aligning it, by the time I took the first photo, the lens had already cooled down significantly. (All the images I took yesterday present the same diffraction pattern on said brightest stars).

I would like to know what I can do about this issue, I kindly appreciate any info or test I could do on the lens to try and pinpoint the exact problem and (hopefully) solve it. 

Also, in case I have to tamper with it, I have seen there is a set of 120 degrees spaced screws next to the aperture ring (which, according to online guides, are useful for fixing the lens in case it doesn't focus on infinity); but there are also 90 degrees spaced screws on top of the focusing ring, what do this do? (I won't immediately use them but I would be interested in knowing about them).

Thank you to whoever sheds some light on the issue.

PS. I have attached a single light from the session, please let me know if it would be helpful if I share others, or try taking more and uploading them.

[david_taurus83]

Not sure what's causing this on your lens but I used to own a William Optics scope that had 3 similar spikes on bright stars. Its fairly common and I think on the WO scopes anyway, it is/was caused by lens retaining clips at the edges. I have also seen examples of this on pictures taken with Takahashi scopes. I had a Samyang myself and never noticed these artifacts though I never shot below f4.

[dieg00]

2 minutes ago, david_taurus83 said:

Not sure what's causing this on your lens but I used to own a William Optics scope that had 3 similar spikes on bright stars. Its fairly common and I think on the WO scopes anyway, it is/was caused by lens retaining clips at the edges. I have also seen examples of this on pictures taken with Takahashi scopes. I had a Samyang myself and never noticed these artifacts though I never shot below f4.

Hmm I see. Did you also experience the direction being different on each star? I am asking because if it were retaining clips, wouldn't the direction of the spikes be on the same directions (of the clips)?

[david_taurus83]

34 minutes ago, dieg00 said:

Hmm I see. Did you also experience the direction being different on each star? I am asking because if it were retaining clips, wouldn't the direction of the spikes be on the same directions (of the clips)?

I didn't take particular note of the orientation as I just considered it an artifact of the scope, like how diffraction spikes are to a Newtonian. But I did notice the Takahashi scopes artifacts do change direction around the field of view. I know I've seen an explanation somewhere.🤔

[david_taurus83]

I've just gone back through my messages as I asked a Tak owner this question. He spoke to an optical engineer who suggested it was mild pinching of the optics. So given you were imaging in a colder than usual climate, that may explain your issue. Here's some Tak FSQ85 examples:

 

[dieg00]

8 minutes ago, david_taurus83 said:

I've just gone back through my messages as I asked a Tak owner this question. He spoke to an optical engineer who suggested it was mild pinching of the optics. So given you were imaging in a colder than usual climate, that may explain your issue. Here's some Tak FSQ85 examples:

 

Ahh I see. I think I should try imaging as the lens cools down then.

Would a dew heater help?

Also, at least with the Taks, is this a defect or something expected based on construction?

[nephilim]

I was just about to mention maybe pinched optics. Materials obviously contract ever so slightly in the cold, a dew heater strap may remedy it & is worth a try.

I will add though that I also use the Samyang 135 & have imaged in temps ranging from approx +15⁰c to -10⁰c & have never seen this issue.

Steve

[StarryEyed]

Pinched optics can vary from scope to scope depending on how tight the retaining rings are torqued, theres no reason to assume this is not the case for a camera lens. Do they use a torque wrench? Who knows.

 

[dieg00]

2 hours ago, nephilim said:

I was just about to mention maybe pinched optics. Materials obviously contract ever so slightly in the cold, a dew heater strap may remedy it & is worth a try.

I will add though that I also use the Samyang 135 & have imaged in temps ranging from approx +15⁰c to -10⁰c & have never seen this issue.

Steve

I see. Do you use it wide open at f/2? And how long are usually the exposures?

[dieg00]

1 hour ago, StarryEyed said:

Pinched optics can vary from scope to scope depending on how tight the retaining rings are torqued, theres no reason to assume this is not the case for a camera lens. Do they use a torque wrench? Who knows.

 

Is it fixable? Or something dependant entirely on construction?

[Alien 13]

28 minutes ago, dieg00 said:

Is it fixable? Or something dependant entirely on construction?

The Samyang lens has an awful lot of glass in it so may require some time to reach thermal equilibrium, a simple dew heater like the USB powered COOWOO might help.

Alan

[nephilim]

25 minutes ago, dieg00 said:

I see. Do you use it wide open at f/2? And how long are usually the exposures?

@dieg00 I usually stop it down to f2.8 but only because I find its harder (and it takes longer) to get a good precise focus @ f2. My subs also vary, dependent on the weather. If its intermittent cloud rolling past every now & again I'll keep them short to around 60 seconds as I dont want to ruin a long exposure with a rogue cloud, on a totally clear night (I've forgotten what they are like 🙄) I stick to 3 mins.
I've only been imaging for around 6 months & although I have a guide scope & camera i've yet to start guiding. I'm fairly proficient at setting up, PA etc now so I need to start (I had been putting it off due to it been a lot to take in when I was just starting out, i've no excuse now really). Once i'm guiding i'll go up to 5min subs.

Steve 

[ollypenrice]

What you have is a classic 'first world problem.'  Unless you pixel peep you will see nothing wrong with those stars. The only purpose of pixel peeping is to spoil a good image so why do that?

The reality is that your optics are performing superbly in this exposure. This widefield camera lens has split Alnitak as a double.  Please read that twice. Then take a look at high resolution telescopic images which totally fail to contain Alnitak and are a million miles from splitting it as a double.

Making truly great astrophotos has nothing whatever to do with fixing this non-existent problem. Get lots of data with this wonderful setup and learn all you can about processing. 

If you don't think your lens is wonderful, I'll buy it. I'm serious.

Olly

[dieg00]

3 hours ago, nephilim said:

@dieg00 I usually stop it down to f2.8 but only because I find its harder (and it takes longer) to get a good precise focus @ f2. My subs also vary, dependent on the weather. If its intermittent cloud rolling past every now & again I'll keep them short to around 60 seconds as I dont want to ruin a long exposure with a rogue cloud, on a totally clear night (I've forgotten what they are like 🙄) I stick to 3 mins.
I've only been imaging for around 6 months & although I have a guide scope & camera i've yet to start guiding. I'm fairly proficient at setting up, PA etc now so I need to start (I had been putting it off due to it been a lot to take in when I was just starting out, i've no excuse now really). Once i'm guiding i'll go up to 5min subs.

Steve 

Ok I see. I just tried and at f2.8 or f2.5 I can see no diffraction. May I ask what tracker are you using? And tripod? I'm on the lookout to buy one

[dieg00]

3 hours ago, Alien 13 said:

The Samyang lens has an awful lot of glass in it so may require some time to reach thermal equilibrium, a simple dew heater like the USB powered COOWOO might help.

Alan

I'm gonna buy one then. Do you have any experience with the SVBony one? Or should I go for the COOWOO?

[dieg00]

3 hours ago, ollypenrice said:

What you have is a classic 'first world problem.'  Unless you pixel peep you will see nothing wrong with those stars. The only purpose of pixel peeping is to spoil a good image so why do that?

The reality is that your optics are performing superbly in this exposure. This widefield camera lens has split Alnitak as a double.  Please read that twice. Then take a look at high resolution telescopic images which totally fail to contain Alnitak and are a million miles from splitting it as a double.

Making truly great astrophotos has nothing whatever to do with fixing this non-existent problem. Get lots of data with this wonderful setup and learn all you can about processing. 

If you don't think your lens is wonderful, I'll buy it. I'm serious.

Olly

You are definitely right. I'm pixel peeping too much and I'm failing to see the bigger and beautiful picture. I'm intrigued to what you said about Alnitak being split as a double. Which one is Alnitak B?

Thank you for your words, definitely what I needed to hear. (And now you have made me happy about getting this lens, so sorry but I'll keep it haha)

[nephilim]

12 minutes ago, dieg00 said:

Ok I see. I just tried and at f2.8 or f2.5 I can see no diffraction. May I ask what tracker are you using? And tripod? I'm on the lookout to buy one

I've been using this for the last 6 months or so. Its well priced & also has a good weight bearing capacity for imaging, at 12.7kg its capable of carrying more weight than the HEQ5 Pro (it has a quoted maximum of 11kg) plus its around £300 cheaper than the HEQ5 Pro.  https://www.firstlightoptics.com/explore-scientific-mounts/explore-scientific-exos-2-pmc-eight-goto-mount.html  I also use the Asiair Pro to control everything & it all seems to work very well together.  

[dieg00]

1 hour ago, nephilim said:

I've been using this for the last 6 months or so. Its well priced & also has a good weight bearing capacity for imaging, at 12.7kg its capable of carrying more weight than the HEQ5 Pro (it has a quoted maximum of 11kg) plus its around £300 cheaper than the HEQ5 Pro.  https://www.firstlightoptics.com/explore-scientific-mounts/explore-scientific-exos-2-pmc-eight-goto-mount.html  I also use the Asiair Pro to control everything & it all seems to work very well together.  

Thank you for your answer! I will definitely look into that mount

[ollypenrice]

10 hours ago, dieg00 said:

You are definitely right. I'm pixel peeping too much and I'm failing to see the bigger and beautiful picture. I'm intrigued to what you said about Alnitak being split as a double. Which one is Alnitak B?

Thank you for your words, definitely what I needed to hear. (And now you have made me happy about getting this lens, so sorry but I'll keep it haha)

Alnitak is the most easterly of the three belt stars. Here you can clearly see it's a double in your data and that's without taking short exposures for a full separation. Frankly that is quite extraordinary in a focal length of 135mm.

The only telescope I've used which splits it easily in full length exposures is the TEC 140, which has over seven times the focal length. Reflectors also split it well if you can see past the diffraction spikes created by the main star!

TEC 140 image:

https://www.astrobin.com/full/394022/0/

Olly

[dieg00]

10 hours ago, ollypenrice said:

Alnitak is the most easterly of the three belt stars. Here you can clearly see it's a double in your data and that's without taking short exposures for a full separation. Frankly that is quite extraordinary in a focal length of 135mm.

The only telescope I've used which splits it easily in full length exposures is the TEC 140, which has over seven times the focal length. Reflectors also split it well if you can see past the diffraction spikes created by the main star!

TEC 140 image:

https://www.astrobin.com/full/394022/0/

Olly

Wow I'm so grateful that you pointed it out, as I would have never realized it by myself. It's definitely impressive, and now you have given me a reason to observe that star using my tabletop newtonian as well!