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Diffraction artifact in image?

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#1 IC1101

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Posted 24 January 2020 - 09:31 AM

There is a strange diffraction artifact around the brighter stars in my image. I can't figure out the cause and would appreciate some ideas.
My imaging train is - WO Z61 - Flat61A flattener - UV/IR filter - asi294mc pro

Diffraction Artifact.jpg

#2 WadeH237

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Posted 24 January 2020 - 09:59 AM

That looks like pinched optics.

 

My AT102ED had exactly the same issue when I bought it.  There were 4 nylon screws positioned around the primary lens cell at 90 degree points.  I needed to back off those screws ever so slightly (a small fraction of a turn).  If you are not familiar or comfortable with how to do this, you should contact William Optics so that they can take care of it.


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#3 LarsMalmgren

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Posted 24 January 2020 - 11:16 AM

That looks like pinched optics.

 

My AT102ED had exactly the same issue when I bought it.  There were 4 nylon screws positioned around the primary lens cell at 90 degree points.  I needed to back off those screws ever so slightly (a small fraction of a turn).  If you are not familiar or comfortable with how to do this, you should contact William Optics so that they can take care of it.

No, to me it looks like the diagonal spider vanes are not exactly perpendicular and/or not completely straight in extension on the other side of the secondary holder.

This will make double diffraction lines so instead of 4 you get 8.

The smaller blurry lines in between the 4 double lines are from the primary mirrors turned down edge.

Very typical of budget telescope, in fact most mass produced telescopes today.

 

Any little protrusion into the light path, might it be a screw end or similar, will also produce some of these smaller blurry diffraction lines around the stars.

They are typical more irregular. All of it adds up to become a kind of "fingerprint" of the main mirror and telescope.

 

A pinched mirror normally  results in triangular shaped stars.

 

You should be able to fix the spider vanes, so they are more straight and perpendicular.

This should reduce the large double spikes to just 4.

The rest of the glare/glow is another matter, since it means replacing the primary or re-figuring of it.

No an option for most...



#4 LarsMalmgren

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Posted 24 January 2020 - 11:20 AM

Sorry, i'm talking about mirrors not realizing you have a refractor blush.gif  Oops!

 

A lens can still have a turned down edge.

But the bigger double diffraction spikes are probably from the spacers in between the lenses.

Don't touch them, they need to be there, so no much to do.

 

I still think a pinched lens will tend to produce triangular stars but i'm not an expert in refractors.

Checking for pinching and collimation is never a bad thing to do waytogo.gif


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#5 B 26354

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Posted 24 January 2020 - 11:22 AM

No, to me it looks like the diagonal spider vanes are not exactly perpendicular and/or not completely straight in extension on the other side of the secondary holder.

This will make double diffraction lines so instead of 4 you get 8.

The smaller blurry lines in between the 4 double lines are from the primary mirrors turned down edge.

Very typical of budget telescope, in fact most mass produced telescopes today.

A WO Z61 is a 61mm refractor.  grin.gif


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#6 IC1101

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Posted 24 January 2020 - 01:38 PM

I found another thread with exactly the same issue. It looks like its pinched optics. Very frustrating!!!! I waited a month and a half for delivery of this scope, then another month to get one clear night. I was so happy to get almost 5 hours on M42 just to have it all wasted. Probably will take another month to return this scope and get a new one. You would think WO would test there scopes before shipping them.frown.gif

 

https://www.cloudyni...or-another-m45/


Edited by IC1101, 24 January 2020 - 01:40 PM.


#7 Madratter

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Posted 24 January 2020 - 01:44 PM

This was a very common topic when the Z61 came out. There were any number of threads on it. I don't know if a solution was found or it was a learn to live with it and enjoy the big wide field for cheap kind of thing.

 

EDIT:

 

See this thread with a similar scope.

 

https://www.cloudyni...-on-the-wo-z73/

 

Sounds like at least for that scope, there was a reason/fix.


Edited by Madratter, 24 January 2020 - 01:49 PM.

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#8 LarsMalmgren

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Posted 24 January 2020 - 03:37 PM

What kind of temperatures did you have while using the scope?

 

Coming to think of it, i remember that WO in the early days had problems with the lens being pinched in cold weather due to the metal contracting.

But they fixed it eventually.


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#9 WadeH237

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Posted 24 January 2020 - 04:30 PM

I still think a pinched lens will tend to produce triangular stars but i'm not an expert in refractors.

Checking for pinching and collimation is never a bad thing to do waytogo.gif

A pinched lens will produce triangles if there are 3 contact points.  I actually diagnosed a friend's triangle stars yesterday as pinched optics that were triangles (and he confirmed it after making an adjustment).

 

I would bet a reasonable sum that his scope has 4 screws into the lens cell, and that they are just a bit too tight.  My scope produced exactly the same artifacts until I made an adjustment.  And I have see then exact same pattern on other refractors with the same problem.


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#10 IC1101

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Posted 24 January 2020 - 05:08 PM

What kind of temperatures did you have while using the scope?

 

Coming to think of it, i remember that WO in the early days had problems with the lens being pinched in cold weather due to the metal contracting.

But they fixed it eventually.

It was cold! -8C

 

I had a good look down the tube and around the objective and there are no visible screws at all.


Edited by IC1101, 24 January 2020 - 05:13 PM.


#11 OldManSky

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Posted 24 January 2020 - 06:38 PM

There are indeed 4 sets of two screws for the lens cell. The set towards the front is for locking down the cell, the rear set for collimating. The “fix” for this is generally really simple, and won’t void your warranty. Unscrew the lens shade. You’ll see the 4 sets of screws. Loosen the front screw in each set only about 1/8 turn. See if that takes care of it. If not, loosen another 1/8 turn. You would have to loosen them 3-4 turns to remove them from engaging the cell, so you’re in no danger of the cell coming loose. WO tightens them down all the way to insure the cell doesn’t loosen in shipping, and sometimes that means they intrude in the light path a tiny bit. This is no big deal, and a simple fix.

 

And yes, I did it on my Z61 with the full knowledge of WO. No sweat. :)


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#12 georgian82

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Posted 25 January 2020 - 01:34 AM

I have what I believe to be the same issue as you with my Meade 70mm f/5 Petzval refractor. My scope does have four sets of screws and you can see the diffraction it makes in this picture (zoom in on the brightest star):
https://www.astrobin.../full/zu7nep/C/

I first drew a line following the indentation of the screw to make sure I would be able to go back to the original position just in case, although I think this was not necessary as the screws were tightened to the end with no more room to go any further. I also used that line as a guide for the adjustments (last pic below you can see the screw is now perpendicular to the line I drew). I then followed OldManSky's instructions above and turned the screws about 1/4 of a turn and put the dew shield back on. Now I just have to wait until the clouds disappear to be able to test it out. I will report back.

Meade screws 1

Meade screws 2

Meade screws 3

Edited by georgian82, 25 January 2020 - 01:43 AM.

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#13 IC1101

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Posted 25 January 2020 - 09:35 AM

I have what I believe to be the same issue as you with my Meade 70mm f/5 Petzval refractor. My scope does have four sets of screws and you can see the diffraction it makes in this picture (zoom in on the brightest star):
https://www.astrobin.../full/zu7nep/C/

I first drew a line following the indentation of the screw to make sure I would be able to go back to the original position just in case, although I think this was not necessary as the screws were tightened to the end with no more room to go any further. I also used that line as a guide for the adjustments (last pic below you can see the screw is now perpendicular to the line I drew). I then followed OldManSky's instructions above and turned the screws about 1/4 of a turn and put the dew shield back on. Now I just have to wait until the clouds disappear to be able to test it out. I will report back.


 

Yes that is exactly the same diffraction spikes I get. Do you see these screws impinge into the tube causing the spikes or that they are just too tight and pinch the objective?

 

I emailed WO but of course they are closed until Feb.3 for their New Year. The scope is just one month old so I hesitate to do anything to it until I hear back from them to make sure the warranty would still be valid and instructions from them how to adjust it. Although it sounds really simple.


Edited by IC1101, 25 January 2020 - 09:38 AM.


#14 OldManSky

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Posted 25 January 2020 - 10:53 AM

georgian, I'm sure you'll be fine.  When these things are in final assembly, they sometimes tighten the hold-down screws all the way in to make sure the cell doesn't move around during the long shipping from China.  Which is probably a good idea, but the screws intrude in the light path just a little bit when that's done.  Backing them off a tiny amount is the ticket -- it's what WO told me to do just over a year ago, and after I did it, no more diffraction artifacts at all.

 

IC1101, you can certainly wait if you want to, but this is really simple and won't harm your scope or void your warranty in any way.  It's what WO will suggest to you when they come back from holiday :)

 

georgian, let us know the results, ok?


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#15 georgian82

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Posted 25 January 2020 - 12:29 PM

Yes that is exactly the same diffraction spikes I get. Do you see these screws impinge into the tube causing the spikes or that they are just too tight and pinch the objective?

I emailed WO but of course they are closed until Feb.3 for their New Year. The scope is just one month old so I hesitate to do anything to it until I hear back from them to make sure the warranty would still be valid and instructions from them how to adjust it. Although it sounds really simple.


Seems to me they are just too tight and are pinching the objective or they are far down enough to block some light which is seen as diffractions.

I am not suggesting you do what I just did by the way...I just wanted to show what OldManSky is referring too and see if I can fix the issue I have with my scope along the way 😉
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#16 georgian82

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Posted 25 January 2020 - 12:31 PM

georgian, I'm sure you'll be fine. When these things are in final assembly, they sometimes tighten the hold-down screws all the way in to make sure the cell doesn't move around during the long shipping from China. Which is probably a good idea, but the screws intrude in the light path just a little bit when that's done. Backing them off a tiny amount is the ticket -- it's what WO told me to do just over a year ago, and after I did it, no more diffraction artifacts at all.

IC1101, you can certainly wait if you want to, but this is really simple and won't harm your scope or void your warranty in any way. It's what WO will suggest to you when they come back from holiday :)

georgian, let us know the results, ok?


Yes, seems Sunday will be clear so I will get a chance to get out and test it. I will report back.

Cheers
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