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SW120ED - is this what pinched looks like?

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#1 Nate1701

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Posted 05 April 2021 - 07:52 PM

Doing some AP with this scope. ( I know a triplet is better)

Though I'm not the most experienced - i think it performs really well.

I've been shooting in sub-freezing the past few weeks...  20*F or  -6*C 

I will test this summer and take some shots to compare. But thought i would get some impressions.

Edit: I did use a Bahtinov mask for fine focus. Maybe it needed a tweak or shifted? Doesn't seem like focus problem to me.

 

pinched.jpg


Edited by Nate1701, 06 April 2021 - 11:21 AM.


#2 jeremiah2229

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Posted 05 April 2021 - 07:56 PM

Sure looks like it. When the scope is warm in the house loosen the cell a bit and try again to see what happens.

 

Peace...


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#3 wizbang396

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Posted 05 April 2021 - 08:03 PM

I don't believe it is a focus problem



#4 Redbetter

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Posted 06 April 2021 - 02:56 AM

Get enough scale to show the airy disk pattern and it will be obvious if this is pinch.  


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#5 Mike in Rancho

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Posted 06 April 2021 - 04:55 AM

Did this just start with a cold snap, or is the scope new to you?

 

If it's built anything like my Evostar 100ED doublet (which it very well might be since it's just a bigger brother), it could be imperfections in setting the final aperture circumference.  In my case it was not pinching, not related to temperature, and loosening the lens cell retaining ring did not help.  But, anything that prevents a perfectly smooth and circular aperture will result in funny star shapes and/or obnoxiously defective halos on bright stars.  And even medium stars with enough stacked integration.

 

So, if it does not resolve with temperature, it may take some testing to uncover the source(s).  And you will probably want to, as it will make a train wreck out of otherwise nice AP images.  It took me a few months to resolve, but it's a huge relief once the stars start coming out nicely.



#6 Nate1701

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Posted 06 April 2021 - 05:46 AM

Did this just start with a cold snap, or is the scope new to you?

 

If it's built anything like my Evostar 100ED doublet (which it very well might be since it's just a bigger brother), it could be imperfections in setting the final aperture circumference.  In my case it was not pinching, not related to temperature, and loosening the lens cell retaining ring did not help.  But, anything that prevents a perfectly smooth and circular aperture will result in funny star shapes and/or obnoxiously defective halos on bright stars.  And even medium stars with enough stacked integration.

 

So, if it does not resolve with temperature, it may take some testing to uncover the source(s).  And you will probably want to, as it will make a train wreck out of otherwise nice AP images.  It took me a few months to resolve, but it's a huge relief once the stars start coming out nicely.

scope is new to me. I will do some star testing and compare at warmer temps too.



#7 Jeff B

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Posted 06 April 2021 - 07:42 AM

Assuming the scope is thermally stable, at high power visually, pinched optics will display a non-round airy disk at focus, perhaps triangular, if the pinching is evenly distributed.  If bad enough, you may see spikes too.  Going slightly out of focus, may see "dents" or "divots" in the diffraction ring pattern on one side of focus with "spikes" or "spokes" on the other side of focus, and/or non-circular rings, again triangular in shape.

 

Those are dead giveaways for pinching.

 

Good luck!

 

Jeff


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#8 Nate1701

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Posted 06 April 2021 - 11:21 AM

Assuming the scope is thermally stable, at high power visually, pinched optics will display a non-round airy disk at focus, perhaps triangular, if the pinching is evenly distributed.  If bad enough, you may see spikes too.  Going slightly out of focus, may see "dents" or "divots" in the diffraction ring pattern on one side of focus with "spikes" or "spokes" on the other side of focus, and/or non-circular rings, again triangular in shape.

 

Those are dead giveaways for pinching.

 

Good luck!

 

Jeff

thanks for the tips everyone. Much appreciated



#9 Mike in Rancho

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Posted 06 April 2021 - 04:00 PM

scope is new to me. I will do some star testing and compare at warmer temps too.

Cool, good luck!

 

If you have the space, and maybe extensions to add backfocus if needed, you can also test indoors with an artificial star.  Which can just be a flashlight with a pinhole in foil or black tape.  Also if the 120 comes with a center cap in the main dust cap, like the 100 does, that's a quick way to do an aperture mask test.  Albeit a pretty heavy mask.  You can make others that are closer to your 120mm size.



#10 Nate1701

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Posted 06 April 2021 - 04:55 PM

Cool, good luck!

 

If you have the space, and maybe extensions to add backfocus if needed, you can also test indoors with an artificial star.  Which can just be a flashlight with a pinhole in foil or black tape.  Also if the 120 comes with a center cap in the main dust cap, like the 100 does, that's a quick way to do an aperture mask test.  Albeit a pretty heavy mask.  You can make others that are closer to your 120mm size.

yes it does have the centre cap. good idea.




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