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What causes triangular stars

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#1 Kepler1349

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Posted 18 May 2018 - 06:10 AM

We've observed an odd phenomenon with our 12" Newton (2,5" TS-optics Wynne Coma Corrector, ASI 1600MM-C Pro), when slightly out of focus stars look like this:

 

Screen Shot 2018-05-17 at 22.28.23.png

 

For reference this is what a bright star looks like out of focus

 

Screen Shot 2018-05-17 at 22.20.14.jpg

 

Current hypotheses include: pinch on the primary, flex, refraction from the draw tube sticking in (approx 50mm into the tube), microlensing??

 

Some other tidbits, the vignetting profile seems to change from east to west (meridian), turning 180º, and west seems more in collimation.

Since the it's approx 3kg hanging off of a 12" carbon tube, the likely candidate is some sort of flexture... but we can't seem to find it.

 

Any ideas :-)?

 

(for now the idea is just to image only on the west side)


Edited by joelkuiper, 18 May 2018 - 06:11 AM.


#2 sternenben

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Posted 18 May 2018 - 06:39 AM

Looks an awful lot like pinched optics to me.  


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#3 ChrisWhite

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Posted 18 May 2018 - 06:54 AM

Agreed. For additional reading: http://www.loptics.c.../starshape.html
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#4 Kepler1349

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Posted 18 May 2018 - 08:40 AM

Thanks Chris and Ben, I'll investigate the support of the primary for pinched optics bow.gif



#5 calypsob

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Posted 18 May 2018 - 09:36 AM

Definitely an astigmatism



#6 jhayes_tucson

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Posted 18 May 2018 - 09:40 AM

You didn't say where in the field this occurs.  If this is an on axis image, then you have pinched optics somewhere.  If it's off-axis, you may have coma caused by an error in the coma corrector.  The worst case is that you've got both.  From what I see, the most likely problem is due to a distorted primary mirror.

 

Sorry Wes...but this is definitely not astigmatism.  Astigmatic error produced elongated or cross shaped stars.

 

John



#7 jsmoraes

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Posted 18 May 2018 - 10:15 AM

I have that issue,also . For me it is from coma corrector distance error plus focus. I have a QHY163M, similar to your ASI1600MM and 12" newtonian telescope.
My QHY needs a good focus. You call the effect as triangle, I call them as heart shape.
Although that issues can be from anothers sources as told on topics above.

 

I will receive very soon a kit of spacers to check the coma corrector distance as source. Take a look on image below. There are heart stars shape. It is is on the center of image. I did a comparison with Canon T3, nothing was done with mirror that can cause any pinch occurence.

 

You can perceive on spikes (horizontal) that it had a very small focus error.

 

CopareCanonQHY.jpg

 

Another image with better focus. You don't see the heart stars shape. With coma corretor distance error yet.

ETA was at center-bottom - 1/4 from bottom border.

 

keyhole-qhy-detail.jpg


Edited by jsmoraes, 18 May 2018 - 10:26 AM.


#8 ChrisWhite

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Posted 18 May 2018 - 10:43 AM

You didn't say where in the field this occurs. If this is an on axis image, then you have pinched optics somewhere. If it's off-axis, you may have coma caused by an error in the coma corrector. The worst case is that you've got both. From what I see, the most likely problem is due to a distorted primary mirror.

Sorry Wes...but this is definitely not astigmatism. Astigmatic error produced elongated or cross shaped stars.

John


To add to this, astigmatism elongation shifts 90 degrees from infocus to outfocus.


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