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What in the world is causing my V shaped stars?

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#1 Loaded4bear

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Posted 28 September 2021 - 02:52 PM

I am moving into a new OTA (ES 115mm CF) getting ready for the more clear fall weather.  I ran some test images and am getting these weird V shaped stars – what the heck?  I checked my image train over and over – solid and flush (Hotech flattener, Zwo filter wheel, Falcon rotator, Zwo ASI1600mm pro).  My spacers are dead on 55mm for backfocus.  The pattern of the V stars are all in the same direction anyway so I think I can rule that out since I think backfocus spacing problems doesn't create star images leaning all in the same direction across the entire field. My guiding was reasonable and I wasn't running Synscan P-PEC or anything in EQMOD that may do strange things.  I use Sharpcap for my polar alignment so that makes it pretty much moron proof for me.  No wind that night at all.  I haven't a clue at this point – I even bought a cheshire collimator and checked the glass.  Looks OK by my astigmatism laced eyes.  What do you all think???  I don't know where else to go....

 

Test image 2.jpg



#2 Jim Waters

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Posted 28 September 2021 - 03:06 PM

What's it like without the Hotech flattener?



#3 ngc7319_20

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Posted 28 September 2021 - 03:24 PM

This is not backfocus issue, or at least not dominant.  Looks like bad coma... something is tilted / decentered / wedged / out of collimation.  How do star images look to the eye with eyepiece at high power?  That would test the OTA itself, and then go from there.


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#4 SilverLitz

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Posted 28 September 2021 - 03:36 PM

My old ED102CF focuser would easily get out of collimation causing arrowhead shaped stars.

 

ES's normal focusers are very substandard and definitely out of place for their FPL-53 or FCD-100 scopes.


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#5 michael8554

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Posted 29 September 2021 - 10:16 AM

As the OP said, looks like coma, but it's over the whole frame, and pointing in the same direction.

 

Suggestions to be discussed:

 

ASI1600MM sensor tilt, not unheard of ?

 

Pinched optics ?

 

A short exposure without the Hotech flattener, Zwo filter wheel, Falcon rotator, might be illuminating.


Edited by michael8554, 29 September 2021 - 10:17 AM.


#6 ngc7319_20

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Posted 29 September 2021 - 11:36 AM

As the OP said, looks like coma, but it's over the whole frame, and pointing in the same direction.

 

Suggestions to be discussed:

 

ASI1600MM sensor tilt, not unheard of ?

 

Pinched optics ?

 

A short exposure without the Hotech flattener, Zwo filter wheel, Falcon rotator, might be illuminating.

Probably the easiest way to make coma over the entire field (and mostly pointing in the same direction) is to wedge the air gap on the ES 115 objective lens -- for example if the spacers have migrated during shipping.  Or if some retaining ring came loose and the glasses are flopping around.

 

I don't see sensor tilt as causing this.  Sensor tilt would produce a sharp image somewhere on the sensor, and progressively de-focused images elsewhere (with increasing distance from focused area).

 

This doesn't look like pinched lens cases I've seen.  Usually thats a nice sharp round image with spikes corresponding to the spacers or pinch locations around the edge of the lens.  With a lens it is difficult to make a pinch that effects the entire optical surface -- global distortions at the front and rear surfaces cancel out to first approximation.  (This is different from a mirror where there is no "rear surface" and global distortions are the expectation.)

 

I agree some test that examined the image quality of the ES 115 CF alone would be my next step.  That could be a star test with an eyepiece, or some short exposure image.  My guess is the problem is with the objective lens spacing.  It would also be interesting to look at images inside and outside best focus.



#7 Loaded4bear

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Posted 29 September 2021 - 01:56 PM

Great ideas - thank you.  I found a possible problem area.  The Falcon rotator has a lot of motion slop in it internally.  If I put a little pressure on the camera, the internal mechanism of the rotator shifts quite bit.  Something is loose.  Just got the rotator a couple of months ago.   I contacted Pegasus Astro, sent them a video of the movement, and they agreed that this amount of flex was not right.  This must be a known batch problem as they are sending me parts that I have to install that will fix the problem.  Fun!  I don't know if this will fix it 100% until I get the parts and can image again.  I wish I had the correct spacers to eliminate various components and keep back focus.  I guess we can never have too many spacers!  I mostly believed that because the distortion was pretty even across the entire field that it had to be a tilt problem or something as NGC suggested to do with the glass out of alignment.  I think that would cause the collimation to be out of whack though.  The collimation looked ok.  This is my second ES OTA.  I have found with diligence, you can get very nice images.  I'll post back if the rotator was the problem.



#8 Loaded4bear

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Posted 29 September 2021 - 02:12 PM

Oh I forgot.  Regarding the idea of the focus shift across the field, I thought that had merit.  Then I remembered that I use Voyager LocalField auto focus (fabulous routine by the way).  That looks at the entire frame to find an average focus overall.  I would think that would minimize the ability to see the focus shift of the tilted optical train. If I can get this fixed, I should see my focus numbers improve quite a bit (I hope!).



#9 ngc7319_20

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Posted 29 September 2021 - 02:22 PM

Yeah, I guess something at the back end -- rotator for example -- could be flopping around in a way that mimics coma.  Good point.  As you track it might flop this way, then that way, etc.  Some short exposures might help sort that out.




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