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Extra Diffraction Spike?

reflector optics Orion equipment
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#1 Tom Conway

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Posted 02 December 2014 - 12:42 AM

I've got a really annoying extra diffraction spike in my images.  I'm fairly new to all of this, but believe I've got good collimation.  I've attached two images showing the spike, one of an astro photo in the M42 region, the other using an artificial star I made out of an LED flashlight.  I've inspected the scope carefully and don't see anything obvious.  The spider vanes appear to be straight, and at 90 degrees, not bent or twisted. Anybody have any experience with this sort of thing?  The scope is an Orion F3.9 Astrograph. Thanks!

 

OrionExtraSpike.jpg ArtStarExtraSpike.jpg



#2 precaud

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Posted 02 December 2014 - 08:06 AM

Yeah, I've see it in 2 or 3 eyepieces, and in each case have traced it down to the eyepiece. The spike rotates as the eyepiece is rotated in the focuser. Close examination reveals nothing wrong with them, and their performance is otherwise uncompromised, so it remains a mystery.



#3 schang

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Posted 02 December 2014 - 09:25 AM

What kind of eyepieces that caused this extra spike?  I wonder if the OP used the same type of eyepiece?



#4 precaud

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Posted 02 December 2014 - 09:56 AM

The one that has it the strongest is a 10mm Vixen NPL. It bothered me enough that I wrote the dealer. He sent me another one to compare it to. The second had no extra spike, but was noticeably worse in other respects, so I returned it and kept the spikey one. Whatever the cause, it doesn't seem to have an impact on its performance in other respects, it is quite good.



#5 tazer

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Posted 02 December 2014 - 02:24 PM

Have you verified there are no unpainted or shiny screws in the interior? Perhaps the focuser drawtube (the end of which is probably shiny) is sticking further into the OTA than expected?

 

Mark



#6 GlennLeDrew

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Posted 02 December 2014 - 03:56 PM

To those inferring eyepiece trouble, note that these images are at prime focus--no eyepiece involved.

 

The intensity and narrowness would imply a significant linear obstruction. But the asymmetry, whereby it's confined to one side of the Fresnel pattern, is puzzling. It might be informative to take a well out-of-focus image of a bright (real or artificial) star, so that its donut occupies about 1/10 or more of the field width. A shorter exposure that doesn't saturate will reveal the state of obstructors. And a long exposure might hint at other stuff...

 

The artificial star has much to recommend it, for it permits to leave the focuser in its infinity focus position and obtain a far out-of-focus image. (All mechanical elements should be in their normal-use position.)



#7 Tom Conway

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Posted 03 December 2014 - 01:32 AM

Thanks for the advice.  Yeah, the asymmetry bothered me as well.  I don't really know much about optics, but I would have thought that I'd see the same thing on both sides of the center.  I'll take some more images as suggested, out of focus and different exposures.  I think there are some unpainted surfaces, the bottom of the focus tube for example.  I'll post again when I get more data.  Thanks again!



#8 Jarrod

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Posted 04 December 2014 - 12:47 PM

Could there be a scratch in your secondary mirror?  I had that once and it produced an extra spike.  But I think it may have been symmetrical...



#9 Tom Conway

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Posted 05 December 2014 - 12:37 AM

Could there be a scratch in your secondary mirror?  I had that once and it produced an extra spike.  But I think it may have been symmetrical...

Not sure, but I'm starting to think that there is something inherent in the design of this scope.  I noticed this image from another astro-imager (Dan Watt) using the same scope (Orion F/3.9 Astrograph)  http://www.pocketrub...om/ap/?m=201210   He's done some really good work, great images there.  Here's an excerpt from the California Nebula image on that page:

 

PR.jpg

 

Looks familiar...  But I'd be really happy if I could get images half as good as he's posted.   Thanks to everyone for the help!


Edited by Tom Conway, 05 December 2014 - 12:48 AM.


#10 star drop

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Posted 05 December 2014 - 03:41 AM

I see little spikes around bright stars in my 25" reflector but nothing big like that. I traced them down to the screws on the diagonal holder and the slight gap at the junction of the holder's seam.

Is there a dew heater wire in the optical path?



#11 schang

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Posted 05 December 2014 - 11:44 AM

As I can see from the images, it appears that the extra spikes are not symmetric, and there exists some gaps around the halo of bright stars... I'd be checking the mirror holder, and edge of mirror to see if there are any unusual pattern or defects... 



#12 csrlice12

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Posted 05 December 2014 - 02:17 PM

Cracked mirror?



#13 Thomas Karpf

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Posted 05 December 2014 - 02:27 PM

You can see exactly the same thing in the image of the stars on http://www.telescope...ope/p/99602.uts



#14 schang

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Posted 05 December 2014 - 07:22 PM

Look like the Orion pic shows the extra spark on the other side of the main spike, and has weak spike on the opposite side.   Kind of puzzled ...



#15 Tom Conway

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Posted 05 December 2014 - 11:41 PM

You can see exactly the same thing in the image of the stars on http://www.telescope...ope/p/99602.uts

 

Thomas,

 

Thanks, that's the "big brother" of the scope I'm using, but it sure looks like the same design, with the same issue!

 

Thanks for the reply,

Tom




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