Below is a quick 1.5 hrs RGB capture of M51 that I took with my Newtonian. The help I'm looking for is what you think is causing the messy one-sided diffraction spikes on my stars (see enlargement on large star later). It really messes up large stars, but also impacts the shape of small stars.  Here's a 1:1 view of the large star:  and here's the original file: 
M51 with only 90 minutes of integration |
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Is there something else protruding into your light path? I have heard that an OAG stalk placed too far downwards can cause odd star shapes and diffraction spikes. While I don't have experience with a Newtonian, perhaps check light path and collimation. I also ended up with very peculiar diffraction spike pattern once when my velcro dew shield was deformed and was shaped more like an oval than a circle at its end. Lastly, was there a meridian flip in the middle of your data? While it doesn't look like that's the cause, I have had post-meridian flip diffraction spikes not line up with pre-meridian spikes creating a "double-vision" sort of look in the final stack.
Mainly though I just wanted to comment because it's a beautiful picture you have there! Seriously impressive for such a short integration time.
Clear skies,
Ben
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Looks like a tilted secondary. Fresnel rings image would help diagnosis of the ailment.
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A known cause for 'extra' diffraction spikes in Newtonian images are the primary mirror holders that protrudes into the light path a little. A known cure for this is to add a so called aperture ring on top of them, which covers them. I have used one of these in my 6" Newtonian and it has helped. However, when these holders are the cause one would expect the spike pattern to be more regular with respect to the holders, and yours is not. But maybe you should have a look at them, even though something else protruding into the light path seems like a more likely cause in your case. Possibly, it could be that your holders are not protruding equal amounts over the mirror.
Ulf.
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Something is in your light path, and it can very well be the focuser draw tube. My guess is that your spider vanes are parallell and perpendicular to your focuser tube, and not at 45 degrees. People have been known to solve this issue with a hack saw (on cheaper focusers), or invest in a focuser with a shorter travel length.
cs,
Wim
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andrea tasselli:
Fresnel rings image would help diagnosis of the ailment. Thanks Andrea. Just for clarity, what do you mean by a fresnel rings image? Are you talking about an unfocussed image of a star or is there some other "fresnel rings" technique?
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Is there something else protruding into your light path? I have heard that an OAG stalk placed too far downwards can cause odd star shapes and diffraction spikes. While I don't have experience with a Newtonian, perhaps check light path and collimation. I also ended up with very peculiar diffraction spike pattern once when my velcro dew shield was deformed and was shaped more like an oval than a circle at its end. Lastly, was there a meridian flip in the middle of your data? While it doesn't look like that's the cause, I have had post-meridian flip diffraction spikes not line up with pre-meridian spikes creating a "double-vision" sort of look in the final stack.
Mainly though I just wanted to comment because it's a beautiful picture you have there! Seriously impressive for such a short integration time.
Clear skies,
Ben if your diffraction spikes don't line up after a meridian flip, you have cone error. Gets worse the closer you are to the celestial pole.
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If your secondary mirror is too small, you'll likely see this sort of diffraction. I used to see this sort of diffraction when I imaged through my goto dob, which has a small secondary. In general, any rough edge in the light path will contribute to diffraction. I highly recommend an aperture ring to eliminate diffraction from the mirror clips plus it also masks the extreme edge of the mirror and provides a smooth outer edge.
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Andy Wray:
Thanks Andrea. Just for clarity, what do you mean by a fresnel rings image? Are you talking about an unfocussed image of a star or is there some other "fresnel rings" technique? Yes, that is a way out of focus star.
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As Jacob said,the primary mirror clips are causing it... I made an aperture mask to cover the clips...
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I had the same problem and it turned out to be a turned edge on my mirror.
Dave
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Elmiko:
As Jacob said,the primary mirror clips are causing it... I made an aperture mask to cover the clips... I can see some "dark comet tails" that are associated with diffraction from mirror clips. Looks to be more too it though with all those outward radiating beams from the star. If Dave (above comment) is right about the turned edge, then the solution to both mirror clips and a turned edge is an aperture ring.
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Your issue is being caused by 1/ focus tube sticking into OTA , if you fit a m48 adapter this should draw the drawtube out a bit more and if required trim tube do not cut more than 15mm off tube assuming your using a skywatcher 200p .
2/ fit an apeture mask Deepskydad makes a nice mask that covers the edge of primary too .
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I can't tell from your image but could it be a problem for the star you highlight but not right across the field?
Embarrassing to relate but I did one time report that I had a strange diffraction spike problem with my Newtonian - that just turmed out to be because the earth had turned and everything was lower in the sky than I had figured and now there were tree twigs and a TV ariel in line with part of my image
Tim
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Have you got it right by masking or removing the mirror clips in the meantime?
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It's definitely from your mirror clips. You need an aperture mask. I ordered mine from here: https://www.astrogoods.com/masks.imaging.shtmlIt fixed my stars. I have it on both my 6" and 10" Newtonians  |
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Check this out www.backyard-universe.dethe secondary mirror spiders will significantly improve the spikes as well as the collimating stability and the CNC machined primary mirror masks will remove such weird reflection/ diffraction patterns around stars.  original Skywatcher 200PDS  Skywatcher 200PDS upgraded with Backyard Universe Secondary Spider and Primary Mirror Mask
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