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I've always suffered from excess fuzzy star diffraction spikes with my Skywatcher 200PDS Newtonian. I think I've now got my collimation/alignment quite well sorted as the spikes are no longer lopsided. My two questions would be: 1) What mods could I do to my OTA to reduce the spikes? Flocking, painting, primary mirror ring, baffles? 2) Are there any techniques in Pixinsight that people use to "de-fuzz" stars? Thanks for any suggestions. You can see the effect on the two large and some medium stars in this crop below:   The Bubble (Now with 10 hrs integration) |
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It's hard to pinpoint 1 method.. as in pixinsight, there are a lot of roads that lead to the same end result. I would usually use Starnett++, removing all the stars. - Then use sharpening methods on the star mark layer only, before combining them again. usually reducing their size aswel, to make the nebula pop more. this might be worth a look.. something I spotted recently, but haven't had a chance yet to use it. |
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| The issue you have cannot be cured post-fact. It needs to be sorted upfront by a branch and root analysis of what gives you the weird triangular shape and vaguely lopsided appearance of the PSF. Do take the Fresnel rings image we discussed a long while ago and let's see what's wrong with it. |
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andrea tasselli: Andrea, I don't see the weird triangular shapes as you put it and my PSF looked round when I generated it. Would you be able to point me at the artefacts that you are talking about in this version of the image please. I will try and take the fresnel ring image, although I have struggled to do so, and see if I can identify the issue. FWIW: I thought I had generally countered the triangle shaped stars by getting my secondary better aligned (i.e. evening out the diffraction spikes from the primary mirror clips). |
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FWIW: I can't get the fresnel rings with an out of focus star, but I did get the below. It clearly shows the focus tube interfering, but does it say anything about my collimation? |
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Any "edge" that the light encounters in the OTA will contribute to diffraction, which has the effect of increasing star fuzz. A big culprit is mirror clips, which gives rise to "dark comet tails" eminently from the star. I can easily see this effect in your image. The cure for this problem is to replace the mirror clips with an aperture mask, which also hides the extreme edge of the mirror that may be slightly turned. Another major culprit is an undersized secondary mirror, which I can testify to from personal experience. If the incoming light cone from the primary encounters the edge of the secondary it will create even more diffraction (star fuzz). The appropriate size for a secondary is one that fully illuminates your sensor (i.e. the light cone never touches the edge of the secondary even when off-axis) For me, I have a 90mm secondary on a 250 F4 quattro with an APS-C sensor (QHY268M) and it gives very even illumination. Don't worry about any contrast loss from a bigger secondary obstruction. I can promise you that contrast loss from the secondary is one of the last possible factors affecting image contrast. Although as a general rule, keep the secondary obstruction at or below 40%. Another possibility is the focuser tube. If it's in the light path, it will create diffraction. All of this of course assumes your optics are of decent quality. As for collimation, it's imperative you get this right. The gold standard is a catseye autocollimator and cheshire with a hotspot to replace the white donut at the center of the mirror. FYI, those white donuts aren't always at the exact center. Mine was a couple of mm off. |
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Andy Wray: The "weird" triangular kind-of shape is seen on mid-sized stars easily but minding that "triangular" is something I use for lack of better words, as it is a very very bloated triangle. As the "fuzziness" is on one side only I'd recon is due to a combination of un-simmetrical diffraction, e.g., the focuser tube protuding into the light path, and mild miscollimation which can be arrived at by looking at your image above as the CO appears slightly off-center. The cure for the focuser tube is to shorten it unless you want to replace it (the focuser). As for dialling in the collimation, as others have said, get the Cateye collimation tools and get familiar with the whole procedure. Best money ever spent. DO NOT relay on the laser beyond the initial step of centering the secondary onto the primary. |
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FYI: I checked my scope and the spider vanes were not of an equal length. I fixed that and did as thorough a recollimation as I know how to. The results are below and I am quite pleased. It's by no means perfect, but is heading in the right direction. N.B. This is pretty much the full field of view (just the usual tweak to take out very minor stacking artefacts). Just a quick stack of a total of 2 hrs RGB taken on a cloudy night with minimal processing so that I could see the star shapes.  |
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| As far as processing techniques to manage funky stars.... try separating the stars from the rest of the image and process separately then combine. The less you stretch stars, the less you will notice these kinds of artifacts. I now separate stars and target as part of my routine and it makes a tremendous difference. |
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andrea tasselli:Andy Wray: Andrea, Thank you for your help with this. I'm working hard on my collimation skills, but have also decided to replace the focusser to see if it helps. That drawtube extends more than one inch into my OTA at prime focus which can't be a good thing for my stars. I'll see how that goes before fiddling any more with my OTA. Looking at the above, the drawtube is much more of an issue than any mirror clips ... do you agree? |
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Andy Wray: In terms of pure diffraction effects, this is certainly so. I don't consider the clips (which I have along pretty much everyone who owns a newtonian) to bring much of an effect if properly blackened and of the right size, which they really should be. |
