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Hi all, I've encountered two perplexing issues with my PlaneWave CDK14, that I would love some input on. I am chatting with Bill Dean over at PW on this, and while we both try to get to the bottom of it, I would love thoughts from people on this community, especially if you have seen something similar before. For each, I will describe the situation I encounter them in. 1. The diffraction spikes pointing to the bottom of the image on large stars is wavy; this seems to be the case with the majority of subs. Note that this isn't the side where the dew heater wiring to the secondary runs down the vanes. Also, nothing from the light shroud seems to be interfering in this (or diagonally opposite) area either.  2. Orthogonal issue: Without the light shroud, star halos seem skewed (or there is extra halo) towards the left / bottom-left and the spikes seems offset from the center of the star in the opposite direction. The stars and the diffraction spikes become perfectly normal with the light shroud back on. I am guessing it is some kind of internal reflection, but it beats me as to what / from where it could be that the shroud seems to rectify... Could it be a tilted baffle on the primary, although in that case why would the shroud make a difference?  Any thoughts on this would be much appreciated. Thanks, Ani |
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| The bottom spike isn't wavy, it's double. As for the light shroud my guess is that it slightly vignettes the marginal rays so without it you get the flare, with it you don't. |
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Thanks for your prompt thoughts, @andrea. Regarding #1, you are correct, they do seem like double spikes. I believe diffraction spikes are oriented at 90 degrees to the vanes that cause them, aren't they? In that case, perhaps it is because the two vanes (that are perpendicular to the bottom spike) aren't perfectly oriented at 180 degrees to each other? Or perhaps the wiring down one is skewing it somehow...? Regarding #2, what is the implication that the shroud vignetting the marginal rays mitigates the flares? i.e. what is the underlying cause in your opinion? Thanks, Ani |
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Regarding #1, you are right, the diffraction spike is the complex conjugate of the real image with same periodicity, so at right angle from each other. Often a slight twist or a slight bend in one arm can cause the issue, as well as running a cable down the spider's arm. As for #2, and here I'm speculating as I don't have access to the real thing, is that some of those rays get scattered from something such as the edges of the secondary. with the light shroud you are preventing this light to reach the point of reflection/scattering. |
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Arni, You are correct that the diffraction spikes are formed perpendicular to the straight edge that forms them. Double diffraction spikes from the spider are most often caused by the spider vanes parallel to the spikes pulling the secondary very slightly off-center so that the vanes causing the spikes are not perfectly parallel on either side of the secondary. That causes two spikes and they will often appear equally double on either side of the star. It looks like yours is double only on one side but you should test that by imaging a really bright star so that you get a bright, large spikes. You might also get a similar diffraction pattern if one or both of the vanes perpendicular to the diffraction spike are slightly twisted. In either case, it doesn't take much of an error to cause the problem, which makes it hard to see when you are at the scope looking at the spider. Regarding issue #2, I doubt that that pattern is being cause by a stray reflection. Where in the field are those stars? That looks more like a field effect rather than a stray light issue to me. John |
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andrea tasselli: Andrea, I'm not sure what you mean when you say that "the diffraction spike is the complex conjugate of the real image with same periodicity". To be clear: The diffraction pattern at focus is a component of the point spread function for an obstructed aperture. The PSF is computed by taking the Fourier Transform of the complex pupil function, which includes both the aperture function and the phase distribution over the pupil. You then compute the square magnitude of the result to get the PSF in frequency space. The pattern scales with wavelength and as I recall, the scaling in frequency space goes as lamda/f which you can then rescale to distance in the focal plane. For me, scaling is the slightly messy part and I often have to go refresh my memory with a trip to Gaskill's or Goodman's book to get it right. You are correct that a straight edge (or feature) will diffract light at right angles and at a scale inverse to the size of the feature. The bigger the feature, the less spread in the diffracted light and the smaller the feature, the less light is diffracted but it spread much more further. Thin spider vanes are desirable because the spikes are fainter, but unfortunately they are also longer. John |
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| @John Hayes@andrea tasselli Thank you both for your continued thoughts on this. Regarding #1, we did a "strum" test on the vanes, and one of the vanes (the one with the wiring) perpendicular to the double spike in question was much tighter than the others. I wonder if that is potentially causing either an offset on the mirror, or somehow making it less-than-perfectly-parallel to its opposite vane? Regarding #2, @John, this is an odd one. Larger stars all across the field exhibit this issue, doesn't matter whether it's TL, TR, BL, BR or even the center. And the halos are all skewed towards the left / bottom-left direction, albeit sometime more pronounced on the left side of the image compared to the right. Here's an example of the beehive cluster. Could this happen if the secondary was somehow at an angle to the primary? Again note this all goes away as soon as the shroud comes back on...  Here's a star at the center of the field exhibiting the same issue:  Another data point that I want to add is that SkyWave shows that my collimation is really good:  However, the spacing isn't particularly ideal:  And this is what an example of the defocused star in this case looks like:  Thanks again for all your thoughts, Ani |
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| Ideally secondary support should be made from a single block for medium apertures and never made with thin vanes holding the secondary mirror assembly in position by tension such as this design. This said I don't think the issue in itself is due to tension directly but maybe by induced twisting of said vane because of the tension. Bigger vanes should get rid of the problem altogether and allow a routing of cabling without intruding into the light path. As a test you could release the tension a little and see whether the situation improves. |
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Ani, Did you use SKW to look at the distribution of aberrations? I see what appears to be coma in your intra-focal image. Did you take an extra-focal image as well? I'd have an easier time reading that. Either way, that out of focus image does not show an aberration free system. John |
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Hi @John, how do I look at the distribution of aberrations in SKW? I did take 5-6 images each of around two dozen stars across the sky in order to get to around 50-70 seconds of total stacked time per defocused star, and they all seem to show similar results (with a slightly more whitish partial ring towards the bottom-left). All of them, including this, are extra-focal (on the Optec Gemini, typically around 89000 steps, when the focus is at ~39000 steps). Yet, the collimation scores always seem perfect (~9.4-9.6), as does astigmatism (~9.0), but the spherical/spacing aberration is always off (~6.0). Thanks, Ani |
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I was looking at a few other CDK images of larger stars and I definitely see some of them having a dual diffraction spike; for example including in your superb image of the Horsehead Nebula (https://www.astrobin.com/full/1xk09q/C/) with your CDK20 @John. Would my hunch be correct that in your case, the vane with the dew heater wiring was perpendicular to the vertical axis of this image (the dual-diffraction spike is towards the top)? If so, I wonder if this is a “feature” not a “bug” of the CDK vane design (where one vane carries the heater wire and is different from the other three in thickness)!   Thanks, Ani |
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I think we finally have a better idea of what is going on with the dual diffraction spikes. @Samuel A Miller helped measure the vane thickness on mine and a couple of other CDK14s which don’t appear to exhibit the issue. We found that in mine, the vane that carries the dew heater wiring is 2.70mm thick, while the other ones are 1.6mm thick. In the scopes without the issue, the wiring vane is ~1.6mm thick and the other 3 vanes are ~1.2mm thick. So a pretty significant delta in the thickness is likely what is causing the issue. @Wei-Hao Wang I know you were curious about this too, and I would be interested to hear if you can measure the vane thicknesses with your CDK20 and see if they are grossly different. The reason for the second issue is still at large, but at least the first one seems to have an explanation. I’ve sent this info to Bill Dean over at PW, and will share what I discover as to why there is difference across the CDKs for the wired vane. Ani |
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| Very odd! Did they use the wrong part on yours? |
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Ani Shastry: Hey Ani, My CDK14 shows the same "wavy" behavior on very bright stars. If you find something out about this, let me know. -Bill |
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I had the same problems with my CDK14: Split spikes and elongated stars/halo caused by the shroud. 1. I talked to Bill at PW 2021 for that point too. I had adjust the spider vanes. The screw became lose. Noticed also the effects of the "thick cabled one". 2. The shroud is causing serious trouble. Avoid it when ever possible. Since here not avoidable, we have modified Rouz' basic idea. We are using this solution on 3 CDK14. Also always keeping fans on, when using shroud is used helps. CS Rudiger     |
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Hi Ani, Like you, I've just started to use the CDK14. So far I have not seen the double/wavy spikes yet, but will test this out on a very bright star. The second problem is something I have noticed as well, although not as significant as your examples. When I had my shroud on, I noticed somewhat of a double set of spikes, although now I dive deeper into it, it looks like I have one/two extra spike and with stacking before/after meridian, it copies to both sides. I use the Rouz Astro spreaders, similar to the ones Ruediger is showing, so one would expect this is not caused by the shroud. When taking off the shroud, similar to you I see a somewhat eccentric halo/blob in the other direction, while the original spike is still somewhat present, further proving the point that the shroud does not cause the spike. Under better focus conditions/seeing, the image becomes more symmetrical. But overall star dimension is larger than under similar conditions with shroud. Here are images of the various situations:  So it looks like it is a choice between using Shroud (tighter stars, extra spikes) or no Shroud (less spikes, more bloated stars). On a single image the effects don't seem to be strong, but when stacking a series of images, the differences become more pronounced. As to possible explanations, and this is just hypothesising, two thoughts came to mind: 1. On the primary mirror is an aperture ring to cover the edges of the mirror. On my scope this ring is not flat at all. I've brought this up with Planewave (through the dealer) and they say it is just cosmetics. I checked with another CDK14 in the observatory (few years older) and that one has the same bulging aperture ring. It is hard to understand why in an expensive high-precision optic instrument one would screw in a bulging aperture ring, but that's a different matter. Would it be possible that this has something to do with the halo/eccentric 'blob'? Do you have the same bulging aperture ring in your scope?  2. Why would the eccentricity show up when you take off the shroud? If it is some kind of reflection, the shroud would absorb reflections going outward, not inward? But, depending how exactly you put on the shroud, it also blocks the 1/4" screw holes in the front ring. Which made me think, is there any way these 1/4" holes can cause any trouble? In many truss designs the front ring is just solid or has very wide holes. Is there any chance these 1/4" screw holes can become little point sources of light, and causing a weird kind of interference? If so, could this cause these halos/blobs, spikes, etc? I know, it is a far-fetched idea, but just wondering if people ever looked at this. I guess we could just tape them off to test. Anyway, I am very interested in this discussion and look forward to hear if we can somehow find some answers to the issues identified. Willem Jan |
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Bill Long - Dark Matters Astrophotography:Ani Shastry: Hi Bill, I think we've narrowed down at this point that the split spikes are very likely coming from different vane size on the wire carrying vane. I'd be curious to see if you can have someone measure both your CDK14 and CDK20. But how to fix that is still up in the air, and I will let you know what I hear from PW. This is what the wire-carrying vane (left) and the others (right) look like on my CDK14. That's not good.  Ani |
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Ruediger: Hi @Ruediger, Thanks so much for your input, I really appreciate it. Regarding #1, were you able to address the thick cabled issue, or are you living with it still? Regarding #2, I do have the Rouz Rods, but I had since taken them out (due to arcane L-350 balancing reasons) that I need to put back on at some point. Your rods seem longer than his, and they likely do a much better job I would assume. I unfortunately run into issue 2 I described earlier, where the star halos are skewed if I don't use the shroud. That's quite unacceptable, and so I need to use the shroud. I do run the fan always when imaging as well. The bigger issue that I have seen is the shroud protruding into the light path at the front of the scope... I wish PW had made some kind of a screw or clamp that held the light shroud back from the front aperture.  Thanks, Ani |
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Willem Jan Drijfhout: Hi @Willem, Thank you so much for your response. It's good (and sad) to see another person seeing skewed halos with their CDK14. Both your points are actually excellent callouts. @Samuel A Miller, would you be game for seeing if my CDK14 has the bulging aperture ring (picture in Willem's post) on mine as well? Agreed that this seems in a QC issue on such high precision instruments.The second point there is a very interesting one Willem! I do wonder about that... It would be an interesting experiment to see if somehow taping those front holes makes a difference. Let me see if I can do some experiments in this area. Thanks so much again for your input Willem, really appreciate the thoughts! Ani |
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Willem Jan Drijfhout: I have discussed this effect with PWI Bill also in 2020. I called this effect "daemon stars", since the look like having to horns. We found out, that it directly proportional to temperature difference between M1 and air. You can reduce the effect by having the fans running and having equal temperatures. CS Rüdiger  |
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Ani Shastry: #1 No. Since I have painted my vanes with Mousu Black, which quite thick, the problem became less prominent. A proper and precise vanes alignment reuced the problem almost to not visible at all. After painting not visible any more, resp other problems/limits/restrictions are dominating. #2 When Rouz was working on his solution, we did parallel our tests. He published his solution with different rods. We use some which do not require guiding. the have 1/4" joints. We also use carbon and aluminum ones. For a while I had used a generic plastic light shroud for 16" SCTs and wrapped that around. This avoided also the problem, but it is too heavy. |
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Willem Jan Drijfhout: There are a couple of other considerations on this issue. One relates to the mount being used, the other to modern processing methods. 1) For those using Planewave mounts (my 14 is on an L-350), you will be aware that although excellent in almost every way (more so than their optics) they have one Achilles heel and that is wind. They simply do not like even moderately windy conditions. The shroud will act like a sail in those conditions and make that problem even worse. For that reason and after a bit of shroud/no shroud testing, my shroud came off after a few days and now lives in my accessory box. 2) Bright star aberrations are not great but given the fact that it is almost universal practice these days to remove stars early in the processing and then replace them toward the end, it has become much less of an issue than it otherwise would be. From my point of view, this is only an issue if it is an indication of a severe enough optical problem that it might also affect the object of interest. That will vary with the user and the degree of the problem. Since stars are point sources anyway and are almost never the subject of the image and since diffraction spikes are artifacts of the optics, I do not feel like modifying either of them after the fact is an issue as long as color and a reasonable brightness/size ratio is maintained. FYI, I do see the double spike issue to a very small degree but in my case it is on only one spike, not a pair (probably because it is minor). Looking at other CDK images I have found this is quite common. Sadly, I suspect that this is yet another issue that persists because of "attitude" at PW. As such, it may take an aftermarket solution since PW seems uninterested in dealing with these things (as has been shown even more clearly by the focuser/fans/heaters issues). Too bad since dealing with these issues could make a good scope into an excellent scope with minimal effort. |
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Bill McLaughlin:Willem Jan Drijfhout: For me it was a big issue, since the "horns" are always pointing upwards in the tube. That means during the night -and filter-, the position of them moved. In the end had beautifully colored tiny hedgehogs. 🤣😂 |
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As far as I know, I was the first to point out the problem with the Planewave shroud with this post early in 2021: https://www.astrobin.com/xfedon/I/#rO. The shroud will only affect stars that are off axis and you'll only get two full sets of extra spikes in the corners. This is simply a diffraction problem due to the shroud creating 4 straight edges that can intercept the off-axis marginal rays. None of the other components in the telescope will cause the extra-set of spikes at 45 degrees to the spikes from the secondary spider. The fact that one of the spider vanes is simply thicker than the others, will NOT cause double spikes! It will cause the diffraction spikes (on both sides of the center ) perpendicular to the thick vane to be narrower. In this case, narrower means that the zeros in the diffraction patter will not be as widely spaced as in other set of orthogonal spikes. On the other hand, you will get double spikes if one of the vanes has a wedge shape as appears to be shown in Ani's photo of the wire caring vane. In the CDK20, the wires are flat and bonded tightly to the vane so that there is no wedge. Based on what I can see in Ani's image, they are doing something screwy (and unadvisable) in the CDK14 with the design of the spider. A wedge shaped vane will indeed produce two diffraction spikes. John |
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Thanks so much for the clarity on that @John. Not all CDK14s seem to have the wedge shaped vane but mine definitely does; I can see that @Ruediger's CDK14 also has the same vane structure for the wire carrying vane, and this looks exactly like the underside of mine with a groove running down the middle and I assume there is a bump like in mine on the opposite side of that vane. @Bill Long, would be interesting to see if yours does as well since you get the dual spikes. @John, what are your thoughts around @Willem's description about the 1/4" screw holes on the front rim, or the wobbly aperture gasket by the mirror, potentially causing the lopsided star halos that I described in problem #2? Thanks, Ani |
