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Adding some more data to this thread. I was able to point my scope at Arcturus last night, and here is what I got. It's definitely a split-spike in one direction, that doesn't seem to occur to the other spikes. L filter:  L-filter (45 degrees rotation of imaging train):  Since it occurs in the same vane direction, I don't believe it's anything to do with the image train. And the issue actually looks very curious with the R-filter; it almost looks like there are alternating bands. This over-lapped "phase-shifted wave overlap" like effect also seems to happen with the other vane directions, but those just seem to be tighter. Does anyone have any speculations on what this could be?  Ani |
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Oh no, you also have the donut grid pattern/microlens diffraction. I was wondering if these scopes would do that. Many threads about it already across many different configurations, nobody seems to have a fix. I am still not aware of how it could be possible for a diffraction artifact to not affect both spikes. If it’s asymmetrical, my money is on some other kind of collimation or focus/tilt issue. But, not very much money, because I’m not an expert in diagnosing such issues by any means. |
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Ani, Each spider vane is roughly creating it's own diffraction spike. I say "roughly" because with a spider vane that only goes through half of the aperture, a small amount of diffraction "leakage" might get through to the other side. What you are calling waves along the pattern is just the form of the sinc^2 diffraction irradiance pattern that forms the spike. If you imaged with a narrow band filter, you would see very distinct zeros along the pattern. With the red broadband filter, the zero is "muted" by the wavelength spread, which is why it's not very visible with the Lum filter. The full spectrum does not show such distinct dips in the irradiance, which only become more visible when you add in the color. The reason that the dips appear "shifted", or more correctly at an angle for that one vane is that there is a wedge shape on that vane. It could also be explained if there are two very closely spaced vanes at slightly different angles with different thicknesses. The key point is that the physical thickness of the vane is what determines the spacing of the zeros in the Sinc^2 function. John |
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John Hayes: Mine definitely has the wedge shaped vanes and does show the double spike issue although it is a bit more subdued than some others I have seen.  This does make me wonder if one could create a "snap on" or "slide on" 3D printed cover for the vanes that would make them rectangular at the cost of slightly thicker. I would give that a try except that my printer is at home and the scope is at a remote site. The vane with the wires might require a customized version, of course. Anyone up to try something like that? It is sad to have to modify systems that are made by "professionals" and cost this much, but that is another issue.  |
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Oh no, you also have the donut grid pattern/microlens diffraction. I was wondering if these scopes would do that. Many threads about it already across many different configurations, nobody seems to have a fix. Yeah, I do have the microlensing issue, but I have a hunch that it is potentially addressable by swapping the direction of the filters. The bazillion threads on CN never came to any real conclusion on it. I am swapping out my filter wheel and filters later this month, and will try orienting my filters in the opposite direction. Conventional wisdom on the interwebs seems to say to orient the reflective side towards the camera, which is what I have currently, but that logically never made sense to me as with the filters being so close to the sensor, any reflection off it would bounce right back off the reflective side of the filters again onto the sensor. Then again, I almost never aim at super bright stars, so I don’t think it’s an issue in the practical case. |
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John Hayes: Thank you so much @John. Your detailed explanation makes a lot of sense to me. |
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Bill McLaughlin:John Hayes: Yeah, agreed that this is ridiculous for such expensive and high quality instruments. @Bill, I am going down to SRO (where my scope is at) at the end of the month. I don’t have a 3D printer, but I will see if I can request a friend who does have one to print out the parts for me. I also don’t mind asking the techs to do something more janky in the interim as a pure test, like adding a large cardboard + at the front aperture just to see if that mitigates the split spike issue. Edit: I have been thinking about that groove at the bottom of the wire-carrying vane. I wonder if that channel was actually meant to carry the wires, but PW ended up manufacturing it incorrectly for the CDK14s (putting it on the opposite side of the vane) and so the wires unfortunately can’t be passed underneath the vane in that groove as it was intended… So now they just pass it on the other side and wrap it up as tight as they can still causing a wedge. I would be curious to hear from CDK17 and CDK20 owners if they see a groove on the bottom side of the wire carrying vane. |
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Actually, I think I have a simpler explanation for my "edited text" above. The specs for the CDK actually show that the groove in the vane is an open groove through which the wires are supposed to pass through: In some cases, they do a good job of actually passing in the wires in that channel, and then "tape" it up mostly nice and flat. This is another CDK14 from SRO:  And in other cases, like mine, @Ruediger, and others, they just did a shoddy job with passing the wires on the side and creating an unnecessary "wedge" when taping:  |
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Ani Shastry: Interesting. If the cause, the good news is that a fix should not be hard. They should maybe have just made the vanes thicker and made the one for the cable a sandwich with a rectangular hole/groove. In fact, I have a small manual machine shop at home so I may just make new vanes at some point. The main problem is that I won't be able to measure until June and would not be able to install until the next June.  I do wonder when John says "wedge" which dimension is he referring to? |
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I think he means the "bump" that runs along the longer axis of the vane on the flat portion from the secondary to the outer edge. The other axis (front to back of the telescope) is also a "wedge" technically as it also tapers from the secondary to the periphery of the aperture, but all CDKs have those shaped vanes. But I will leave it to @John to clarify.    Ani |
