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Hello All, I recently acquired a used, but seemingly well-maintained Meade SN10 OTA with the hope of using it photographically. Both mirrors and the corrector plate appear to be in good shape. Two test images are in my gallery (though the first does not include stars from the actual test). The brighter stars (in particular) in my first actual light frames were flared, some to the degree of being spiked. Afterward, I did find the three mirror front supports to be excessively tight, so I loosened them. I then made sure that the diagonal was as perpendicular to the plane of the focuser (as possible), and re-collimated. The secondary structure from the factory already accounts for any offset. In a third test, I still had flared stars. It does not appear that the mirror bevel is rough at all ... though I have not ruled out an aperture mask. Thinking that flared / spiky stars are typically caused by pinched optics, I am wondering if the primary mirror could be pinched in any other way - perhaps by main mirror clamps / supports? Perhaps there are some here in the community who have experienced this issue and - better yet - resolved it ?? Jay |
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Hi Jay, The Meade Schmidt Newtonian has a spherical primary and a corrector placed near focus. As such there will be the field aberration coma, that will flair off axis stars, even in the absence of other mechanical issues like a pinched mirror. Here is a reference https://www.telescope-optics.net/SN.htm https://www.telescope-optics.net/Mak-Newton.htm#approximated Regards, Dave |
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Thanks, Dave. 1. When you say, "the field aberration, coma," I assume you mean regular old coma, which I understand ... and would think that whatever coma a Schmidt-Newtonian has should be less than a traditional Newtonian reflector, and certainly minimized in the smaller FOV captured by my ASI533MC 11.3x11.3mm sensor. 2. The flaring present in my test images is in the brighter stars across the entire image and (having just looked again) appear to be in a triangular orientation. To me, that indicates a pinched primary. I'll need to remove the primary cell again and look more carefully at how the mirror is supported. 3. I have also just looked at a few other images (here) captured with Meade Schmidt-Newtonian scopes. A few have better-than-decent stars (by today's standards). Unless I have a lemon, I should (hope to) be able discover the problem(s) and correct it. I did review the telescope-optics.net article on Schmidt-Newtonian optics. Nothing described there resembled the degree to which my stars are distorted. I would settle for decent and let BLX do its thing .... Jay |
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| I got pretty good stars from my former 8" Schmidt-Newtonian until I mistakenly took the corrector plate off to clean it. Even though I was careful to reassemble it in the exact same orientation, I thereafter had flared stars. Turns out it's devilishly hard to retighten the screws that hold the corrector plate in place so that pressure on the corrector plate is completely even. Just the slightest over- or under-tightening of the corrector place screws would introduce pinches in the optics. I spent a lot of time trying to re-adjust but I never got it exactly right. |
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| Well, best of luck. It looks like coma, and that the center of the field is toward the bottom left corner in your M51 image. As expected with this field decenter the the coma is much worse toward the upper right corner. Thus a simple adjustment of the fold mirror, to center the field on your camera sensor, should fix the problem... Dave |
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| I'm afraid I have to concur with the previous poster. Despite the heavy processing it has all the hallmarks of field coma. Fact is that a SN isn't that much better than a traditional newtonian in terms of field correction (about 1/2 of a similarly sized newtonian). It was merely devised in order to use easy to make good spherical mirrors so you would still need a coma corrector for any wide field imagery. Collimation must be done properly, which is in focus and possibly at high magnification, after a first pass using the traditional out-of-focus techniques. Needless to say, it is best done the old fashioned way; with an EP and Mk.1 eyeballs. |
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andrea tasselli: I used a coma corrector with my SN8 - a Baader MPCC - although the MPCC had an odd reflection that appeared in my flats and over-corrected the image so I’m not sure I’d recommend that one. |
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Keith Egger: It is got to be a coma corrector (and flattener) designed specifically for the SN, not for Newtonians. |
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| Please upload a single stretched sub and provide a link. |
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andrea tasselli:Keith Egger: When I had one there was no coma corrector designed for the SN as far as I know. The MPCC worked well for the SN except for the reflection in my flats - the stars were round across the chip of my Atik 383L+ at least. |
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Keith Egger: This tails with what I know of SNs. I never heard of such a CC but I can only presume that you could design one, if you were so inclined. Given that SNs weren't much of widespread use even in their heydays I'd safe to assume that none exists today. Obviously you could try and see what modern CCs would do (of the 4-element kind) if you have a spare one laying around. |
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Thank you @hbastro , @Keith Egger and @andrea tasselli . I'm sure that in some combination of your advice / suggestions lies the reality. It is also becoming increasingly clear (unlike the skies) that there is no easy fix - like many of the issues we deal with .... Last night, I was imaged (Test #3) M101 with the SN10 and was able to WBPP 51x120s Light Frames. The stars were as good as I have been able to get ... but the reality is: even at its best (or at least as well as I've been able to tune it), my FRA600 is able to capture better detail. I knew going in that this investment (albeit comparatively minor) had a higher risk / lower reward factor. I was just looking for a scope in the 1000mm range ... and the amount of photons the SN10 captures is impressive. It's probably time to invest in another refractor .... Again, THANK YOU ALL, Jay |
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There is a solution for the field flattner. Here is a Zemax layout for one I designed almost 20 years ago that can be adapted...  Dave |
