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I yesterday finished printing a tri bhatinov mask for my EHD 9.25 and when I mounted it I was shocked that my central mirror/corrector plate is not very centered. When I look at the position of the 4 grubscrews for centering the plate this seems to be on purpose, but I wonder if others have similar non-centricity. When measuring (+/- 0.1mm) I see that distance from the corner of the big holding nut for the secondary mirror to the inner of the tube is: 85.3mm for top 88.3mm for bottom 85.8mm for left 87.6mm for right Should I be concerned? Or is this normal? I have issues with round stars in the corner with my Hyperstar, it felt to me that they got worse when I mounted the Celestron Dew Heater, but I mounted it with the corrector plate in direction of the sky, no way thet the plate can have shifted by 1.5mm (I think). I also tried to compensate for Tilt in the same timeframe so the root cause is not clear. Here's a picture, I measured the Tri Bhatinov mask, it is perfectly centered, it is really the corrector plate/secondary mirror that is off center:  |
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| Hi! I havent measure it, but I think that if you check that your collimation is perfect, that should be not an issue. |
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| never measured mine. I have a 3D printed mask too, but I don't think it matters if the secondary is off centre a bit |
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Agree that if collumation is ok, forget it, from my Tak 180 ED days. There are many ways of measuring but an easy way in PI is just to look at: This is will vary with star field etc etc... there is also CCD Inspector and NINA... but if you have PI this is easy... there is also Aberation Inspector/Spotter under the image analysis scripts. I am interested in this as have been thinking again of a longer FL scope... but I want one I do not have to spend my life tweaking.... good luck and tell me what you find... am in the market sorta... but would probably get the 11". |
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| It’s not a big deal. You can center it with the screws that are on the outer perimeter of the OTA where the corrector plate is. It will help you get the best collimation of the telescope. |
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If that is true, measure all your screw positions and take some good images with PI so you can backtrack... IMHO... perfection is the enemy of good  |
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Honestly, I am not keen to move the corrector plate at all, I only know that my stars are not that good and that I need adjustments (Tilt beeing the obvious one as backfocus is already corrected with HokusFocus), but to start with the basics, for a precision instrument I find it quite strange to see that the plate is adjusted 1.5mm off center. It may be totally normal but still.... it's just a little much for my liking. Michael |
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Agree that if collumation is ok, forget it, from my Tak 180 ED days. There are many ways of measuring but an easy way in PI is just to look at: Fantastic! I didn't know about this tool in PI. Thanks! |
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Just do not let it drive you crazy with try to get everything perfect... results will very with seeing, set of stars, sky glow, etc etc...but you can tell if there is an eccentricity problem in the corners for sure.... Remember the image train is not just the scope... major issues often arise around the columation of the focuser as well as the flatness of the camera sensor... not to mention connections... but if this image is OK, you are good.... Try to use some PI masters ... only works in B&W if you look at several different filters that may tell you if there is a systematic problem... will see it across all filters... normally there is one corner that is worse that most... the little corner issue on mine reverses with the Green filter.... make sure all frames are oriented to the same compass direction... the masters should be OK... but the originals may be flipped... |
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The secondary mirrow is not centered on the physical tube. It shouldn't be. It is centered on the opticl axis of the primary mirror. Which is a floating mirror and will never be perfectly orthogonal with the mechanical tube assembly. It flips back and forth as it pushed in and out of focus. Both the primary and secondary is spherical, so physically being off centered with the tube is not an issue . It can still be collimated. However, as to how on center it needs to be with the in baffle tube fleatener in the Edge scopes, I am not sure. Celestron rotates the secondary to best match the primary mirror, then centers the secondary on the primary optical axis. If you center the secondary to the physical tube, you will more likely un-center the secondary off the primary optical axis. More likely your secondary is exactly where its supposed to be. Lynn K. |
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If that is true, measure all your screw positions and take some good images with PI so you can backtrack... IMHO... perfection is the enemy of good obsessing over image stats can make the experience not fun anymore. I absolutely agree you need to get it good enough visually and enjoy the hobby rather than spending night after good night just tweaking the hardware. |
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Hi Lynn K, thank you for the explanations, I guess I will relax for now and continue with my tilt & collimation efforts. Michael |
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Glad it helped Michael. I own a non-Edge C11 & C9.25, and a Edge 8. I have also owned non-Edge C8, two C6 and a C5. I recenter all the secondaries except the C5 & Edge 8. Celestron does not do/did not do a great job centering the secondary on the non-Edge scopes. Basically because is is not that crucial for visual and can still be well collimated. However, I think the addition of a flattener in the baffle tube required better centering from Celestron. And as far I know, they do a better job on the Edge. When checking the secondary centering, put the measuring tools away. Do it visually. STARIZONA use to have a good method tutorial on ther site. Not sure if they still do. Set the scope level. A table will do. Brace you head on the back of a chair back, about 6 to 8 feet away. Look at the reflections of the secondary in the primary mirror. Are they concentric with the outer edge of the primary? If they are, then the secondary is centered. If the secondary is out of collimation, is can confuse the reflections. I always did this after removing the secondary. That way the double refection coming off the secondary didn't confuse me. The Fast Star equipped scopes makes that easy. I also used a cardboard tube mounted on a tripod to help keep my eye centered on the baffle tube. Lynn K. |
