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MoonWhen using DSLR with Newtonian (130mm/1000mm), I am not able to get perfect focus.

However, when I use an eyepiece, I am able to get crystal clear views. So, I think collimation is okay.

The focusing tube is not short. I am able to focus upto a certain point using the DSLR. But the best focus that I am able to get is not crystal clear.

Any suggestions? Thanks!

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    $\begingroup$ How are you connecting the camera to the scope? Are you using a T ring or are you attaching the camera to the eyepiece? $\endgroup$ – Dr Chuck May 24 at 9:17
  • $\begingroup$ Also, if possible please send a photo of your setup showing how the camera is connected. Also please upload an image of the best focus that you can get; there may be things in the image that help to debug the problem. Also mention which kind of DLSR you are using and how you are doing the focus; are you looking at an LCD screen on the camera, or through the viewfinder? (for example this one has both). You need to add a lot more detail if you 'd like a good answer. Thanks! $\endgroup$ – uhoh May 24 at 12:47
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    $\begingroup$ Just putting the DSLR on the focuser works for me, as it is stable. But I was getting severe coma aberration. Then I got to know about backfocus. Then after some adjustment (increasing the distance between CMOS sensor and the corrective lens in the focuser tube) the comas weren't that severe. I am posting a moon picture (which I could focus the best). $\endgroup$ – Mathematicie May 24 at 16:16
  • $\begingroup$ Thanks for the edit, information and image! The Moon's terminator is pretty sharp near the bottom and very fuzzy near the top, I'll bet that the focal plane of the telescope is not parallel to the plane of the camera's image sensor, and you'll have to figure out what more to tilt. If you have great focus over the full FOV of an eyepiece with a large entrance pupil (as big as the camera's sensor) then maybe the tilt of your diagonal is not a problem, but if it's got adjustments on it it might be something to visit as a last resort. $\endgroup$ – uhoh May 24 at 22:35
  • $\begingroup$ Instead, I'd take a good hard look at the way the camera is mounted; is there any looseness in the focusing mechanism or the tubes that the extra weight of the camera body might cause to bend under gravity? Also, once you get any mechanical issues resolved and your camera's imager plane parallel to the telescopes focal plane, remember to take many images and look for the best one (e.g. lucky imaging) since astronomical seeing can make any one image more blurry in some areas than others. $\endgroup$ – uhoh May 24 at 22:38

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