C9.25 star test video
Posted 05 January 2013 - 11:36 AM
I got my first SCT Telescope last week, a used C9.25, and while collimating with a star I took a video.
What I noticed is that when I collimate, the other side of the focus (either in or out) is not perfectly collimated. This can be seen in the video below, where one side of the focus seems out of collimation while the other seems ok.
C9.25 Collimation Video
Here is an image of Jupiter I took afterwards in the same seeing conditions.
I've seen that other SCT users have the same issue.
Any thoughts on why this is happening?
One thing I must mention, I did the collimation with a diagonal , as I do not currently have an SC to 2" adapter.
Posted 05 January 2013 - 01:15 PM
It happens the same to me, only when I apply too much weigh to the scope, with barlow, filter wheel, flip mirror and camera. I think i´ts due to the optical axis being bent.
I once read, that it can be also due to misalignment of the main mirror-secondary-and baffle.
Please correct me, if I´m wrong.
Posted 05 January 2013 - 02:08 PM
Posted 05 January 2013 - 02:09 PM
Posted 05 January 2013 - 02:26 PM
Posted 05 January 2013 - 05:36 PM
Posted 05 January 2013 - 06:12 PM
Always collimate straight through the SCT. When focusing, make your last adjustment by turning the focuser knob counter-clockwise, (this pushes the mirror up thereby 'holding' the weight. Use the focusing knob for only doing a 'rough' focus.
Next by an extremely good quality dual-crayford focuser (such as a moonlight for example.) Also when imaging, do not use a diagonal. It just adds another potential problem.
And as someone I think already stated, when you get to hanging a lot of weight off the back of the scope, (camera, filter wheel etc.) it can 'bend' the light path (focuser sag) making it appear as if collimation is off.
I hope this helps...
Posted 05 January 2013 - 06:23 PM
Posted 05 January 2013 - 07:12 PM
I think i´ts due to the optical axis being bent
Please correct me, if I´m wrong.
You are not wrong, you are correct. Paul and Tim seem to be spot on, too. There is some artifact at the top, might be a binary. But, I don't think the latter is the source of the problem.
I've seen this effect, too, and aligning it helped. The optical axis is not coincident with the movement of the focal plane. They are not orthogonal to each other. At focus it might look fine, but as you pull back from focus the optical axis "falls" toward the bottom as evidenced by the shadow offset. Its possible added weight can cause this misalignment.
There may be arguments whether or not this is a condition worth fixing, many already have mentioned it. But, it should induce a tiny bit of coma. Personally, I would align the error. I'd bet your in-focus stars flare in the direction of the shadow offset outside focus.
Posted 06 January 2013 - 03:20 AM
I will try again without the diagonal once I have the adapter. I will also check the centering of secondary holder in the corrector and try to inspect the reflections from in front of the scope.
Posted 10 March 2013 - 11:16 AM
I spent an hour last night trying to fix this on my scope, ha, ha. Sounds like I may have been collimated as the images resembled your video. I think my diagnol may have been the cause of this. Great shot of Jupiter!
I'm going to recheck the next stary night to see if I get the same without the diagnol.
Again, thanks for sharing.
Posted 11 March 2013 - 12:41 AM
Posted 13 March 2013 - 07:58 AM
Posted 13 March 2013 - 08:55 AM
I had a scope doing the same thing. If it is a used scope it is possible the previous owner had the corrector out and it is not shimmed perfectly, could be the reason he sold it, I can see the flair on Jupiters Moon, your not Crazy.