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C9.25 star test video

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#1 Petros


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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.
Posted Image

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.

thanks :)


#2 Petrus351


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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.

#3 Mirzam



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Posted 05 January 2013 - 01:30 PM

Welcome to mirror shift.


#4 Darren Drake

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Posted 05 January 2013 - 02:08 PM

The optics seem quite good and I think the Jupiter image proves it.

#5 pstarr



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Posted 05 January 2013 - 02:09 PM

Stand and 6' or so in front of the scope and look at the reflections of the components. Check to see that all the reflections are concentric to one another. By moving toward or away from the scope, you can expand or contract the reflections so they are just touching one another. The secondary holder has some play in the corrector. In the C-9.25 I had, the secondary was not centered properly and needed readjustment to get all my reflections concentric. It wasn't off much but even a little makes a difference. I found performance and collimation much improved after I corrected this problem. Do this test with the focuser empty, no diagonal or eyepiece. Note, I was very familiar with disassembly of these scopes and how to align the various components. Don't try this unless you are sure you can handle it.

#6 rflinn68


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Posted 05 January 2013 - 02:26 PM

I just worked on a C5 that was doing this. I switched the diagonal out to one that was known to be good and it tested the same then on both sides of focus. Try the test again without the diagonal and see what the star test does.

#7 Cepheus Elf

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Posted 05 January 2013 - 05:36 PM

That's a great image of Jupiter and the figure of the optics appear to be very good in the video. I'm not sure about discrepency of collimation between intra and extra focus. My C9.25 does not show this, but I have a 150 f5 newt which shows a similar problem.

#8 Timthelder


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Posted 05 January 2013 - 06:12 PM

I think you may have been trying to collimate on a binary star. I notice another set of rings at 12 o'clock in the first bit, which moves to about 2-3 o'clock (mirror flop) in the second bit.

A tip:
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...

Cheers, Tim.

#9 gonzosc1


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Posted 05 January 2013 - 06:23 PM

the prove is in the image not the star test,,,,, don't over think it!!!! look at the moon in that shot,,its perfect!

#10 Asbytec


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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.

#11 Petros


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Posted 06 January 2013 - 03:20 AM

Thanks for your suggestions and replies,
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.

#12 Tenorotti1


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Posted 10 March 2013 - 11:16 AM

Interesting, thanks for sharing!

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.

#13 nevy


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Posted 11 March 2013 - 12:41 AM

If you use a Crayford focuser & a diagonal wouldn't it be best to collimate with them attached? And if you collimate without them then attach them afterwards wouldn't that make the collimation out ?

#14 jerry10137


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Posted 12 March 2013 - 08:17 PM

What equipment did you use to get that shot of Jupiter?

#15 mgwhittle


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Posted 13 March 2013 - 07:58 AM

It's showing that you are out if collimating on both sides of focus. However, you should be just using one side of focus to evaluate collimating, either side is fine, but pick a side and work with that to get collimating correct. Also, you are defocusing too much, if, what you are presenting on the video, is what you are using to determine alignment. You should be defocusing just enough to show the poison point and center that up in the fresnel rings.

#16 orion61


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Posted 13 March 2013 - 08:55 AM

looks to me like the secondary is not perfectly centered either in the corrector or the corrector itself is not centered.
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.

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