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Equipment Discussions >> Cats & Casses

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Petros
member


Reged: 11/04/06

Loc: Germany / Greece
C9.25 star test video new
      #5606398 - 01/05/13 11:36 AM

Hello,

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.

thanks

Petros


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Petrus351
sage


Reged: 12/17/12

Loc: Spain
Re: C9.25 star test video new [Re: Petros]
      #5606611 - 01/05/13 01:15 PM

Hello:
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 its 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 Im wrong.

Edited by Petrus351 (01/05/13 01:19 PM)


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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: C9.25 star test video new [Re: Petrus351]
      #5606638 - 01/05/13 01:30 PM

Welcome to mirror shift.

JimC


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Darren Drake
Carpal Tunnel
*****

Reged: 10/09/02

Loc: Chicagoland
Re: C9.25 star test video new [Re: Mirzam]
      #5606701 - 01/05/13 02:08 PM

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

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pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: C9.25 star test video new [Re: Petros]
      #5606703 - 01/05/13 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.

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rflinn68
Carpal Tunnel


Reged: 03/09/12

Loc: Arkansas
Re: C9.25 star test video new [Re: pstarr]
      #5606728 - 01/05/13 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.

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Cepheus Elf
member


Reged: 08/01/10

Loc: Rainy, Cloudy Lancashire UK
Re: C9.25 star test video new [Re: rflinn68]
      #5607001 - 01/05/13 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.

Edited by Cepheus Elf (01/05/13 05:40 PM)


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Timthelder
super member


Reged: 06/12/12

Loc: Bowling Green KY
Re: C9.25 star test video new [Re: Cepheus Elf]
      #5607086 - 01/05/13 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.


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gonzosc1
sage


Reged: 11/08/12

Loc: South Carolina
Re: C9.25 star test video new [Re: Timthelder]
      #5607106 - 01/05/13 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!

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: C9.25 star test video new [Re: Petrus351]
      #5607164 - 01/05/13 07:12 PM

Quote:

I think its due to the optical axis being bent
Please correct me, if Im 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.

Edited by Asbytec (01/05/13 07:46 PM)


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Petros
member


Reged: 11/04/06

Loc: Germany / Greece
Re: C9.25 star test video [Re: Asbytec]
      #5607737 - 01/06/13 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.


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Tenorotti1
journeyman


Reged: 01/03/11

Re: C9.25 star test video new [Re: Petros]
      #5723797 - 03/10/13 12:16 PM

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.


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nevy
professor emeritus
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Reged: 02/07/12

Loc: UK
Re: C9.25 star test video new [Re: Tenorotti1]
      #5725102 - 03/11/13 01:41 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 ?

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jerry10137
sage
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Reged: 01/26/13

Loc: Texas, USA
Re: C9.25 star test video new [Re: nevy]
      #5729024 - 03/12/13 09:17 PM

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

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mgwhittle
Pooh-Bah
*****

Reged: 08/24/11

Loc: Chattanooga, TN
Re: C9.25 star test video new [Re: jerry10137]
      #5729881 - 03/13/13 08:58 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.

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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: C9.25 star test video new [Re: mgwhittle]
      #5729994 - 03/13/13 09:55 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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