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Odd Shadow In Collimation Image

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#1 briansalomon1

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Posted 25 November 2021 - 04:51 PM

I've owned only refractors for many years and recently bought an Obsession 15UC. Collimation appears to be fairly close but I have an odd shadow in the barlowed image that I'm wondering about. I suspect it may be the center marking on the secondary.

 

When I collimate during actual use, I'll follow the advise from other users to hang the paracorr/eyepice on the focuser for best result, but I feel like the collimation is solidly on in the pictures, it's just the shadow I'm wondering about.

 

https://www.cloudyni...uc-collimation/

 



#2 Vic Menard

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Posted 25 November 2021 - 05:11 PM

Looks like a dust bunny to me--try rotating the laser and/or Barlow and see if the artifact moves.


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#3 briansalomon1

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Posted 25 November 2021 - 07:13 PM

Rotating the collimation tool does not change the position of the shadow. Here's a closeup of the secondary mirror center dot.

 

https://www.cloudyni...1101-135923400/

 

The laser coming from the focuser isn't hitting the center dot, but the returning beam from the primary is.

 

It looks like I might be able to move the focuser over slightly so it does, but I'm not certain the dot is actually centered.


Edited by briansalomon1, 25 November 2021 - 07:46 PM.


#4 Bill Weir

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Posted 25 November 2021 - 07:40 PM

It looks like a dried out arachnid carcass in a web across the truss although that secondary mark is very odd. Have any acetone to clean it off with? I mean cleaning off the secondary.

 

Bill


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#5 briansalomon1

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Posted 25 November 2021 - 07:54 PM

It looks like a dried out arachnid carcass in a web across the truss although that secondary mark is very odd. Have any acetone to clean it off with? I mean cleaning off the secondary.

 

Bill

It looked odd to me the 1st time I looked at it. Since it's new from the factory, and I'm new to this type of alignment I'm leaning towards trying everything else before I remove the dot.

 

I need to do some reading and have Mr. Menard's "Perspectives" on order.

 

Having used only TeleVue scopes for 30 years, I'm used to that level of finish, but I don't think the center dot has to look nice in order for the scope to work correctly. The collimation tool beam remains motionless when rotated even at 40' away, so it's dead centered.

 

I'll take my time and over the next couple of days get the focuser aligned to the secondary center dot and proceed from there. 


Edited by briansalomon1, 25 November 2021 - 08:27 PM.


#6 Asbytec

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Posted 25 November 2021 - 08:27 PM

It looks like I might be able to move the focuser over slightly so it does, but I'm not certain the dot is actually centered.


I bet the return beam is "imaging" the center dot just like it does the primary center mark. The diagonal center dot won't be at the geometric center, it is offset to the optical center because the diagonal mirror is likely physically offset. If you move the focuser, you'll probably have to move the diagonal to recenter it under the focuser. So the center dot will be in the same place relative to the focuser. Or accept your diagonal will not be optimally placed. You don't need the diagonal center dot to center and align the focuser axis. Not sure what else you can do to get rid of the weird shadow than just cleaning the dot off the diagonal.

#7 Moxized

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Posted 25 November 2021 - 09:19 PM

I'm of no help in this as I have never owned any other reflectors, but I have to say I have never, ever, seen a secondary marked in any way, and I just cannot imagine the purpose for it. Is this just a thing for larger Dobs?


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#8 Tom Stock

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Posted 25 November 2021 - 09:57 PM

Thats clearly the secondary dot (scribble) visible as a result of the barlowed laser.


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#9 Jon Isaacs

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Posted 26 November 2021 - 01:15 AM

If the secondary mirror is clean, I would not worry about it.  The primary is properly aligned. If there is something on the primary, it's in the shadow of the secondary.

 

Jon


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#10 Garyth64

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Posted 26 November 2021 - 08:48 AM

Thats clearly the secondary dot (scribble) visible as a result of the barlowed laser.

After looking at the other photos that were posted, it looks like someone tried to "scribble" the center of the secondary mirror to try and define it's center.


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#11 Vic Menard

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Posted 26 November 2021 - 09:28 AM

After looking at the other photos that were posted, it looks like someone tried to "scribble" the center of the secondary mirror to try and define its center.

Factory "new" UC Obsession... can't say I've seen that before, and I've seen a lot.  foreheadslap.gif


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#12 Tom Stock

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Posted 26 November 2021 - 10:30 AM

I would remove that spot if it were me. It really serves no purpose, but mostly because it would annoy me.


Edited by Tom Stock, 26 November 2021 - 10:31 AM.

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#13 Garyth64

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Posted 26 November 2021 - 10:41 AM

No matter who, when, why, or how, it is there.

 

scribble.jpg


Edited by Garyth64, 26 November 2021 - 10:42 AM.

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#14 Tom Stock

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Posted 26 November 2021 - 10:58 AM

Alcohol and a q-tip?  



#15 briansalomon1

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Posted 26 November 2021 - 11:00 AM

The shadow is definitely the center spot of the diagonal mirror. Following Vic's procedure where I center the spider before proceeding with the collimation (which I hadn't done) I repositioned the focuser so the laser is striking closer to the center spot and realigned.

 

https://www.cloudyni...8023b5dc8de91fd



#16 briansalomon1

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Posted 26 November 2021 - 11:07 AM

I would remove that spot if it were me. It really serves no purpose, but mostly because it would annoy me.

When I opened the shipping container, it alarmed me. Now that I've had the scope a few weeks I'm coming around to accept that it's probably something I can just remove with some acetone.



#17 briansalomon1

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Posted 26 November 2021 - 11:24 AM

By the way, I've found that a water bottle (the type used on a bicycle) can be adapted to serve as a good cover for the diagonal mirror. It needs a thin strip of self adhesive tape to shim the diagonal for a snug fit.

 

But I haven't found anything yet to cover the primary. The oil drain pan Obsession provides isn't going to last very long.

 

https://www.cloudyni...d4597326e795085

 

Has anyone found something that works better for the primary mirror cover?




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