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C8 Visual Back Focusing Problem

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#1 acat

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Posted 24 September 2020 - 06:21 PM

I have an old Celestron C8 (black tube) mounted on a new HEQ5 mount. I usually use the scope with the 0.63 Celestron FL reducer, and it works quite well, except that the scope is in need of collimation. I've read some articles on the process which seems relatively straightforward. Most articles have indicated that it's best to use high magnifications when doing this, in the range of 200-300X. The 2000 mm FL of the C8 used with a 9 mm FL eyepiece gets me in the range.So I thought the FL reducer was counterproductive for this exercise and removed it. Now I'm a bit baffled because with the Celestron Visual back alone I cannot focus the scope! Looks like I need another inch or so of travel to move the eyepiece out to get things in focus! How can this be? I've tried it with and without a star diagonal without success. Do I need some add-on device such as a helical focuser or perhaps an extension tube of some sort? I've seen adverts for focusers for 1.25 " and 2 " eyepieces and thought this might be an essential accessory to solve this problem.



#2 KBHornblower

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Posted 24 September 2020 - 06:30 PM

Does the scope not have a working focus knob separate from the visual back?  On my old orange tube C8 that knob works by moving the primary mirror and gives an enormous range of back focus for various accessories such as a diagonal, a focal reducer, or a camera.  Please let us know what you have on your scope.


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#3 KTAZ

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Posted 24 September 2020 - 06:38 PM

You absolutely should be able to achieve focus with the eyepiece inserted directly into the visual back. However, it will be a long distance (many turns) from what it was with the .63 reducer.



#4 Jethro7

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Posted 24 September 2020 - 06:59 PM

Hello Acat,

You should be able to come to focus unless you have a secondary focuser. These scopes have a tremendous focal range, out of curiosity have you run the focuser all the way in then all the way out.  Just wondering if the primary mirror and the focuser is getting stuck. I do not need any other devices to do a Collimation other than a high power eyepiece and a bright star. On my C8

 

HAPPY SKIES TO YOU AND KEEP LOOKING UP Jethro


Edited by Jethro7, 24 September 2020 - 08:45 PM.

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#5 acat

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Posted 25 September 2020 - 10:33 AM

Thanks very much for the advice and suggestions as to what might be amiss. I thought I'd do a "reality check" again after these comments this morning. Previously I had done this in the afternoon when temperatures were fairly high by sighting on a terrestrial object. I thought that the heat might be a contributing factor; it's cool here now. Still no luck I tried several eyepieces with and without the star diagonal. I cannot turn the focus adjustment sufficiently clockwise to bring things into focus. It seems to come close so I can discern the image but that's all. Near the end of the travel the image becomes shifted as though the mirror is tilting off axis. Don't want to force it. There are three hex head screws holding the focuser in place. Has anyone experience removing this to perhaps  "break things loose" (without breaking them, that is)? Any suggestions where to go from here?



#6 acat

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Posted 25 September 2020 - 10:43 AM

I should add that I can bring an object into focus by carefully moving the eyepiece out of the visual back about 3 inches or so, and just hold the eyepiece in space.


Edited by acat, 25 September 2020 - 11:18 AM.


#7 KBHornblower

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Posted 25 September 2020 - 11:23 AM

Something is jamming your focusing mechanism short of where it needs to move the mirror to achieve focus.  For starters, look for the possibility that a too-long accessory screw might be fouling the edge of the mirror.  If that is not it, see this Cloudy Nights thread about disassembly.

 

 

https://www.cloudyni...echanism-issue/



#8 acat

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Posted 25 September 2020 - 12:47 PM

I did some reading on the Celestron website where they describe the screw which pushes on the mirror cell sometimes slipping off the cell causing an abrupt change in focus, hence the interest in external focusers. They also describe a trial and error approach to getting it re-centered. So I took the 3 screws off that hold the focuser in place to see if something could be done. This cap comes off easily enough. However the bolt which connects the focusing knob (more like the handle of a micrometer) seems firmly attached to the mirror cell via a threaded rod. With the securing cap removed the mirror can be rotated slightly about its optical axis (as much as the opening allows) and pushed in and out to change focus. Unfortunately pushing the mirror back using the freed-up focuser seems to just hit the inner back side of the telescope. There doesn't seem to be anywhere for the mirror cell to go! It's hard to understand how it ever could have worked. I'm probably missing something. Using the star diagonal with eyepiece on the re-assembled focuser I can see that I am very close to being in focus but still about a half inch or so too close to get there. Looks like either telescope repair or an external focuser is called for.



#9 acat

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Posted 26 September 2020 - 11:25 AM

KBHornblower:

 

Thanks very much for the link. That user has described perfectly the symptoms that I am seeing. I'm going to look into this being the cause of my problems and report back. Cannot thank you enough!



#10 acat

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Posted 26 September 2020 - 01:10 PM

That was it! The too long screws for the mounting bar penetrated into the tube assembly preventing the mirror cell from moving all the way to the back. THANK YOU, THANK YOU, THANK YOU!!!!!!




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