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Celestar 8 focus and Collimation problem

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#1 Kirkman1

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Posted 25 September 2019 - 08:15 PM

I have a Celestar 8. Bought it used. The mirror looks
Great. I’ve tried to Collimate several times but images
Seem dull and not much defined compared to my 8 dobsonian
Could there be another problem? This scope should
Do better than it is. I don’t want to give up on it.
I called Celestron and I could drive it up to Torrance
And they would clean and Collimate it and advise on any repairs
But would that be more than the scope is worth? Any advice?

#2 Mike G.

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Posted 25 September 2019 - 08:43 PM

could be someone has moved the corrector out of place.  that would definitely cause bad images. there's usually a mark on the edge, I think it typically goes at 3 o'clock position but you should do a search to verify.  in any case, Celestron repair could probably get it producing good images for you.  my C8 (fully flocked) throws up amazing detail on clear nights.



#3 markb

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Posted 25 September 2019 - 10:08 PM

I have to second Mike G, there are rotational alignment marks on the secondary (sharpie or marker 'stripe' on the edge of the secondary, the corrector (scribed tiny numbers on the front surface at the very edge of the corrector, sometimes accompanied by a marker swipe on the ground edge, and reading correctly viewed from the front surface of an un-flipped corrector), and, allegedly (I haven't looked for mine yet, but will) an edge sharpie/marker index on the primary edge.

 

if you live close to Celestron, by all means let THEM do the corrections. Most people complain about the crazy shipping costs, not the repair cost.

 

The correct alignment, from trusted sources that disassembled factory fresh units and made notes, is to match all three marks (presumably you can ignore the primary unless you suspect that, or the rear cell, has been removed- its alignment mark should be 'tied' to the Ota body) with the 3 o'clock position viewed from the corrector (big) end of the ota. That should place it on the opposite side from the focuser knob. My C11 had a corrector retaining screw hole at that spot, making it easy to match the marks.

 

Side to side, or concentric, position of the corrector plate is said to be less critical. Many folks just center it, but it should be matched to the centerline of the baffle and visual back with a laser collimator and secondary perforation 'target'; I use a concentric circle holographic projection. Possibly no longer easily located. Recent Celestrons have no shims, but a sticky rubber gasket keeps the plate where the factory placed it. The most recent ones have nylon centering screws.

 

I bought a used C11, alleged to be fine, that was an absolute horror, soft and extremely disappointing. It was either dumped on me, or they genuinely did not realize how soft images were. Or they blamed it on the C11's claimed sensitivity to seeing conditions. In fair seeing, I'd still use the C11 over a smaller scope, personally (visual, not photgraphic).

 

Every surface was 180 degrees out of rotational alignment.

 

Rotational aligning the corrector and secondary to the incorrect 3 o'clock (some folks claim viewed fron the rear) reference, the scope went from soft to acceptably sharp, but not yet stunning.

 

The scope lives in my future AZ residence, and I ran out of time to try the later confirmed as correct, 3 o'clock reference point viewed from the big end, so I expect images will further improve. Tabletop daylight concentric circle collimation normally matches nighttime collimation, and they don't,  yet. I expect they will once I can finish the job.

 

Incidentally, I also believe, based on my progress photos and recent online photos of unaltered scopes, that someone ALSO misindexed the secondary within the holder, and with respect to the Fastar notch, when Bob's Knobs were installed, another alignment issue to resolve.

 

So check those rotational alignment marks. If they appear correct, check for a loose primary (apparently not that rare, see the Wilmslow site for their C14 primary issue) and check that the baffle centerline still matches the secondary perforation center. The rear cell could have shifted, or have been misinstalled.

 

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I developed much of my views from wilmslow astro's great C14 tuning account, other sources, and trying to figure out why a Meade 8 was impossible to collimate. That scope had a misaligned baffle AND a secondary non-concentric to the mounting puck, that required placement of the secondary and the corrector as far to one ota edge as possible, to allow the optical center to match the mechanical, baffle, center. Even with a failing silver coating on the secondary it give solid images now. Incidentally, adjusting the rear cell is not possible since Meade epoxies them on. So frustrating, but a great learning experience. And the factory rotational alignment marks were rotationally correct and close to the final position ( everything had to be placed to maximize secondary placement in line with the baffle centerline).

 

Final note: I have suspected the rotational alignment in most SCTs s partially or entirely to align the optical and mechanical centers of the scope, compensating for small errors or variations in the rear cell/baffle centerline, the variations in corrector location, and the slight (we hope) variation in secondary centering caused by non-concentric gluing to the aluminum puck and variations in the secondary holder. AFAIK no effort is made in manufacture to 'cancel out' optical errors, although marching 'power' variations, and hand correcting to reduce SA is done. I hope that was correctly stated.


Edited by markb, 25 September 2019 - 10:15 PM.



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