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Adjusting Celestron 9 x 50 RACI Finderscope

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30 replies to this topic

#26 Lasko

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Posted 22 February 2021 - 10:29 PM

Alas, when I say mine are not tight they often are not touching at all.

I've seen that happen too and it shouldn't be that way.  I would try rotating the whole thing a little counter-clockwise, either adjusting the OTA bracket itself, or the finder bracket mounting screws.  You should be able to impart a little twist to it by tightening the back mounting screw more than the front one.  Then you should have to push the back end of the finder further into the tensioner to center it.



#27 rocketdmc

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Posted 23 February 2021 - 07:07 PM

Is it not possible to use a different mount that uses two rings with three screws in each ring? You would have to replace the shoe with a common finder shoe.



#28 Lasko

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Posted 25 February 2021 - 09:31 PM

Is it not possible to use a different mount that uses two rings with three screws in each ring? You would have to replace the shoe with a common finder shoe.

True, you could definitely do that.  There was some decent weather today and a chance to experiment with the different quirks of this finder.  I started with the finder centered in its bracket, and 8mm eyepiece centered on the corner of a rock about 1 mile away.  The finder OTA bracket was as close to square as I could get it.

 

The two screws that mount the finder bracket on the OTA bracket can cause a wide swing in left/right adjustment depending on how tight each one is.  It seemed closest to left/right alignment with the target when the back screw was pretty tight and the front was just snug enough to be secure.  Tightening the back screw made it swing to the left, and tightening the front the opposite.  Now that I did it a few times while watching it closely, it is probably reasonably repeatable but will still be misaligned some amount when it's not left on the scope.

 

The vertical alignment was below the target, meaning I needed to push the back of the tube down.  This was good because that put more tension on the spring holding it all in place. 

 

If you're having trouble with this finder definitely look at the brackets and check how much you can move the adjustment by tightening or loosening the bracket screws.  At least get it as close as possible by adjusting the bracket before moving the adjustments for the finder tube.

 

Of course this brings up the question of how in the world do you put a bracket on a finder that can make it difficult or impossible to align with the scope?  I fear only Celestron knows the answer to that.  I really like it when works, it is just very frustrating to set it up and have the adjustment so far off every time.  I think next time will be better with some idea of why it seemed so random before.

 

Now to go see if the heat plumes are gone and it's time to look at something other than a rock.



#29 BradBB

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Posted 28 February 2021 - 06:00 PM

Now that I realize how precisely the mounting system would have to be machined to have it removable with repeatable accuracy, I have given up. Removable riflescopes are machined very precisely. This mount is a fairly crude casting, so there is no hope of removable accuracy. It may be fine if left installed permanently, but for us mobile astronomers, it will remain a pain. The trouble will be, of course, that first star in my two star auto alignment. If I can get it in the eye piece then I can align the finder and be good for the evening. I agree that the spring retaining hex screw is not meant to be adjustable. On my C8, the finderscope base is held to the OTA with just two screws. There are slots in the base, so there is plenty of side to side adjustment, but it allowed only a tiny bit of rotational adjustment. For me, it worked last time, barely, with very light tension on the thumb screws at alignment. It is a nice scope,  but the mounting system needs much improvement, at least for a mobile astronomer. It would be best left attached.



#30 charlesgeiger

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Posted 01 March 2021 - 02:22 AM

I investigated replacing the whole assembly (the silver slotted unit with adjustable spring assembly) with a replacement cap screw with the right thread pitch).  jeffreym in another thread regarding this topic measured the thread pitch to be M8-1.25 (metric thread with odd pitch) and another person, windowpane, found a metric cap screw with this very thread pitch and replaced the spring tension unit.  It worked much better but suspected he had a problem with the focuser not fitting properly into the O-ring in front. 

I plan to attempt to find a nylon or delrin cap screw with this thread pitch and remove and replace the stock metal unit.  I believe the unit I have is properly fitted into the rubber O-ring in front and I believe the right length M8-1.25 nylon or delrin replacement should remove most of the problem.  Remember too that the finder scope does not fit exactly into its base each time you install it onto your main scope so that might be another consideration.

I do believe a hard fix without a spring system will work much better and if I can find the part in McMaster Carr or somewhere else I will try and let you folks know.  At any rate, if I get the part I will try myself and get back here to this forum.

Charlie


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#31 CathyA

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Posted 30 March 2021 - 12:15 PM

GREAT help!! I have my two adjusting screws snug, but not tight, and the hex screw tight. I would NEVER figured this out were it not for this post!!!!

 

Thanks so much!!!!!




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