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Thread Size on Back of C90

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4 replies to this topic

#1 Observer2006

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Posted 20 October 2020 - 05:28 PM

Hi, so I purchased a old orange Celestron C90 back a couple months ago, it's a great scope that's super fun to use, but the previous owner had lost the dust cap that goes on the back, and replaced it with a pink pill bottle cap that only kind of fits. The stupid thing is driving me nuts, and doesn't even screw on right in the first place. I was looking at replacing it with one of the 3D printed dust caps people in the classifieds sell, but realized that I have no idea what sized thread that is on the back of the scope. Does anyone know what size it is?



#2 RichA

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Posted 20 October 2020 - 10:56 PM

How accurate does it need to be?  You could measure the width of the threads and count them, that'll give TPI. 



#3 Kasmos

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Posted 21 October 2020 - 01:53 AM

The hole fits a .965" diagonal so you might plug it with a junk eyepiece, until you find a decent cap.


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#4 kwangett

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Posted 21 October 2020 - 12:32 PM

I recently bought an orange C90 that also was missing the cap, and got one of these that works well enough:

https://agenaastro.c...mm-yellow.html 


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#5 Geo.

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Posted 23 October 2020 - 08:40 PM

The C90's rear external thread, is: 1.375" * 24TPI and was used for mounting photo accessories such as the tele-extender for eyepiece projection AP.




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