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Focuser Travel & 2" Diagonal on Celestron 80ED

Started by soddy, 03/20/2009 02:51AM
Posted 03/20/2009 02:51AM Opening Post
Hi! I purchased a Celestron 80ED and replaced the stock diagonal with a 2" Celestron #93519. Tonight I noticed that with some eyepieces (and particularly with my Ultima barlow) I can't get focus because the focuser will not travel inward quite far enough. It appears that the problem may be with a threaded ring on the 2" diagonal that is not necessary with the 80ED but actually prevents sliding the 2" diagonal closer to the end of the focuser. The threaded ring is 'captive' to the diagonal and therefore can't be removed. Is this diagonal incompatible with the 80ED? What exactly is the problem, and how can I fix it without cutting off the ring on the 2" diagonal?

Thanks for your help.

Walt
Posted 03/20/2009 09:17AM #1
Walt Soderman said:

Hi! I purchased a Celestron 80ED and replaced the stock diagonal with a 2" Celestron #93519. Tonight I noticed that with some eyepieces (and particularly with my Ultima barlow) I can't get focus because the focuser will not travel inward quite far enough. It appears that the problem may be with a threaded ring on the 2" diagonal that is not necessary with the 80ED but actually prevents sliding the 2" diagonal closer to the end of the focuser. The threaded ring is 'captive' to the diagonal and therefore can't be removed. Is this diagonal incompatible with the 80ED? What exactly is the problem, and how can I fix it without cutting off the ring on the 2" diagonal?

Thanks for your help.

Walt

Walt: I did a web search on the particular diagonal, the Celestron #93519, and it seems that this is a diagonal designed to thread on the rear of a Schmidt-Cassegrain Telescope (SCT) rather than a refractor though it does include an adapter that allows it to be used with a refractor.



http://www.celestron.com/c3/product.php?ProdID=334

I think you have indeed identified the reason that you are unable to reach focus with your diagonal, that threaded ring is what is required to thread onto an SCT.

Looking at your profile, I see that apparently you own a Celestron C-8 so I imagine you purchased this diagonal to fit the C-8 and use it with the C-8, it should work nicely with the C-8. Unfortunately it seems the Celestron CO ED-80 does not have inward focus travel (called back focus) to allow it to reach focus with SCT ring in place. The similar Orion ED-80 I had was similar, not much inward travel.

I don't really see an easy solution, you need a diagonal specifically designed for refractor unless you want to do some machining which I do not recommend. I think your best best is to purchase a used 2 inch Diagonal here on astromart. I have seen several reasonably priced dielectric diagonals in the past few days.

If you purchased this diagonal new for your CO ED-80 then the vendor sold you the wrong part and they may trade it for the proper diagonal.

Jon