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GSO 10" Truss RC - wheres the backfocus measured from ?

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#1 ZL4PLM

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Posted 08 December 2018 - 05:30 PM

I am trying to balance the back focus distance on a GSO 10" Truss RC and the back focus of a CCDT67 reducer

 

where is the back focus measured from ?

 

Its suggested this is 233mm .. but from where??



#2 ollie31770

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Posted 08 December 2018 - 05:54 PM

Does this thread help?  https://www.cloudyni...backfocus-help/



#3 ZL4PLM

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Posted 08 December 2018 - 10:50 PM

nah im still not getting where its from !!!

 

I can make it focus with no extn ring and the moonlite 2.5" focuser  and the CCDT67 and about 70mm from the rear of the threads on the CCDT67

 

that 'seems' to be about right

 

according to the AP document from the rear of flange on the CCDT67 that should give about 0.7 reduction

 

of course I dont have a moonlite draw tube to 48mm adapter!

 

:)



#4 Terry White

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Posted 12 December 2018 - 11:28 AM

For my Astro-Tech 16" RCT, the back focus length is measured from "the end of the focuser attachment port" to the focal point. Since GSO makes all of the store branded RC's, it's a good bet that your GSO will be measured in the same way.



#5 pfile

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Posted 15 December 2018 - 01:10 AM

other than knowing whether or not you'll be able to achieve focus, why is the actual number even important? it sounds like you already know you can reach focus, so what is it you're trying to do? the critical number is the spacing between the CCDT67 and the sensor...

 

i have the same setup and with the TS tip-tilt ring, a moonlite 2.5" CSL with 2" compression ring adapter, a camera + filter that eats 54mm of backfocus, a 3.8mm camera faceplace, and the CCDT67 configured at 0.7x (which eats about 38mm of backfocus optically) i can still focus the telescope. if i hide the reducer and its extension inside the focuser then i have about 30mm of focuser throw. the precise parts adapter i made (L=19.2mm) unfortunately has a diameter that's too large to fit in the focuser drawtube, so with it in place i only have about 1cm of focuser throw.

 

of course, my mirror spacing is probably not exactly stock and i think my native focal length is a little more than 2000mm (probably 2015mm), which may have saved my bacon.

 

rob




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