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Refractor Drawtubes?

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#1 krehmkej

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Posted 02 November 2005 - 10:17 PM

I have an old 80mm f15 refractor OTA with a .965" drawtube that extends out to 9 inches. Seems awful skinny to me. Does anyone know how much (if any) vignetting this might cause?

#2 trainsktg

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Posted 02 November 2005 - 11:11 PM

You would need to do a ray-trace to see for sure. My old 3" Tasco 'Solarama' and Orbit 'Scope' have such draw tubes, the only disadvantage I can see is slop especially if upgraded to heavier 1.25" eyepieces, as all of my older .965" drawtubes are. Cheap adapters are available. ( However to save you all of this bothersome trouble I would be very willing to take this scope off of your hands...they are not worth a whole lot anyways... :whistle: )

Keith in Portland

#3 krehmkej

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Posted 03 November 2005 - 08:53 AM

Keith,

Showing my complete ignorance, I have no idea how to trace rays! As we may not see the sky for weeks (months?)I may just turn a tube adapter on the lathe and replace the whole tail end with a 2.5" tube and add a new focuser, updating everything at once..

#4 trainsktg

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Posted 03 November 2005 - 12:56 PM

Sounds good...just make sure to keep the old focuser assembly for re-sale value.

In order to ray trace, there are probably some really good posts here, but for a basic overview you precisely draw the exterior and interior of your telescope on paper, including baffles (generally 1:1 scale) and 'trace' the light path from front objective to eyepiece. It the light path cuts through space occupied by baffles, drawtube, etc then you have some light loss.

Keith

BTW: I'll still buy the scope if you want to save wear and tear on your shop Smithy. :silly:


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