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WO GT71 Backfocus?

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#1 Bill001

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Posted 28 February 2020 - 01:29 PM

Hi,

 

Does anyone know the back-focus of a William Optic GT71 420mm? That is a GT71 w/o reducer (or flattener) so its focal length is native 420 mm. 

 

Very little info at their website and nothing in their manual.

 

Thanks,

Bill



#2 OldManSky

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Posted 29 February 2020 - 09:48 AM

I have one, and no - I don’t. I use it with the flattener/reducer for imaging, but I have used it with a diagonal and eyepieces for visual. With a 2” diagonal and a 20mm wide-field eyepiece, it focused with the focuser racked out about 3/4 of its travel.

Are you thinking about using a binoviewer on it?



#3 Bill001

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Posted 29 February 2020 - 09:59 AM

I have one, and no - I don’t. I use it with the flattener/reducer for imaging, but I have used it with a diagonal and eyepieces for visual. With a 2” diagonal and a 20mm wide-field eyepiece, it focused with the focuser racked out about 3/4 of its travel.

Are you thinking about using a binoviewer on it?

 

You’re using the GT71 w/ imaging package - 350mm FL.  That package is sold so one can use it as a 420 or 350mm unit. My 420mm unit does work in visual mode  (w/ 2” diagonal and 55 mm plossol; focus rack out 90 of 92mml) but it’s a few cm short for cmos camera.

 

Think I may have this solved. Will let you know


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#4 OldManSky

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Posted 01 March 2020 - 10:52 AM

You’re using the GT71 w/ imaging package - 350mm FL.  That package is sold so one can use it as a 420 or 350mm unit. My 420mm unit does work in visual mode  (w/ 2” diagonal and 55 mm plossol; focus rack out 90 of 92mml) but it’s a few cm short for cmos camera.

 

Think I may have this solved. Will let you know

Well, it wasn’t a package...I purchased the reducer/flattener for photo work separately, but don’t have it on for visual. I suppose the length of the diagonal is a factor...

FYI, with the f/r on, it’s 336mm FL, not 350 :)

Let us know how you get things sorted out.



#5 Bill001

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Posted 01 March 2020 - 12:37 PM

The GT71 is a 420mm FL; an optional “Imaging Package” is available - the .8 reducer/flattener - and yes you are correct that would be a 336 of 337mm  as you said. The GT can be bought as a 420mm w/o imaging kit; buying reducer/flattener separatey later or with imaging kit.

 

Anyway like ZWO’s backlog due to Corona, William Optics is in the same situation. I was lucky to find one last WO GT71 w/o imaging kit and the reducer/flatter kit is also backordered.

 

What I’ve done is measure the FL back from the primary lens. I also checked that location by imaging the Sun on a piece of paper to find the focal plane. Doing that w/ focus rack out 90cm I measure where the cmos should be. I ordered a Stellarvue 80mm extender. That should give me the extension I need, hopefully.

 

Will tell you if that works. 

B



#6 Bill001

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Posted 04 March 2020 - 04:00 PM

Findings:

 

The WO GT71 420 mm FL ie w/ no reducer/focuser:

 

Native, the WO did allow focus of a visual view using a 2” diagonal and a 55 mm plossol. The focuser rack was at 90 of 92 mm travel.

 

My Canon EOS required about 1.5 cm spacers. With the 80 mm extension the focuser racked in at 21 mm of 92 mm travel.

 

My ZWO Asi178mc required about 3.5 cm spacers. With the 80 mm extension the focuser racked in at 55 mm of 92 mm travel.

 

The Stellarvue 80 mm extension did the trick. Guess I could have got by w/ a 60 mm extension.


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#7 OldManSky

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Posted 04 March 2020 - 08:58 PM

WO has the Flat6AIII in stock on their own website...

 

https://williamoptic...nt-not-included

 

:)



#8 Bill001

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Posted 04 March 2020 - 09:13 PM

WO has the Flat6AIII in stock on their own website...

 

https://williamoptic...nt-not-included

 

smile.gif

Thanks. They ship from Taiwan so I will wait til suppliers here in the USA catch up w/ their backorders....




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