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Binocular telescope close focus?

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#1 jprideaux

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Posted 17 April 2021 - 06:34 AM

I’ve been very interested in buying a binocular telescope (BT). The one thing holding me back is that I also enjoy fairly close focus terrestrial viewing in addition to infinity focus astronomy and could better justify the purchase if it could also be used for some close focus work. I realize that BT individual eyepiece focusers only give you so much travel so the focus deviation from infinity is limited meaning that you cannot focus all that close. Has anyone experimented with adding some short extension tubes to each eyepiece and examined the ability of it to focus closer? If one were to do this, would there be issues with getting the two images to converge, vignetting, or other optical aberrations? Also, would there may be some balance issues introduced mechanically in the BT mounting system that would need to be addressed?

I have enjoyed close focus work using a refractor with diagonal and binoviewer. Just wondering if a BT could be better or if a single telescope is better for deviating from it’s initial design (meaning getting a closer focus by moving focal point out by adding extension tubes).

Just trying to rationalize parting with more of my hard-earned money!

#2 wrvond

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Posted 17 April 2021 - 06:42 AM

The minimum focus on my Oberwerk BT-70-45 is 50 feet, so I've never felt the need to get closer. How close is close to you?



#3 Mark9473

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Posted 17 April 2021 - 09:12 AM

Just wondering if a BT could be better or if a single telescope is better for deviating from it’s initial design (meaning getting a closer focus by moving focal point out by adding extension tubes).

You can add extension tubes to the eyepieces. Works well. I've done that with a binoviewer.



#4 jprideaux

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Posted 17 April 2021 - 07:16 PM

If I get a BT, I’ll definitely experiment with adding extension tubes to the eyepieces. I would like the option to get closer than 50 ft. I would assume that the close-focus distance for larger than 70mm is more than 50 ft.


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