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Secondary size for bino-Newton?

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#1 mcmaker

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Posted 17 January 2020 - 02:50 PM

Hi, I'm finishing building a 6 inch f/5 binoscope but I'm unsure about size of secondary mirrors. Bino is calculated for 1,25 inch diagonals for now but it has 2 inch focusers. Of course I shortened the primary-secondary distance by factor of length of the diagonal light path which is about 75 mm (about 3 inches).

 

I'm pretty sure that stock secondary mirrors won't be able to deliver 100% of light and the bino will need a bigger secondary. Can you help me calculate size of the secondaries?


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#2 Steve Dodds

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Posted 17 January 2020 - 03:06 PM

With 2" focusers you are going to have a hard time getting it to both eyes.  Most eyes are about 2.25" apart



#3 Garyth64

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Posted 17 January 2020 - 03:38 PM

How far inside the f.l. is the secondary?

 

Knowing that, you can calculate the size needed.

 

If your secondary is 7" inside the focus, and you use the diameter of the Moon as the diameter of your image plane, you would need a secondary of 1.6".  (Moon diameter will be .27")

 

For a larger image plane diameter, let's say .5", you would need a secondary about 1.78".


Edited by Garyth64, 17 January 2020 - 03:47 PM.

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#4 TOMDEY

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Posted 17 January 2020 - 03:52 PM

Are you adapting two existing stock Newt OTAs (with 2-inch focusers) to bino use... using 1.25-inch Star diagonals to fold the beams to your eyes? Your 75mm up there seems consistent with that layout. I calculated it for 7 and 8 inch total folded distance from SM to focal plane and get OK visual performance with 2-inch diagonals: The 7-inch case gives you a 20mm unvignetted field. the 8-inch case provides a 15mm unvignetted. The trade-off is that bigger diagonals vignette less, but at the expense of more overall light loss due to the bigger central obstruction. If your OTAs already have 2-inch folds... just keep those and get on with life.

 

Your biggest challenge will be comfortably achieving ergo-adjustable image merge.    Tom 



#5 mcmaker

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Posted 17 January 2020 - 05:49 PM

I could show the photo, it would clarify the situation wink.gif I'm pretty deep with this project (like over 150 hours spent making binoscope and heavy duty AZ mount on tripod) and overcame most problems like IPD adjustments and collimating (merging both images)

 

With 2" focusers you are going to have a hard time getting it to both eyes.  Most eyes are about 2.25" apart

 

 

 

Nah, like I said, diagonals would be 1,25 inch but I'm using 2 inch focusers to avoid vignetting :) 

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Edited by mcmaker, 17 January 2020 - 05:54 PM.

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#6 eroyer

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Posted 18 January 2020 - 04:36 AM

Hello,

congratulations on your binoscope construction. I made a software for computing the parameters of this kind of binoscope.

For example, for 150mm primary mirrors, if we assume a distance of 400mm between the primary mirrors and a distance of 100mm between the tertiary mirror and the eyepiece, I get this light curve at the focal plane with a 58mm minor axis secondary and 36mm tertiary.

 

binewt150.png

 

This is just an example. The software is open source. You can download it at http://www.astrosurf...newt13beta9.jar and play all the parameters as well as a lot of other useful stuff.

 

If you don't mind showing other pictures, I really like to see how people solve the bino specific problems mainy IPD adjustment and co-collimation.

 

Eric


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#7 mcmaker

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Posted 18 January 2020 - 05:11 AM

Wow! Thank you Eric, I wouldn't think that there is such a marvelous help like your program!


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