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Obsession New Truss Poles for Binoviewers

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#1 jragsdale

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Posted 11 July 2020 - 03:44 AM

There's something about binoviewers on a giant scope that just must be done. When I recently became the owner of a 20" f/5 classic Obsession much to my surprise, I busted out the binoviewers first thing but was disappointed that I could only use them with a 2.5x powermate, even a 2x barlow or 1.6x OCS didn't work. I ran some numbers (and had a friend run some numbers too) and we came up with a 4" shortening of the tubes. From 70" to 66". But rather than cut them and forever be stuck with using an extension tube when not using binoviewers, I opted to have a whole new set made so I can go back and forth depending on the night. So I called up a local metal company and they were able to cut the 8 tubes of aluminum to 66", 1.25" OD for around $80. Not too bad! I ended up getting .065" wall vs the stock .049" in order to keep the weight balance similar, and it's a good thing I did, as it is a little more prone to taking off when you remove the eyepiece than it was before. The upside of this is it can take a whole bunch more weight at the eyepiece before it ever starts to think about taking a dive. 

 

I used William Optics Binoviewers with the stock 20mm eyepieces that came with them and it gave amazing wide field views of some DSOs and planets. I found that when I wanted to switch to a regular eyepiece, or use the binoviewers with a barlow, that a 2" diagonal gives the perfect amount of backfocus. So, while a little weird looking, it is actually much more ergonomic for viewing the low altitude planets right now with no impact on the sharpness of views.

 

The only downside to this setup, is the mirror is nearly perfectly sized on these 20" f/5 Obsessions with a 3.1" mirror, so I'm losing a little aperture with this mod. To really take it to the next level I probably should upgrade to a 3.5" mirror.

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#2 jragsdale

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Posted 11 July 2020 - 03:45 AM

New shiny tubes!

 

(Hint: Acetone wipes the printed on text that comes on stock aluminum tubing off like it is nothing)

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#3 Marcopolo_M

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Posted 11 July 2020 - 04:09 AM

Do you find the delivery of light to both eyes with a dimmer image is more than compensated for by the fact that the image is going to two eyes? Which gets the most benefit, globulars, plantetary, moon, galaxy, nebulae?


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#4 ButterFly

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Posted 11 July 2020 - 05:13 AM

Very nice.

 

Having a second secondary live with the shorter tubes is not that big a deal at all.  Just one more swap for extra goodness - totally worth it.  The real question is whether you would ever go back to cyclops.  Get back to us in a year on that one.


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#5 jragsdale

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Posted 11 July 2020 - 10:04 AM

Do you find the delivery of light to both eyes with a dimmer image is more than compensated for by the fact that the image is going to two eyes? Which gets the most benefit, globulars, plantetary, moon, galaxy, nebulae?

Since most of my viewing is from my suburban backyard, most of the targets I'm going after are solar system or brighter DSOs so it made sense for home viewing. I also have the option of the diagonal/eyepiece configuration when I want to hunt the dimmer objects, but planets in a binoviewer are simply stunning! Last night on Saturn the Cassini division almost never went away from seeing.



#6 Pinbout

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Posted 11 July 2020 - 03:14 PM

Very nice.

 

Having a second secondary live with the shorter tubes is not that big a deal at all.  Just one more swap for extra goodness - totally worth it.  The real question is whether you would ever go back to cyclops.  Get back to us in a year on that one.

you need a bigger "real" 2ndry. and the 2nd 2ndry is probably optically inferior. I tested my WO 2ndry and although it was smooth it was not flat.

 

plus that 3 reflective surfaces that reduce brightness/ contrast... and we all know its about contrast that reveals those very fine details in nebula.



#7 jragsdale

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Posted 11 July 2020 - 04:26 PM

plus that 3 reflective surfaces that reduce brightness/ contrast... and we all know its about contrast that reveals those very fine details in nebula.

The bino setup will be used mainly for solar system objects so not too worried about loss of brightness. 



#8 tmiddendorf

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Posted 16 July 2020 - 01:12 PM

Anyone know the stock length for the 1.25" poles on a 20" f/5 obsession ?



#9 jragsdale

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Posted 16 July 2020 - 01:44 PM

Anyone know the stock length for the 1.25" poles on a 20" f/5 obsession ?

The documentation says 70" +/- 2", but when measuring I found them to all be nearly exactly 70.5".



#10 tmiddendorf

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Posted 16 July 2020 - 02:14 PM

The documentation says 70" +/- 2", but when measuring I found them to all be nearly exactly 70.5".

Thank you. 



#11 Jon Isaacs

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Posted 16 July 2020 - 03:32 PM

For anyone living in Socal or Phoenix-Tucson, Industrial Metal Supply has black anodized tubes in 6 ft lengths in 0.75", 1" and 1.25" diameters for around $10 each.

 

They also have other colors and though its unlikely anyone reading this forum would qualify, they offer a senior citizen discount... :)

 

https://www.industri...Products/Metals

 

Jon



#12 jragsdale

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Posted 16 July 2020 - 03:46 PM

For anyone living in Socal or Phoenix-Tucson, Industrial Metal Supply has black anodized tubes in 6 ft lengths in 0.75", 1" and 1.25" diameters for around $10 each.

 

They also have other colors and though its unlikely anyone reading this forum would qualify, they offer a senior citizen discount... smile.gif

 

https://www.industri...Products/Metals

 

Jon

That's a good price!

 

Verify the side wall thickness too, you would want .065" for shortening the tubes on a 20" f/5 to keep the weight approx the same.



#13 Mason Dixon

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Posted 18 July 2020 - 08:47 PM

I've got the same problem with my 25" F5.5 and my Seibert BlackKnight BV's, how did you calculate the size for the new truss poles?


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#14 jragsdale

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Posted 19 July 2020 - 03:41 AM

I've got the same problem with my 25" F5.5 and my Seibert BlackKnight BV's, how did you calculate the size for the new truss poles?

I didn't, just guessed as binoviewers usually take up about 100mm of backfocus so settled on 4" reduction. 



#15 Don H

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Posted 21 July 2020 - 03:14 PM

I didn't, just guessed as binoviewers usually take up about 100mm of backfocus so settled on 4" reduction. 

I think making the tubes a bit longer to avoid the addition of the star diagonal would be the way I would want to go. Then just insert the binoviewer and look straight through, without additional reflections.



#16 jragsdale

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Posted 22 July 2020 - 10:35 AM

I think making the tubes a bit longer to avoid the addition of the star diagonal would be the way I would want to go. Then just insert the binoviewer and look straight through, without additional reflections.

That is how it works now. Binoviewer without barlow comes to focus right about mid way on the focusers limited 30mm of travel. But when you add a barlow it pushes the focal point much further out, necessitating the addition of the diagonal.



#17 Pinbout

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Posted 22 July 2020 - 11:21 AM

That is how it works now. Binoviewer without barlow comes to focus right about mid way on the focusers limited 30mm of travel. But when you add a barlow it pushes the focal point much further out, necessitating the addition of the diagonal.

sell those inexpensive binos and buy some proper denk II's they have everything you need to get a straight thru image without new poles.

 

the 1.25" nose on the WO bino's is not doing your scope any justice.

 

https://www.youtube....s&index=51&t=1s


Edited by Pinbout, 22 July 2020 - 11:24 AM.



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