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How much shorter should truss poles be?

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#1 A Walker

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Posted 15 February 2013 - 02:27 PM

Hi all,

I have a 12.5 inch f6 dob and would like to make a second set of truss poles to allow my Denk standards to come to focus without the use of an OCS or similar. How much shorter should the poles be? I guess I am asking for the optical length of the Denk standard which for some reason I think might be around 105mm although I could be completely wrong. Any tips from those who have done this already?

Kind Regards
Andrew

#2 faackanders2

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Posted 03 March 2013 - 06:05 PM

Ask Russ at Denk, but I think it might be 3" if you don't want to use an OCS, or 1.5" if you want to switch from Newtonian to Multipurpose OCS without needing extensions.
Ask Russ at Denk!

#3 Mike Harvey

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Posted 03 March 2013 - 10:58 PM

Hi Andrew...I just happened across your post and wanted to say "that way lies madness!" :)
I started on the quest for binoviewers-with big-dobs about 15 years ago and "cutting the truss tubes" was one of the first projects (there were no OCS devices then). MANY lessons were learned the hard way.

You could be looking at cutting your tubes more than 5 inches, depending on how high the profile of your focuser is. Once you do that you've moved your secondary much further into the light cone from the primary. That means you'll be reducing your effective aperture. You'd have to diagram the light path to know how much it will be affected, but it could be significant.
You might be much better off just getting the lowest amplification-factor OCS available (Denkmeier has a 1.2X).
Hope this helps.

#4 faackanders2

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Posted 04 March 2013 - 08:07 PM

For Newtonians, the "Multipurpose" 1.2x/1.8x/2.3x OCS (which now has a 45mm clear aperture option) requires the most infocus and may require an extension. The "Newtonian" 1.4x/2.0x/2.5x OCS has normal focus. The "Multiplier" 1.8x/2.6x/3.2x OCS has the most outfocus.

I noticed Russ stopped listing it in the accessories, but he still may offer the focal reducer power switch which lowers the Netwonian to 1.0x and the Multipurpose to 0.87x (but requires an extension or two).

P.S. As mentioned in other responses, shortening your trusses most likely would require a larger secondary to reduce effective aperture loss. Going the OCS and power switch/reducer route, allows the option to use single eyepieces for wide views as the telescope was originally designed.

#5 BBryce

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Posted 16 March 2013 - 01:57 AM

In order to focus with my maxbright without ocs, I use an oversized secondary mirror for my 12" and a set of truss poles with two positions.

In binoview position, the secondary cage is lowered about 4.33" (110mm). In mono, either I reverse my set of truss poles (upside down) or I use an extender tube.

Best of both worlds !

Clear skies,

Bryce

#6 faackanders2

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Posted 17 March 2013 - 01:30 PM

In order to focus with my maxbright without ocs, I use an oversized secondary mirror for my 12" and a set of truss poles with two positions.

In binoview position, the secondary cage is lowered about 4.33" (110mm). In mono, either I reverse my set of truss poles (upside down) or I use an extender tube.

Best of both worlds !

Clear skies,

Bryce


Extension for single eyepieces would be easiest and quickest; especially going back & forth between binoviewing and single eyepiece. But it viewing for long periods in single eypiece/momo mode it probably would be best to just use the longer tubes w/o extender (if the extender vigenettes light incoming to the single eyepiece). Nice to have options though!

#7 faackanders2

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Posted 17 March 2013 - 01:37 PM

In order to focus with my maxbright without ocs, I use an oversized secondary mirror for my 12" and a set of truss poles with two positions.

In binoview position, the secondary cage is lowered about 4.33" (110mm). In mono, either I reverse my set of truss poles (upside down) or I use an extender tube.

Best of both worlds !

Clear skies,

Bryce


Does anyone know of truss scope manufacturers that sell their scopes with two sets of truss tubes and an oversized secondary; exactly for binoviewer and single eyepiece use?
If and when they come out with 3" focusers and eyepieces (for wider TFOV) would this increase the problem, or would the oversized seconary for the binoviewers work for the 3" eyepiece as well (assuming they could make a 30mm 100 AFOV, 40mm 82 AFOV, ad 50mm 70 AFOV set)? Or make the swconary for the widest of the bunch.

#8 johnnyha

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Posted 17 March 2013 - 01:44 PM

I don't think they will because again, there is a significant loss of aperture in fast scopes at 1X due to the long length from the clear aperture to the eyepieces of the binoviewers.

#9 faackanders2

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Posted 17 March 2013 - 01:51 PM

I don't think they will because again, there is a significant loss of aperture in fast scopes at 1X due to the long length from the clear aperture to the eyepieces of the binoviewers.


Significant aperture loss, even with larger secondaries?






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