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Strut Newt Binoviewer Strategy

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#1 careysub

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Posted 31 October 2013 - 03:25 PM

I recently acquired a 13.1" F/4.41 Newtonian that is build with three straight split struts, not a tube nor a truss. The struts connect to three wood rings: the bottom ring which supports the mirror cell, a middle connector ring, and the top ring with holds the focuser.

One nice thing about this design is that the 3 top strut piece are only 22" long, and can be swapped for three shorter struts if I need more in-travel.

That being the case I can set this scope in "binoviewer" mode with shorter struts to get all the light path I need without needing barlows or other optics to extend the focal plane.

The swap need only take a minute (plus the recollimation time) so it could easily be done for only part of a viewing session.

This changes the game for Newt binoviewing - and makes it less clear to me on what the best options are for binoviewers and associated equipment.

For a "starter system" I picked up a William Optics binoviewer used, which is listed as having a 4" light path.

#2 GlennLeDrew

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Posted 01 November 2013 - 04:36 AM

I'd be curious to know if you're utilizing the full aperture when not usung the OCS (if you do indeed do this.) I would guess that the limiting f/ratio would be nearer to f/5, or an effective aperture of about 11.5".

#3 careysub

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Posted 02 November 2013 - 12:27 PM

I'd be curious to know if you're utilizing the full aperture when not usung the OCS (if you do indeed do this.) I would guess that the limiting f/ratio would be nearer to f/5, or an effective aperture of about 11.5".


I haven't had an occasion to take the WO BVs out with this telescope yet (probably not until after Black Friday) but I would use it with the 1.6X Barlow nosepiece that came with the WO BVs.

I'll have to try it with and without the Paracorr. From what I read BVs don't like F/ratios under 5, so I would guess using the Paracorr would be all but obligatory.

If I make an upper strut set 4" shorter I can use it without the Barlow I believe. Would that really be an advantage to speak of?

These are older model WOs, and have path obstruction that prevents them from being Seibert supercharged.

How are you calculating the bit about the focal ratio and effective aperture?

Thanks,
Carey

#4 BBryce

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Posted 02 November 2013 - 01:00 PM

Hi,

I also use a 12" Dobson f5 with a set of shortened truss tubes, about 110mm (4.3"), exactly the light path of my Siebert BN25.

So I don't need any ocs/oca for reaching focus and the views with a pair of Panoptic 24 and D14 are to die for !

I no longer use the cyclop mode !! Viewing wide fields with both eyes without any magnification due to the use of some ocs is just marvellous when it comes to newton.

I can use the full aperture of my scope without almost any loss of light, maybe a bino with more CA will give more light or so, baader markV or Siebert echelon 40mm.

But the BN25 has been designed with the same F/ratio than the MarkV, I don't have any vignetting with my pair of Panoptic 24mm.

So I can do no other than encourage you to use your newt in bino mode without ocs, you'll never go back to mono mode !

Clear skies !

Brice

#5 Pinbout

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Posted 02 November 2013 - 01:07 PM

to go native I raise my primary up 4", and it is exactly 4", to use my WO binos. but to get the full aperature I had to go from 1.5 2ndry to a 2.14 in my 8"f6 dob.

using a 1.6x ocs will require a different 2ndry to primary distance than 4" shorter or normal length.

if I use my 2x barlow I don't have to move anything from standard view setup.

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#6 careysub

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Posted 03 November 2013 - 08:50 AM

Thanks for the encouragement, I will be using the WO BVs for the time being, but I want to make plans for an upgrade in the future.

Any suggestions about what BV brands/systems/components make sense for a "binoviewer-ready" Newtonian?

The existing market assumes you either have a refractor or SCT (and thus takes a diagonal, e.g. the Denkmeier powerswitches), or that you have a conventional Newtonian and thus need an OCS/Barlow. The relatively short focal ratio (5.06 with Paracorr) may also affect what equipment is most suitable.

A premium binoviewer, plus filter slide, plus Paracorr, plus two EPs adds up to quite an optical stack extending from the focuser. I wonder if a conventional Moonlite is up to it.

#7 Pinbout

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Posted 03 November 2013 - 08:56 AM

On that optical path do glenn's laser aperature test to see if your loosing aperature cause the secondary size.

A moonlite is rated to 5lbs if it has a motor 8lbs

#8 careysub

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Posted 03 November 2013 - 09:13 AM

Adding up all of Denkmeiers weights that would be used with a tricked-out Binotron-27 (D14 EPs, filter switch, power switch, no OCS, no diagonal, no Newt spacer), plus the Paracorr, it comes out to nearly 4 lb. That is within the nominal weight capacity, but I wonder if the long cantilever would compromise it.

#9 johnnyha

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Posted 03 November 2013 - 05:26 PM

Most people don't use the Paracorr with their binoviewers. It's really not necessary once you add an OCS and are viewing at 1.7X+. Even the B27 at 1.4X does well without it.

#10 careysub

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Posted 03 November 2013 - 08:15 PM

Most people don't use the Paracorr with their binoviewers. It's really not necessary once you add an OCS and are viewing at 1.7X+. Even the B27 at 1.4X does well without it.


Ah - thanks for emphasizing the point of my original post. My planned configuration is NOT what most people binoviewing with Newts use!

I will not be using an OCS, I can adjust the primary to focuser distance so that I can get the back-focus needed without special glass. I probably would be using the Paracorr, at 1.15x, instead.

Experience with this is definitely thinner of the boards here - so I am looking for advice to assist in planning future purchases and making mod decisions (possible pitfalls, etc.).

#11 Pinbout

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Posted 03 November 2013 - 08:54 PM

I probably would be using the Paracorr, at 1.15x, instead.



how do you do that with the field stops more than 4" away? unless you buy two and put them after the bino? :foreheadslap:

#12 careysub

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Posted 03 November 2013 - 09:04 PM

I probably would be using the Paracorr, at 1.15x, instead.



how do you do that with the field stops more than 4" away? unless you buy two and put them after the bino? :foreheadslap:


Double a-ha! Yes, of course that would not work! Rookie error avoided!

But the EPs for a BV are 1.25", so Siebert coma correctors (which operate at 1.25x) would be needed if you wanted coma correction.

But using it at the native F/4.42 would create field illumination issues wouldn't it? I found an older post about Denk's having their fully illuminated field drop to zero at F/4.42.

Do some BVs handle steep light cones better than others?

#13 Pinbout

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Posted 03 November 2013 - 09:21 PM

you can always do the emperical testing and see the results of glens aperature test.

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my seconday was too small.

#14 BBryce

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Posted 04 November 2013 - 11:02 AM

Do some BVs handle steep light cones better than others?


Mark V and Siebert BN25 both work perfectly down to f4 without any OCS.

I also don't use any paracorr for binoviewing with my f5 dobson fitted with a CR2 moonlite, as I use some well-corrected eyepieces such pano 24 & Denk14.

#15 Pinbout

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Posted 04 November 2013 - 12:31 PM

I was looking at m31 last night and I never used the binos on it. even from my bleached out white sky it was a lot better than the night before going mono, and my bino scope's primary isn't as good as my truss scope.

#16 careysub

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Posted 08 November 2013 - 12:25 PM

The Echelon 40 ($1200) and Elite 45 ($2000) Siebert binoviewers were mentioned on another thread.

Expensive, but if one is considering (at some remote future time) dropping $1750 on a Mark V, then going $2000 on a Siebert binoviewer is an option to be considered. The Echelon is a comparative bargain. Seems like with their very large CAs, they would do well with a fast Newt that has a special no-OCA binoviewer mode.

Any problems with these?

#17 careysub

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Posted 08 November 2013 - 01:44 PM

Post deleted by careysub

#18 BBryce

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Posted 08 November 2013 - 02:22 PM

Yep Echelon 40 is my future and ultimate upgrade for my dobson !

#19 Sorny

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Posted 14 November 2013 - 02:51 PM

What 2" eyepieces will you be able to get your nose between? Have you considered the extreme optical path a 2" binoviewer has? You'll need to move the primary several inches, and increase the size of your secondary to a size approaching that of a R-C to get adequate illumination.

Mind you I think a 2" binoviewer is very cool, but a 2" binoviewer has every problem a 1-¼" binoviewer does, and then a whole heap more.

#20 careysub

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Posted 17 November 2013 - 06:20 PM

What 2" eyepieces will you be able to get your nose between?


The Echelon 40mm VP is a 1.25" binoviewer. It has a 2" initial light path, but ultimately it delivers the light to 1.25" EPs.

The reason for interest in this device is the steep light cone of a fast Newt that gets vignetted in BVs not designed for it. Denks provide zero fully illuminated field for F/4.42 for example.

Have you considered the extreme optical path a 2" binoviewer has? You'll need to move the primary several inches, and increase the size of your secondary to a size approaching that of a R-C to get adequate illumination.


In my case I don't have to move the primary, I just lower the focuser. The scope uses straight struts that are split with a connector ring (the upper half could be thought of as a tall, disassemble-able cage). Substituting a set of shorter struts for the upper ones lowers the EP.

Crunching the config with Mel Bartel's diagonal calculator, the situation does not look bad at all, though a larger secondary will be needed.

I currently have a 2.6" diagonal for a 13.1" mirror, giving me an excellent CO of only 20% and excellent illumination with every EP combo I plan to ever use.

The Echelon 40 has a 5.8" light path. Placing the focuser board surface at a radius of 7" touches the entrance aperture, but just barely, no significant light loss. 7+1.4+5.8=14.2", plugging all this in I find that a 3.5" diagonal will give excellent illumination in any 1.25" EP on the market (0.4" fully illum field, max light drop 0.12 mag from illuminated field). The CO is still only 26.7%, much less than my C8 (34%).

Mind you I think a 2" binoviewer is very cool, but a 2" binoviewer has every problem a 1-¼" binoviewer does, and then a whole heap more.


It does not have vignetting of the light cone due to an insufficient clear aperture.

#21 David L

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Posted 17 November 2013 - 07:35 PM

I am in the process of making an extra set of trusses for my 16 Lightbridge so I can go "native", but not sure how the situation will be with the secondary size yet. Just bought a Denk binoviewer. Dave

#22 Pinbout

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Posted 17 November 2013 - 08:04 PM

I raised up my mirror 4" and had to change the 2ndry two sizes larger to get no vignetting with the glenns laser test. But visually I couldn't tell a difference.

#23 CarlDD

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Posted 17 November 2013 - 09:31 PM

Hi,
Could you remind me of the link to "Glens Laser Test"
Thanks in Advance

Best Regards
Carl

#24 Pinbout

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Posted 17 November 2013 - 11:18 PM

kindof like the aperature flashlight test for binoculars.

put in something like a 7mm eyepiece in the focuser. measure the length while focused at infinity.

then indoors, while holding the laser about an inch away from the eyepiece, shot it thru the eyepiece while its focused at infinity and project it onto a "wall" and measure the circle to see if your getting the full aperature.


here's my undersized 2ndry for my binoviewers

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