On Aug 24, 1:55*pm, brucegooglegroups
wrote:
Anyone use binoviewers with a Dob? Recommendations? Clear Skies. Bruce

To get a binoviewer to work with a fixed objective focal length 10"
Newtonian, the short answer is you need to use a 2x shorty barlow to
project the prime focal plane to the focal plane of the binoviewer.
The length of the optical path of a typical binoviewer is about 5 to 5
1/4 inches or around 150mm. Since the distance between the telescope
tube and prime focus on a 10 inch Newt is around 140 mm, there is no
way to get the binoviewer to focus without repositioning the prime
focal plane using negative or positive projection.

Looking at the Orion and TeleVue binoviewers webpages, both either
sell a corrector lens or a recommend the use of a shorty barlow to
project the prime focal plane into a range that the binoviewer
requires.

TeleVue Binoviewer Page
"Bino Vue comes standard with the Flat Coupler and 2x Amplifier
installed on the body. This permits the Bino Vue to be used in all
telescopes with just a moderate focuser intravel requirement."
http://televue.com/engine/page.asp?ID=70

Orion Binoviewer Page
"Includes thread-on 2x Barlow lens so you can choose two levels of
magnification . . . Reflectors and short-focal length refractors may
require the short 2x barlow lens"
http://www.telescope.com/control/pro...oduct_id=52071

Another option, if you own one, is a TeleVue variable positive relay
lens - brandnamed the TeleVue Powermate. Put the TeleVue Powermate in
the optical path. That will let you raise the primary focal plane to
enough back focus so the binoviewer will work.

http://www.televue.com/engine/page.asp?ID=42

Use of a negative barlow or a positive relay lens to reposition the
prime focal plane will increase magnification between 2x to 3x. So
you will want to use the highest focal length set of eyepieces that
you have, e.g. two 32mm fl eyepieces, to get a true-field-of-view that
is equivalent to that seen using a 10mm or 15mm eyepiece.

I recommend first doing an initial focus test on a daylight
terresterial target like a distant mountain top using a single 32mm
e.p. Do two tests:

1) The 2x barlow using the barlow body (3x-4x mag); and,

2) Unscrewing the negative barlow lens off the bottom of the 2x shorty
barlow and rethreading the barlow lens directly on the bottom of your
binoviewer (2x-3x mag).

- Canopus56

P.S. - Optical math diagram for a binoviewer on a fixed focal length
Newt

http://members.csolutions.net/fisher...primefocus.png