After last night's SLAS meeting, a question came up from a club member about getting a binoviewer to work with a fixed objective focal length - limited backfocus Newtonian. The following are my reply notes, which may be useful to other club members that own binoviewers. =================== I. Short answer: With respect to your question on getting your binoviewer to work with a fixed objective focal length 10" Newtonian, the short answer appears to be that you will need to use a 2x shorty barlow to project the prime focal plane to the focal plane of the binoviewer. Looking at the Orion and TeleVue binoviewers, 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/product/~category_id=101607clearance/~produ... Use of a negative barlow 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 reocmmend 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). 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 II. Long answer: 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. This is particularly true where a rack and pinion focuser is about 90mm to 100mm about the level of the telescope tube. To compensate for this physical limitation, the primary focal plane of the objective is projected, or moved, to a new location further out-focus of the telescope tube. This effectively increases the backfocus of the telescope. One reason amateurs prefer variable objective focal length telescopes like SCTs is you do have sufficent backfocus to do pretty much anything. Unlike a fixed objective flocal length Netwonian telescope, you do not have to mess with negative or positive projection of the primary focal plane to a new location. Positive or negative projection effectively gives you more backfocus at the expense of increased magntification. Since the objective len's primary focus is fixed, the primary focal plane has to be projected to another location. If you use a negative lens (a barlow) to project (or move) the primary focal plane to a new location, the technique is called negative projection. If you use a positive lens, the technique is called positive projection. Attached (omitted in in this newsgroup post version) find an optical layout diagram with image math notes for negative projection with the binoviewer and three pages on negative projection image math from Edmund's publication 9078 "Photography with your Telescope". We can discuss those at some point in the future if you are interested in further study on negative projection. Clear Skies, Kurt