21705Re: how to use binoviewer
- May 2, 2011
A barlow or reducer is NOT a standard "included" item with a new bino viewer as there are numerous sizes, focal lengths, lens confg., etc with regard to telescopes. There isn't a "one size fits all" barlow/reducer. Manufacturers wouldn't want to eat the cost of producing/including several quality adapters in order to sell one binoviewer - and customers wouldn't be happy to pay 100's of dollars more (especially with quality reducers)for a range of barlow /reducer sizes when only one will work with their particular set-up. Some bino sellers do offer a barlow or reducer with the purchase - but it shouldn't automatically be expected. The abscence of ana barlow or adapter doesn't make the manufacturer or seller of a binoviewer unscrupulous. Below is an excerpt discussing barlows/reducers from the BEST!!! article regarding binoviewers I know of(also includes ratings of bino's to each telescope type[reflector, refractor....] & best eyepiece model/brand/size performance configuration. Then broken down again by type of use/ viewing interest [planetary vs. nebula -etc.]).It's entitled"Enjoying The Sky With Two Eyes"and can be found athttp://www.weatherman.com
COMING TO FOCUS WITH YOUR BINOVIEWER, and BARLOWING:
..... requires that you rack your focus way in to account for the extra travel path of the light to reach your eyepieces. This travel length averages 5.5 inches. In fact, on most scopes, you simply CANNOT reach focus without using a barlow ahead of the binoviewer, to counteract this. That doesn't sound like a big problem at first, but because the distance is also greater between that barlow, and the eyepieces after travelling through the binoviewer itself, the barlowing is "magnified" to somewhere between 3 and 4 times, on a regular 2X barlow! For some scopes, this will strap you to such high power, that wide field and low-medium power viewing is impossible. There are several exceptions, and tricks to help dodge this issue: (I really got good at dodging this!)
1. On many refractors, the (no-longer-available) Astrophysics/Zeiss binoviewer will come to focus w/o a barlow. Without modification however, it will not come to focus w/o a barlow in most reflectors. There seems to be less focus travel required for this unit, perhaps because of the special color-correcting lens that comes with it.
2. On many refractors the Televue binoviewer will come to focus w/o a barlow if used "straight through", and not in a star diagonal. Inconvenient, but it works.
3. Reflectors have, and can be custom made or re-configured to come to focus with a binoviewer. In fact, on truss Dobs, you can get an extra set of truss poles made some 5.5" shorter, for instance, to accommodate binoviewer use. Of course, this can be inconvenient switching back and forth, and requires some design consideration as well, involving the light path. (ie.. making sure you diagonal is large enough) I didn't particularly like this solution.
4. Cadiatropic scopes such as Schmidt Cassegrain that come to focus by moving the primary mirror will likely have no problem coming to focus with a binoviewer... in most configurations. (provided you don't try to use it with a reducer-corrector which already throws off your focus position substantially)
5. You can use a lower power barlow to try to minimize the magnification. A 1.8X Televue or a 1.5X 2" Vernonscope Barlow (called 2X barlow, but that is when it is used ahead of a star diagonal, in reality it is 1.5X) will help. I have done some crafty "mixing" of barlows to reach 2.6x. Using a regular 2x barlow often results in magnifications close to 4x when employed with the binoviewers.
6. Televue has come out (9/98) with a 2X magnifier that is parfocal! (No in-travel) It only magnifies the image exactly 2x and allows the Binovue to be used in almost all scopes since any extra in-travel is minimal. Televue has now (1/99) made this special barlow "standard" with every binoviewer they sell. This means that you no longer are strapped to the 3.5X - 3.8X that came from using their regular 2x barlow with the unit, as it was previously supplied. The only disadvantage is that their new barlow (or "magnifier", whatever) creates some vignetting which is noticeable on lowest power eyepieces only, mostly when a fast scope is stopped down off-axis.
7. Zeiss/Baader has a 1.7X magnifier that reduces focus travel significantly. It does not completely solve the in-travel problem however... leaving nearly 3" more in-travel needed. If you can figure out how to mate it to your particular binoviewer, (it's internal) and work with other considerations as outlined above - you may be in business. Markus Ludes is currently selling a 2.6x that he apparently claims works to make many binoviewers parfocal. I have not tried that option.
8. As mentioned above, the Takahashi Twin-View has a built-in 2.1X barlow that reduces focus travel and
allows it to reach focus in many (but not all) scopes.
9. Harry Siebert has put a lot of effort in getting his binoviewer AND the Televue binoviewer to work with reduced magnifications. For instance, he sells a corrector/barlow that only has need for 1/4" of in-travel, which means it can be used with almost all telescopes. It is long at 3 1/2" so it is not great in a diagonal, but works wonders in a fast dob. In fact, are you sitting down................... The magnification from the unit is only around 1.35X !!!!!!!!!!!!! It works exceptionally well, and I am now using it instead of my 2x televue magnifier/barlow/corrector with the Televue binoviewer that I own. I frankly couldn't believe my eyes when I used it. It is that good.
Even with all of the above, finding the exact focus position that works with your scope mated to your binoviewer with, or without a barlow can be a bit ...
I should mention that hurdles? mentioned in the excerpt above isn't indicativeof the overall article. the article is PRO binoviewer -very,very much so!
Hope this clears things up a bit.
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