Here's my take on binoviewers.... First, I do NOT own one of the admittedly (very) expensive binoviewer setups. (Noting also that you have to include the price of set[s] of matched eyepieces...) I made one from a surplus Bausch & Lomb bino microscope head, making my own adapters for 1-1/4 inch eyepieces. But I have sampled some of the commercial units. Yes, the prisms, mirrors, or beamsplitters in the binoviewer give a little less than 1/2 of the total light to each eye... Noting also the typical additional transmission losses in the Barlow lens (nosepiece) that many of the binoviewers come with (to compensate for the extra optical path length through the device). Most of the commercial binoviewers are not typically useable on Newtonian reflectors (as the telescope comes from the factory), because of the needed extra optical path distance. (You have to move the primary mirror forward.) This is less of a problem on SCTs and refractors. U.S. Navy tests on the comparative efficiency of wide-field binoculars, conducted immediately after the close of WWII, showed a GAIN of approximately 1.4X (i.e., square root of two) using BOTH eyes to view. So the loss of magnitude is not as severe as one would think. The "3-D depth effect" is mostly irrelevant, except perhaps viewing the lunar terminator.... Although I seem to see an effect where the fainter stars "recede" to the background, and brighter objects come "forward". This is no doubt an illusion in the brain of the viewer from the combined images from both eyes. In my opinion, most of the usefulness of a binoviewer comes from (1) the convenience and comfort of viewing with both eyes, and (2) the ability to see ALL of the detail in the field in a "single glance", without using averted vision and having to scan around the field. Some additional faint details thus become apparent. I found the binoviewer helpful when doing visual asteroid occultations, in which I was observing >>continuously<< for periods exceeding 20 minutes, attempting to detect possible asteroid satellite events. Such prolonged viewing is >>very uncomfortable<< without the binoviewer. However, the price paid (according to my estimate) was an approximate 1 magnitude loss in detection of faint stars. (Estimated using my 8 inch Meade SCT at f:10.) With telescopes smaller than perhaps 12 inch aperture, a binoviewer is not going to be very useful on deep sky objects, due to the need for additional light gathering power. Since the optics in the binoviewers are derived from standard microscope units, it is NOT possible to use eyepieces with f.l. longer than about 22 mm, due to the restriction of the prisms. There are in effect, NO binoviewers for 2 inch eyepieces (with the exception of ONE custom binocular refracting telescope, with a CUSTOM price!!)
Gene Lucas (17250) David Hofland wrote:
Thank you very much Tom. I anticipated both answers but it helps to ask as I confess to being surprised on a few occasions in my life finding out that things I "knew" were totally wrong. :-)David Hofland Director, Student Services College of Nursing and Health Sciences Jacksonville State University 256.782.5276 ----- Original Message ----- From: "Tom Polakis" <tpolakis@xxxxxxx> To: <sac-forum@xxxxxxxxxxxxx> Sent: Monday, September 15, 2008 1:55 PM Subject: [sac-forum] Re: Couple oddball questions...David,Actually, they are both very smart questions. And there is such a thing as a dumb question.For limiting magnitude, you should pull out all the stops, and use averted vision. You mentioned "at the zenith" which is actually only a point exactly straight overhead. That's a good approximation where to look for your faintest star, but you might have to compromise using the charts you have. Try to find a field whose altitude is at least 45 degrees, or better yet 60 degrees. It's best not to do the limiting magnitude test around Polaris, as you are certainly losing some starlight and contrast that low in the sky.Binoviewers can help with the aesthetic aspect of telescopic views, but they can only harm seeing faint detail. Even if there was a hypothetical *zero* light loss in splitting the beam, each eyepiece is seeing half of your scope's light-gathering area. In other words, a binoviewer makes a 12-inch telescope behave like two 8.5-inch telescopes. So each eyepiece loses 0.75 magnitudes relative to cyclops observing.The best expert on binoviewers I know of, Ed Zarenski, says that the commonly accepted magnitude *gain* using two eyes is about 0.3 magnitudes. So even if you "re-gain" all this light thanks to your physiology, you still lose nearly half a magnitude when you look through a binoviewer.But be careful who you tell this to. I have learned the hard way that it can be difficult to convince those who have dropped a grand on a binoviewer that it reduces their limiting magnitude.Tom
