ORION PREMIUM LINEAR BINOVIEWER
Recent discussions motivated me to get one and evaluate it first-hand. I quickly browsed CN history, noted a blend of both spot-on and misunderstanding, all the more reason to start fresh, slate clean, and simply experience it for myself. Here's what I've discovered!
This make and model is produced in Taiwan and marketed under several brands. There are others operating on the same principles. The Orion and TS-Optics sites provide decent qualitative descriptions regarding the principle of operation and advantages. Not much quantitative... which is fine. Marketing, sales and tech blended for user-consumption, without getting prohibitively techno.
Here are images of the gizmo and the cartoon I was able to find. That was annotated in Chinese, but I was able to understand what they were explaining --- fascinating! I annotated in English and added my comments. Now for some (back-solved) techno-speak, for you who enjoy such: The light from the telescope comes in from the left, headed toward the input focal plane, which is virtual, because that positive meniscus singlet lens element intercepts the light, before it gets there. This forms a ~60% compressed real image at my annotated fp conjugate. My first knee-jerk response was "Yikes! that must be a terribly-aberrated real image, compressed to 60% with only a singlet". But, not to worry. The other elements take care of that. As long as their aberration contributions cancel at the final image presented to the eyepiece(s)... all is well! The 1st five elements reimage the telescope's pupil (aka the objective mirror or lens) upon the disjoint pupil splitter. I coined that descriptive moniker to indicate its function, and contrast it with the more traditional beamsplitter approach, which does not require said conjugate relationship. The final doublet reimages the pupil back to where it started, and the focal plane to where the eyepiece would expect to find it, in the absence of the binoviewer --- aberration-free, at 1x, and original F# feed. That's really all there is to it! If it seems cryptic, not to worry... the lens designers take care of all that. The generalized optics context is "periscopes", where the designer has to stuff light thru skinny meandering, articulated tubes, without badly compromising performance. Our predecessors got very good at such work during the 1st and 2nd WWs... we are the beneficiaries!
One unavoidable consequence of this disjoint pupil-splitting approach is that each eyepiece gets all the light from half the telescope's lens or mirror. In contrast, the traditional approach, where each eyepiece gets half the light from all the telescope's lens or mirror. In each case it's a 50:50 split, but executed quite differently. Each has its advantages and disadvantages.
round pupils half-moon pupils
50% luminance 100% luminance
tb res preserved tb res preserved
lr res preserved lr res degraded
net info preserved net info preserved
50/50% BS coatings 100% mirror coatings
neg relay Barlow 1x positive relay
I'll just quickly mention that Emmy Noether's Invariance Theorem imposes other conditions, that are unavoidable. But resulting in a rather astonishing advantage for the disjoint-pupil binoviewer. Teaser >>> At very low magnification use, the images will be twice as bright and luminous as with the traditional BV. I'll be happy to expound on that, if anyone is interested. Could allow for enjoying e.g. M31 with a big Dobsonian, at "abnormally low mag" and seeing it "twice as bright" as a plain mono eyepiece.
I ordered it via Amazon Prime; it arrived a day and a half later, free delivery, but punitive NYS sales tax. Nice carry case and packaging. Heavier and more compact than I had visualized... and looks more precise, sturdy and promising than what I envisioned. Dank rainy day. I opened a window in one of the spare bathrooms and set up the trusty old Televue Genesis F/5 APO Refractor, aimed at tree twigs about 500 feet away. Put in a good Star Diagonal and set focus on the twigs, using the 10mm TV Delos... 50x. OK, that looked good, and reminded me of the residual color (as one should expect) from a decent fast APO... green/magenta blush thru-focus. So far, so good.
I resurrected my annoying habit from decades working at the Research Labs at work. Just before field-testing, anticipate/guess what is apt to be right and wrong with what will follow. (Then, after the fact, compare actuals with anticipated.) Well, I guessed that, as nice as this thing looked and felt, that its performance would be somewhere between quite and terribly disappointing. I expected that I would indeed find half-moon pupils, but ragged and/or blurred. I expected the chromatic aberration to be terrible, because of the aggressive positive relay. I expected field curvature, bad collimation, vignetting... especially at the aggressive F/5 feed, which most all binoviewers abhor.
So, I popped it in. Put in a pair of Delos 10mm @ 50x ... and looked at the twigs again! Astonishing was that they were in perfect focus! When they say ~zero focus shift~ they're serious! I finessed the IPD and L/R focus just to make sure, and then further scrutinized the experience. Well, the image-merge felt comfy, good collimation. Both sides balanced for brightness, neutral color, resolution, field flatness, chromatics (only residuals or scope itself)... I was looking thru an open window, and occasional thermals from that wafted by, but nothing to indicate that the binoviewer was in any way deficient. At that point I knew it was a keeper and exceeding my expectations. And reminded myself that native F/5 is a pretty aggressive feed for a binoviewer.
Then scooted over to the Eyepiece Vault to extract any pair of premium eyepieces that were at hand. Here's in order the list of all tried:
eyep AFOV FS mag pupil
>Delos 10mm 72deg 12.7mm 50x 2.0mm
>Nagler T6 13mm 82 17.6 38 2.6
>Nagler T6 9mm 82 12.4 56 1.8
>Nagler T6 5mm 82 7.0 100 1.0
>Delite 18.2mm 62 19.1 27 3.6
>Delos 17.3mm 72 21.2 29 3.5
>Night Vision 26mm 50 18.0 19 (na) --- not tried yet
They all performed well. The Delos 17.3 (biggest field stop) had degraded imagery at the left/right edges, but perfectly fine over the rest. I'm guessing that's the combination of field and F# stressing the system. The Nagler 5mm was challenged for resolution, but that was the open window in cold weather, not the scope, binoviewer, or eyepieces. Occasionally the thermals would subside and then both sides looked nice and sharp. My eyes felt strained at 100x. I was still merged OK, but felt like my eye muscles were stressed to maintain image vergence. At lower mags this was not the case. I didn't have time to reseat the eyepieces, etc. ... but short efl eyeps push collimation and seating challenges.
Overall, this 1st trial is very encouraging! I haven't yet tried it on the stars, dark-adapted, faster feed, or on the Newtonians, with their centrally-obstructed apertures. This should be a hoot to poke into the focuser on my giant telescope. My 36-incher is F/3.75 (F/4.3 with essential ParaCorr)... Hmmm... ? Also, not tried yet... my pair of Night Vision Eyepieces! With the Zero Focus Shift, I can pop them directly into the receivers, image(s) falling directly onto the GaAs photocathodes... expecting spectacular performance.
[Perspective: I know there have been tons of discussions here on binoviewers, but I chose to not bias my 1st light too much with what others have already decided I should think or discover. I appreciate the history and wisdom archived here, but take Richard Feynman's advice to heart... to rediscover what others and no others have learned in the past, in one's own way. What it was, what it should be, will otherwise contaminate our experiments.]
Bunch of pictures >>> Tom
Edited by TOMDEY, 26 January 2020 - 12:40 AM.