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Binoviewing The Deep Sky - An Invitation to Research
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Binoviewing The Deep Sky
An Invitation to Research
Binoviewing is usually reserved for dedicated lunar and planetary observers. But could it be an overlooked tool for the rest of us? Do binoviewers on a telescope outperform mono-viewing for deep sky work by boosting contrast and reducing visual noise? They offer a spectacular visceral experience, but are they useful for serious work?
I'm Josh, an avid visual astronomer living under dark-ish skies of suburban Lynchburg, Virginia. (The Milky Way is faintly visible with slight detail on a moonless night.) My passion is the deep sky, and I've recently stumbled into a way that noticeably improves the experience: using binoviewers. It was looking at the dark nebula B86 ("The Ink Spot") in Sagittarius that showed me such increased contrast and detail that I had to investigate further.
The following is a report of detailed observations on one evening, inspired by casual views a few nights before. There will be more to follow, and I welcome your findings if you have any to add. I'd love to hear. I'd also like to collect some more data over different nights, and through different telescopes to see if it supports the initial findings This is a cursory report, with information to add. I do hope you'll be part of the project.
The Myth to Bust: "Binoviewers are only for lunar and planetary work." The binoviewer is a lot of extra glass for starlight to travel through, including a beam splitter and an extra eyepiece. Throughput must surely be reduced. I'm a fan of minimal glass rigs. This seems the opposite.
Things I'm learning about binoviewing: Using two eyes is supposed to reduce noise and increase contrast by the square root of 2 (41%). The image scale seems larger, and the view (can be) more relaxed. (There's a lot more to learn, too.)
Evidence gathered thus far: Two sessions, using two telescopes, on popular summer deep sky targets.
I noticed three important things.
1. Contrary to popular opinion, light loss/throughput reduction was minimal, if noticeable at all.
2. Contrast was slightly to noticeably enhanced on many objects, including: the 11.2 magnitude galaxy NGC 6207 near M13, emission and dark nebulae.
3. The experience was more immersive than mono viewing once the rig was calibrated.
4. A maybe: My slower f/7.5 dob seemed to show more of an increase in contrast with the binoviewer vs mono than with a faster f/5 reflector, but this needs more investigation.
Equipment and Methodology
Telescopes
12.5" f/5 dobsonian of the "Portaball" design (a ball scope), with a Zambuto primary mirror(.992 Strehl ratio) and Antares secondary mirror, 4 vane spider.
8" f/7.5 dobsonian with Zambuto primary (also a .992 Strehl ratio), similarly excellent secondary with a curved spider.
Eyepieces
Televue binoviewers w/University Optics 24mm Konigs (effectively 12mm with the barlow of the binoviewers)
12mm Televue Type 2 Nagler comparison eyepiece in the 12.5" f/5
12.5mm University Optics "volcano top" ortho eyepiece in the 8" f/7.5
Magnifications: 132x in the Portaball (both the binoviewers and Nagler)
128x in the 8" dob with binoviewers, 123x with the ortho eyepiece
I logged observations with the 12.5" Portaball in a hayfield with no porchlights, etc. After returning back, I confirmed findings with the 8" f/7.5 from my back porch, which isn't as dark on the ground due to neighboring porchlights, but roughly 1/4 mile away from the hayfield, so the skies were nearly identical.
Eyepiece Impressions
Setting up my prized 12.5" Portaball dob in the back hayfield under the clear, moonless skies of Thursday, June 27th, here's what I noticed. (Transparency was average to good, seeing was fair to poor.)
The Ring Nebula
I started here to get a magnitude test. As long as I've been using binoviewers, I've thought they were best for lunar and planetary use. (And they are indeed stunning for those applications.) However, did throughput actually decrease to the point where it made a difference for deep sky work?
M57 floated like a cosmic cheerio in a bowl of starry milk. The 12th magnitude star of the "coast" of the ring seemed a good test of magnitude.
Through the Nagler (mono vision), the star vanished with direct vision, and popped out easily with averted vision.
It did the same with the binoviewers! It felt like the Nagler edged it by 5%, but that could easily be imagination. I'd like to do a more detailed study of the throughput, but the difference in this test, if any, was negligible.
NGC 6207
This gem of a galaxy shines at the 11th magnitude near the great Hercules cluster M13. It's a treat to observe, and a fascinating distance comparison: roughly 30 million light years distant vs. M13's 25 thousand.
It appeared slightly brighter in mono, showing more obvious elongation and detail in stereo.
Messier 20/The Trifid Nebula
There's no bad way to observe M20, but here again, binoviewers showed more contrast with the lanes in the Trifid, as well as the distinction between the two main clouds of nebulosity.
Messier 8/The Lagoon Nebula
Binoviewers showed a hair more contrast in the dark veins in the spectacular view. The poor seeing added an unexpected effect: the twinkling of the stars added a ghostly candle flicker feel to the billowing clouds of gas.
In both M8 and M20, the larger image scale and more relaxed posture of binoviewing added to the "wow" factor. While feel is hard to quantify, it's worthy of pursuit.
B86 "The Ink Spot"
A previous observation of B86 sparked this project. An earlier view through the 8" f/7.5 dob had showed a dramatic increase in contrast and detail. Tonight's view through the 12.5" f/5 showed improvement with binoviewers, but not as much as the previous time.
Perhaps sky conditions weren't as good, or the focal ratio made a difference. I went back to my porch, and tried the 8" later that evening. While hard to tell, it did seem to make more of a difference in the smaller, slower scope. A test through a slow refractor is in short order.
In both scopes, it was easier to notice the extent of B86. It's a delightful sight, a blot in front of the small open cluster NGC 6520. A notch out of the cluster's starlight is the first clue of B86. With the binoviewers, the notch expands, takes shape, and is visibly darker than the background sky.
It looked like a giant squid got startled. Maybe Scorpius scared him...
Messier 17/The Swan Nebula
Of all the sights in the summer sky, M17 is a favorite. Again, the binoviewers edged the mono Nagler with contrast and image scale.
Still, I feel that an OIII filter and a lower power would be the best view. The 2x barlow built into the binoview unit makes that tricky. 40 mm plossls
Messier 13
Wow. Who am I to write about falling into half a million stars? It's a sight that must be experienced.
Shortcomings in observation + next steps
This report has several weaknesses:
1. Only one comparison eyepiece with each scope (12mm T2 Nagler in the Portaball, and 12.5mm ortho in the dob).
2. Only one evening of serious observation, and one of casual.
3. Only one optical design–the Newtonian reflector–tested.
· 4. A limited number of objects observed.
Next steps include:
1. Trying this test in a refractor. I'll be using my TS 102mm ED f/11 and possibly a 102mm f/7.
· 2. Hearing about your observations.
· 3. Studying the science of eyesight more.
Annoyances and reasons why not to binoview
If deep sky observation depends on any edge, real or imagined, why wouldn't one binoview? Several reasons:
1. It's more expensive, both the unit, and having twin eyepieces.
2. Balance can be an issue, especially for dobsonians. Switching from a low power mono eyepiece to moderate power binoviewers is a pain if the scope moves around.
3. Some tuning is required to make sure each eyepiece is focused right for each eye.
4. Some folks can't merge the stereo image, or don't have the same use of both eyes.
5. Others report difficulty in using averted vision with two eyes.
6. Eyepieces fog up easier with binoviewing.
7. My unit is a 1.25 configuration, leaving out the 2" eyepieces.
8. The magnifications in my configuration are limited to moderate and high.
Conclusions
It's not for everyone, and it's not for every time. But, these tests show binoviewing has been an overlooked tool in my collection for serious and enjoyable deep sky observing.
Please send thoughts, observations, and questions my way. Correspondence is most welcome. Joshurban@protonmail.com or PO Box 783, Rustburg, VA 24588
Feel free to forward and repost this report, with author attribution and website link (www.Joshurban.com). Let's see what we can uncover.
Clear skies,
Josh
Josh Urban is a writer and speaker living near Lynchburg, VA. He's been an avid deep sky observer for twenty five years.
- scottinash, lwbehney, mrkhagol and 2 others like this
17 Comments
Thank you for the report, Josh. With the increasingly cooler weather (good for seeing, bad for endurance!) I was looking for something to "push" the seeing window and was going to trying my binoviewer. Frank
Thank you for sharing your experience. You answered many questions I've had about binoviewing. But I would like to see how binoviewers would do on dso's with a 5 or 6 inch refractor.
Great question, Jon! I did some preliminary viewing after this was written with a 102mm, but found things too dim with that platform. Would love to try it with a 5 or 6! If you've got one near Lynchburg, VA, bring it by and we'll do some looking!
Thanks for the article! Here's my take from the cheap seats:
In our bortle-4 skies using the Arcturus binoviewers with the stock eyepieces on the Startravel 102, most DSOs below magnitude ~6.5 are, to my eye, close in brightness, but more detailed and colorful, than when viewing them with my Celestron X-Cel LX 25mm or my Celestron Omni 32mm Plosl. For objects above said magnitude, the monoculars edge out the binoviewers, especially with focal lengths of 12mm and below.
Thanks for the report! I'm wondering if there's a light threshold where they become useful for DSOs. From doing an "unofficial" session with my 102mm f/11 TS optics ED refractor, they were underwhelming. The 8" dob seemed best, but at this point, it's anecdotal. Thanks for seconding my findings with an 100mm-class instrument.
I think one crucial difference is that the Startravel 102 is f4.9 VS f11 for the TS optics ED.
The issue isn't with the extra optics, it's that by necessity, each eye only receives 50% of the light.
It's certainly not my experience that the light loss doesn't change. It's DRAMATICALLY dimmer in a binoviewer than monoviewing. Binoviewing DSOs in my 14.7" feels like monoviewing with a 10" but with a bit more contrast.
What you gain from binoviewing is signal reinforcement and therefore contrast, but in my experience this does not make up for the 50% light loss to each eye.
This is not qualitative either - I can see fainter threshold galaxies monoviewing than I can binoviewing at the same magnification/exit pupil.
The Orion Nebula looks more contrasty binoviewing, but I can trace the fainter extents of the nebula out farther when monoviewing.
For me binoviewing DSOs just doesn't work and is net worse than monoviewing through the same instrument.
Howdy, CrazyPanda, good to see you on here. You bring up excellent points, and a question that I don't know the answer to: does the brain "re-add" the split beam so it's nearly the same as the unsplit?
In other words, is the equation: Original light, Divided in Half, 50% in each eye = 50% dimmer perceived image
OR:
Original light, Divided in Half, 50% in each eye, summed back together by brain (with slight loss) = 3% dimmer perceived image
And if so, is the boost in contrast enough of a benefit in some cases to offset it?
Word on the street has the first equation right. My experience with the 8: f/7.5 dob and a dark nebulae hews closer to the second.
Perhaps there's also a factor of knowing what I'm looking for. If I see a dimmer, but clearer, picture of an object in stereo, and then switch back to a brighter mono view, maybe that's a way to get more mileage out of it.
We both subscribe to the "ever last bit helps" school of deep sky observing, and this stereo observation of B86 has puzzled me. I'm realizing I've been overlooking the optical train after the eyepiece, too. (Eye and brain.)
I'm looking at this article as an open-source call to research. Appreciate you sharing your findings here!
I have read somewhere that binocular summation adds about 1.4 times the perceived amount of light grasp. That would mean objects should be perceived as about 30% dimmer than when viewing them with one eye. If that holds true for a given observer, then using a binoviewer would be equivalent to reducing your telescope aperture by 83% in light-gathering capacity.
1/1.4=.71; 100-71= 29 OK. Where does the 83% come from?
You are correct that the light gathering affect is 29% less, but the specific telescope aperture reduction to achieve that 0.71 effect is the SQR(.71) = 0.84. I rounded it down to 0.83 based upon my 30% dimmer assumption and hence SQR(0.7). The brightness diminishes at the rate of the square of the aperture reduction. So a six inch refractor using binoviewers would probably have the light gathering equivalent of a 120+ mm refractor, but would have better contrast and better resolution than the 120 mm refractor and would also manage a higher magnification at any given exit pupil. That would help contrast also.
Omg josh? we just talked today.
I don't think you mentioned the specific brand /model of binos you were using, that could be insightful to your results. I've also heard some talk about using binoviewers to compare different EPs, if you ever get the chance that could make an interesting write-up. -A
To investigate the effect of F-ratio try putting an aperture mask over your 12.5" F/5. An 8.3" mask will match the f/7.5 of your other scope. And you could try 6.25" to investigate f/10.
With the Portaball build it may not be easy to quickly swap the mask in and out, but you might be able to with your other scope, and test a 6" mask for f/10.
I'm one of those that struggles to merge both images on binoculars, microscopes
and binoviewers (phoria).
Make sure you don't have trouble using plain binoculars, otherwise there's some risk the
same issue may reappear on binoviewers. In that case, make sure you have a solid refund policy before swiping your credit card.
For those who don't have the issues -> enjoy the view, it's really worth it.
Great research project! CrazyPanda has an extremely valid point on the brightness being halved for each eye and the resulting dimness affecting how much of the “faint fuzzy” you can see.
That being said, using two eyes can help mask the physiological limitations of an individual eye. Astigmatism frequently varies between individual eyeballs (my right eye is slightly more astigmatic than my left, but merged eyes cause the single eye astigmatism to disappear (brain processing). Dioptrix is great…for the eyepieces that accept it. Another potential (?) benefit of two eyes: blind spots caused by the optic nerve in each eye could be slightly different in size/shape/structure, so binocular overlapping coverage could be a possible advantage.
I’ve also found “halving the brightness” works pretty darn good when you have a brighter object in the same FOV…that might be part of the apparent increased contrast gained from binoviewing. I look forward to reading more as you continue the research!
Yes indeed. Jupiter is too bright in any aperture > 6 inches and binoviewers really help calm that down to tolerable levels of light intensity.
My own binoviewing experience is with a Denkmeier Binotron 27 with an Obsession Classic 15" f4.5 reflector.
With this setup, where there is no dearth of incoming light, there is no question that the binoviewer provides a superior visual experience for nearly every kind of DSO. Binocular vision both magnifies objects and increases contrast and therefore enhances detail. Any reduction in the number of photons reaching each eye simply doesn't matter to the brain: if I can see it with my 15" scope, it will look bigger, brighter and more detailed with the binoviewer. Bright objects like the Dumbbell and Swan Nebulas seem to float before you in 3D and when the Andromeda Galaxy fills your field of view, the reduction in brightness probably helps to reveal the subtle gaps between the spiral arms. Dim galaxies in the Virgo Cluster - Messiers and the dozens of smaller NGCs between them - are easier to distinguish from each other, and even the far distant galaxies of Stephan's Quintet are easier to pick out. And then, of course, there is the immersive nature of the experience, which you can't put a number on.
Some features of the Denkmeier in particular that enhance the experience are the ability to switch between 3 magnifications using sliding tabs as well as the ability to switch from unfiltered viewing to one of 2 filters also using a slide mechanism. The eyepiece holders also adjust focus independently and the workmanship overall is top notch. Customer service is unmatched - personal and generous.
The downsides:
1. in my scope, the main issue is the absolute field of view. The 21mm 65º EPs yield an absolute field of view that's a little less than half a degree at the lowest power, and the 32mm 50º EPs FOV is only marginally bigger (because of the Barlow lens and the added distance needed to come to focus, the functional focal length of the EPs is cut by about half). Since I only do star-hopping with a Telrad, I often use a single 20 or 31mm wide angle EP to search for small dim objects before switching to the binoviewer to study them - the wider field keeps guidestars in view and for me the "2D" view with a single eye helps non-stellar objects stand out more in a busy field. Obviously, this also limits how I can view extended objects like the Veil or Rosette Nebulas - and it also means I have to move my scope more often to keep them in view.
2. The weight. It's heavier even than my TeleVue 31mm Nager and I do need a counterweight - a filled 16oz water bottle - at the bottom of my scope when I use it below 45º azimuth. Remember, this is an Obsession Classic, all wood construction, so the mirror and rocker box already have heft. I have also used it on my 8" Orion Dob with a heavier magnetic counterweight without any issues.
3. The cost. The Denkmeier is an expensive purchase, but for a serious observer, it could be considered a bargain. Complete with 2 pairs of EPs and extenders to reach focus in my large Dob, the total cost was about the same as 2 large Televue EPs (+/- $1500). And, it transforms my 15" Obsession into a completely different kind of instrument - basically the equivalent of a pair of high quality 130-300 x 270 binoculars!