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Using one tube of a binocular telescope for high magnification?

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#1 Binofrac

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Posted 20 November 2023 - 06:36 AM

Hello esteemed binocular telescope users,

 

Most of my observing is through binoculars. Occasionally though I'm using my 102/f11 refractor for solar system objects, and sometimes with a binoviewer. I've long wondered if I could be satisfied with a binocular telescope instead. The higher than normal hand held binocular magnification would be handy occasionally but I would mostly enjoy bigger aperture low power views. For odd times though it would be nice if I could use the same instrument to mono view at a higher magnification. Where I am my useful magnification tops out around 120x and I'm wondering if a BT would allow this if I used a higher power eyepiece in just one side. I know that normal binocular use would not reach this. I'd be especially keen to hear from users of the Oberwerk BT's in this regard.

 

I can't imagine this hasn't been asked before but but I've searched hard and haven't found it.


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#2 TOMDEY

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Posted 20 November 2023 - 07:18 AM

It's unlikely, but worth a try.

 

Most (nearly all) Binocular telescopes peak out around 100x because of squirrely ~collimation~ (image merging) issues, not necessarily deficient wavefronts. But that limitation most often feeds back into loosening the wavefront requirement to something less (often far inferior) than diffraction-limited. So, as you allude, the lion's share of binocular telescopes comprise two somewhat junky mono telescopes, perfectly fine for the default intended casual low mag / Rich field application.

 

Easiest thing is to just try higher mag one side (and then the other) just to see how it performs. I previously had 4-inch Oberwerk that exhibited terrible astigmatisms, even objectionable at low mag. I realize that is a statistic on only one, but am convinced this is average performance. I had the same experience with my Vixen 125mm binocular --- junky images, even at low powers.

 

I later bought and enjoy the APM ED APO's (two of them) which seem a decent step up... but I still use them at 100x or less, not expecting much beyond that, even monoscopically. The home-brew binoscopes made from two premium telescopes can deliver exceptionally good performance. But the image-merge is still the Achiles heel.

 

I got beyond all of those issues with my JMI RB-16 (16-inch) reflective Newtonian Binoscope. Custom matched Fullum Primary Mirrors and premium secondary folding flats and star diagonals. So yes, I can indeed go high mag, even binoscopically. This is because the JMI has on-the-fly image merge, just by pushing a couple of buttons while looking in.    Tom


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#3 Fiske

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Posted 20 November 2023 - 07:31 AM

Hello esteemed binocular telescope users,

 

Most of my observing is through binoculars. Occasionally though I'm using my 102/f11 refractor for solar system objects, and sometimes with a binoviewer. I've long wondered if I could be satisfied with a binocular telescope instead. The higher than normal hand held binocular magnification would be handy occasionally but I would mostly enjoy bigger aperture low power views. For odd times though it would be nice if I could use the same instrument to mono view at a higher magnification. Where I am my useful magnification tops out around 120x and I'm wondering if a BT would allow this if I used a higher power eyepiece in just one side. I know that normal binocular use would not reach this. I'd be especially keen to hear from users of the Oberwerk BT's in this regard.

 

I can't imagine this hasn't been asked before but but I've searched hard and haven't found it.

This is a great question, Martyn. Thank you. waytogo.gif

 

One difference between your 102/f11 refractor is that binocular telescope OTAs have much lower f-ratios (typically in the 5.4-5.6 range), which is quite a bit more demanding optically. Plus, erecting prism assemblies are more complex optically than mirror diagonals. This doesn't mean that it's not possible to use one side of a BT as a refractor, only that the optical performance may be somewhat different from a stand alone refractor. And not necessarily as good at higher magnifications.

 

But, it's something I can easily try, and never have. lol.gif

 

So I'll give it a whirl and share some impressions.


Edited by Fiske, 20 November 2023 - 07:32 AM.

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#4 Binofrac

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Posted 20 November 2023 - 10:02 AM

Thanks Tom and Fiske,

 

One day if I ever reach that mythical retirement I'll build a binoscope from a couple of Skywatcher ST102 scopes. That should pretty much do everything I want.

 

Tom- I've seen your binoscope on previous posts and it's very impressive. I also took note of one of your posts that showed how to get a right angled binocular view with a couple of tank mirrors which I might try soon.

 

Fiske- Your binocular reviews are most informative and I'd be very interested to know how you get on. I understand about the different optics so I thought I'd reach out to those who may be able to give it a go and see if mono high power viewing works.


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#5 dnayakan

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Posted 20 November 2023 - 10:18 AM

Binoculars have their design parameters biased towards lower magnifications. They tend to be fast (small f-ratios) with erecting prisms and are much easier to configure for larger exit pupils.

 

The challenge at high magnifications is the whole collimating images across two pupils issue. The eye-brain combination is used to having the pupil completely suffused with light - in other words, the pupils of your eye act as the aperture stop. Small exit pupils create a situation (especially in astronomy where one’s pupils are wide open) where you have two thin tubes of light hitting a wide open pupil and the afocal tube is focused on some point on the retina. With a monocular telescope, this is less of an issue but with binoculars, the twin tubes have to be focused on specific parts of the retina - otherwise, the brain will have a lot of trouble merging the images since it is using the statistics of its experience to merge the images (the spot of light on this part of the retina of the left eye matches with this part of the retina of the right eye). Toss in the large variation in anatomy across people and you really need a bespoke approach to manage the challenges.

 

These challenges are majorly diminished at larger exit pupils which is why binoculars tend to be biased towards lower magnifications and larger exit pupils than telescopes. This relaxes other design constraints and you can make instruments faster (smaller f-ratios), lighter and easier to manage.

 

Premium systems that tackle these issues exist - personally assembled twin premium refractors placed in a rigid support with something like the Matsumoto EMS system to tweak things into optical alignment on the fly. They are eye-watering-ly expensive. It is probably unreasonable to expect a mass-manufactured approach to deliver the same results - like the difference between an off-the-rack suit and a tailor-made, bespoke suit. The latter is going to fit better because it is matched to the idiosyncracies of your body. The off-the-rack is matched to some average body shape and may be good enough for some people but is unlikely to be ideal for anybody (the trouble with averages - no family has 2.5 children). 

 

The issue is also mitigated by the fact that things like binocular summation go some way to reduce magnification needs. I am usually able to spot things at about 0.6-0.75 times the magnification in binocular vision, compared to monocular vision. I would also think that stereopsis (3D effect), which is one of the charms of binoculars is much reduced with higher magnification. 

 

The image in the Tak FC100 is certainly finer than a single tube of the OB BT (both 100 mm aperture), especially at high magnification. Not that the OB is shabby and as far as I am concerned, why haul the bulk of twin tubes and not use them. I’m able to use upto 100x with ease and have gone as high as 160x with care. Given the seeing limitations where I live, there is little impetus to try for more.
 


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#6 Terra Nova

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Posted 20 November 2023 - 10:42 AM

Martyn, tackling this problem from the opposite extreme, have you ever thought that you might be happier using both eyes to look through a single optic at higher power? I’m talking about getting a binoviewer for your 102mm F11 refractor. Like acquiring a binocular telescope, a binoviewer presents its owner with the same problem of having to acquire duplicate sets of eyepieces, but almost eveI know who has one (a binoviewer) loves the experience. And I’m sure the high magnification view through your telescope with a binoviewer would be much more satisfying than peering with one eye through a binocular telescope at the same object and at the same high magnification.

 

Disclaimer, I am completely talking through my hat here to a degree as I have neither a binocular telescope or a binoviewer, (tho I have looked through both, and also known many devotees of both)


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#7 Binofrac

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Posted 20 November 2023 - 10:44 AM

Binoculars have their design parameters biased towards lower magnifications. They tend to be fast (small f-ratios) with erecting prisms and are much easier to configure for larger exit pupils.

 

The challenge at high magnifications is the whole collimating images across two pupils issue. 

Thanks dnayakan,

I understand the issues of high binocular viewing but am asking about the magnification limits if only one tube is used. I'm trying to justify the expense of something like an Oberwerk BT by assessing how wide it's capabilities are. Almost all the time it would be used for normal binocular viewing and would be the only instrument with me. It might therefore be nice sometimes to take the magnification higher in monoview perhaps on the moon for example as the mood takes me as part of the evening's viewing and then revert back. It would mean a more flexible use for a BT without also having to take a telescope with me. 

 

I'm keen on knowing a bit more your experiences at up to 160x with your BT. That would be in excess of my requirements.



#8 Fiske

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Posted 20 November 2023 - 10:47 AM

My experience is completely consistent with the analysis provided by dnayakan.

 

In fact, I have been looking for a lighter, more easily managed option for observing double stars that are difficult for a BT (under 4-arc seconds and/or with significant magnitude difference between the components) and am pondering a longer f-ratio refractor like the SkyWatcher EvoStar 100m f/9.

 

Possibly on an iOptron CEM26 mounted on a Manfrotto 161MK2B tripod I already own.


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#9 Fiske

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Posted 20 November 2023 - 10:49 AM

BTs are optimzied for low to moderate magnification levels. It is possible to use them at higher magnifications, but there are considerable technical and ergonomic challenges. wink.gif


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#10 Binofrac

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Posted 20 November 2023 - 11:00 AM

Martyn, tackling this problem from the opposite extreme, have you ever thought that you might be happier using both eyes to look through a single optic at higher power? I’m talking about getting a binoviewer for your 102mm F11 refractor. Like acquiring a binocular telescope, a binoviewer presents its owner with the same problem of having to acquire duplicate sets of eyepieces, but almost eveI know who has one (a binoviewer) loves the experience. And I’m sure the high magnification view through your telescope with a binoviewer would be much more satisfying than peering with one eye through a binocular telescope at the same object and at the same high magnification.

 

Disclaimer, I am completely talking through my hat here to a degree as I have neither a binocular telescope or a binoviewer, (tho I have looked through both, and also known many devotees of both)

Cheers Terra,

 

Yes I have a binoviewer and like it but it produces high magnifications only due to the barlow required (yes I've thought about cutting my beautiful refractor but can't stand the thought).

 

My thoughts are this- low power viewing- hand held bino used a lot, Medium power viewing- Binoscope used not quite as much but has a better aperture, High power viewing- hardly at all but these infrequent times may possibly be accommodated by monoviewing at a higher power as part of a binoscope viewing session. It would allow greater flexibility without taking too much equipment.

 

Here's an experiment- for binoviewing greater than the BT would normally allow, would a binoviewer work in a single tube? I imagine it would be very dim but if I had a BT I wouldn't be able to resist giving it a go.


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#11 jrazz

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Posted 20 November 2023 - 11:00 AM

I use my BT-100 regularly up to 140x (4mm eyepieces) and have used it up to 187x (3mm) and 224x (2.5mm) though I find the latter two do not give more detail unless the seeing is perfect and even then I usually stick to 140x.

 

Can you use it?

Certainly!

 

Is it difficult? No, not at all. 

But it does take some patience and commitment to collimation and seating of the eyepieces.

It also requires some tolerance by the user on image misalignment. Some can handle more misalignment than others. I've found that if you can handle those "3D" repeating image pictures that were the rage 20 years ago you usually are able to handle more misalignment. 

 

I spent quite some time tweaking the collimation of my BT. I also am very careful about how I place my eyepieces since and misalignment there easily can cause misalignment in the image. The result of this is that yes, I can and do use my BT at high magnification but truthfully, I find the single tube telescope far easier to use when going to higher mags. 

 

 

 

Interestingly, though my telescope is easier for high magnification I've found that the lower bulk and weight of my BT causes me to use it far more. The difference in resolution is smaller than you think. I've split 1.5" doubles with my 100mm BT while my 130mm APO tops out at 0.9". Not a huge difference even though it's far easier to use 400x on the APO. Magnification isn't everything and resolution depends on other thigs as well.  


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#12 dnayakan

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Posted 20 November 2023 - 11:35 AM

I’ve been lucky with the OB 100 SD. It seems to fit my face well enough with the available adjustments. 
 

The single tubes are good but certainly not at the level of the best telescopes I have looked through. In use, I can see a little bit of flaring around the brightest objects if I look through a single tube. Some of this (but not all) is my own aging eyes and obviously it is substantially reduced in binocular vision (where the neural processing cancels out idiosyncracies from each eye). However, not all of it is my eyes but none of the problems are what I would call objectionable in the uses I put the BT to.
 

A pair of Nagler 5mms gets me to 112x, a pair of Tak 4mm TOEs gets me to 140 and  a pair of Nagler 3.5s gets  me to 160. I also have the OB 22 mm, 14mm and 7mm on hand yielding 25x, 40x and 80x. Just for context, I am usually able to merge images in most normal binoculars fairly easily (at least, easier than some friends of mine manage). Have also spent most of my life engaged in various visual pursuits (painting, photography etc.), so I’m reasonably sensitive to visual issues. I have no trouble with merging images at lower magnifications and almost none even at 112x. I can merge images at 140x and 160x but need to be more conscious about head placement. If not, I’m likely to see double images (some of this is idiosyncracies in my eyes - I know because when I slide the object from the right visual field to the left, the images merge into one). I also have to be careful about keeping the eyepieces properly aligned in the collets - I generally try to get the holder vertical when placing eyepieces, I find it easier to keep everything square that way. However, that means I cannot do the usual start with low magnification, center object and swap out for high power eyepieces dance. On the upside, it has made my dead reckoning and star hopping skills better. 
 

I will say that high powered binocular viewing, with its challenges, can be a pretty charming experience. I can happily spend an entire night wandering around the moon. Even at the high powers mentioned, it is still more dimensional in binocular vision rather than monocular vision. Twin 100mms are reasonably potent for deep sky in the skies I observe from. 
 

To your point, I never use it in telescope (one tube only) mode. The binocular mode helps cancel out some of my eye’s problems and the view through one tube at high magnification is not as fine as using one of my telescopes. Sort of have to accept the instrument-observer (me) combination for what it is and what it is best placed to do. 


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#13 Fiske

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Posted 20 November 2023 - 12:02 PM

Do not be fooled by Jordan saying that 140x+ BT observation  is "not difficult." (Jordan has super powers.)

 

lol.gif

 

As a bespectacled observer, it is extremely difficult for me. And that isn't the only issue. Eye position aligning two tiny exit pupils even without glasses is not a trivial exercise. 


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#14 areyoukiddingme

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Posted 20 November 2023 - 02:04 PM

Interesting discussion. I've been thinking of getting a second Astronomics 125mm F7.8 and kludging those together.

 

The merging issue sounds rather difficuilt.

 

Do people have a sense for how it compares with binoviewing? 

 

It sounds like the key challenge would be to be able to make tiny adjustments in the collimation of one of the eyepieces to get a good merged image.

 

The binotrons have an interesting mechanism that makes collimation by oneself relatively straight forward. I would think that a variation on that might solve the high-power merging issue.

 

I have developed a bit of horizontal nystagmus, and merging can sometimes be difficult with my binoviewers, especially when I am tired and I'm cranking the powers up (200+). 

 

A standard cheat is to jiggle the scope so that the image of a bright star or even Jupiter moves around the field chaotically with my eyes tracking its movement. This seems to be a good way of tricking the visual system into believing that the image in both eyes is in fact the same image. 


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#15 TOMDEY

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Posted 20 November 2023 - 04:29 PM

Interesting discussion. I've been thinking of getting a second Astronomics 125mm F7.8 and kludging those together.

 

The merging issue sounds rather difficult.

 

Do people have a sense for how it compares with binoviewing? 

 

It sounds like the key challenge would be to be able to make tiny adjustments in the collimation of one of the eyepieces to get a good merged image.

 

The binotrons have an interesting mechanism that makes collimation by oneself relatively straight forward. I would think that a variation on that might solve the high-power merging issue.

 

I have developed a bit of horizontal nystagmus, and merging can sometimes be difficult with my binoviewers, especially when I am tired and I'm cranking the powers up (200+). 

 

A standard cheat is to jiggle the scope so that the image of a bright star or even Jupiter moves around the field chaotically with my eyes tracking its movement. This seems to be a good way of tricking the visual system into believing that the image in both eyes is in fact the same image. 

I'll chime in on this 'collimation" = coalignment later (going to walk the dogs now before sunset). There is a profoundly simple invariant equation that addresses this. (Hardy & Perrin, 1932) For some reason understanding of these optical alignment topics has become ancient arcane forgotten grist that flourished during the WWII era. People compute and bloviate more, but understand far less than back then.    Tom (recovering bloviator) --- now for walking the canines...


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#16 MT4

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Posted 21 November 2023 - 01:23 AM

Hello esteemed binocular telescope users,

 

Most of my observing is through binoculars. Occasionally though I'm using my 102/f11 refractor for solar system objects, and sometimes with a binoviewer. I've long wondered if I could be satisfied with a binocular telescope instead. The higher than normal hand held binocular magnification would be handy occasionally but I would mostly enjoy bigger aperture low power views. For odd times though it would be nice if I could use the same instrument to mono view at a higher magnification. Where I am my useful magnification tops out around 120x and I'm wondering if a BT would allow this if I used a higher power eyepiece in just one side. I know that normal binocular use would not reach this. I'd be especially keen to hear from users of the Oberwerk BT's in this regard.

 

I can't imagine this hasn't been asked before but but I've searched hard and haven't found it.

 

I don't have any Oberwerks but I think the answer to your central question is most likely a YES.

 

Where I am, local seeing is so bad that most of the time I can't go above 150x.  (With great regret I sold my Tak TSA-120 last month because I couldn't bear limiting such instruments to mags in the low 100's.   I quickly missed the Tak quality and got myself a Tak FC-100DZ. smile.gif).  I have a BT and that's a SkyRover 120mm.  I've used it to reach 101x, 132x and just once 165x.   Note that I mainly use those mags to look at the moon and the planets though stars do show up fine.

 

So if your local seeing limits you to 120x, you could get yourself a 100mm BT.


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#17 Binofrac

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Posted 21 November 2023 - 04:11 AM

Thanks very much everyone,

 

You've all been really helpful. I'm in a continual quest to keep my kit to a minimum to save several trips to and from the car and lots of carrying. The question for me was whether a good BT would do most of what I wanted- deep sky at low wide power and solar system to about 120x in the same instrument. It looks like this would be fairly easily achievable even without possessing the Jordan super power. I never thought I'd be able to successfully binocular view at such a high power. I understand it may be difficult but my binoviewer can be difficult too at higher powers with tight eye placement tolerance hassles and eyepiece jiggling.

 

For my goal of one instrument does it all it would've also be good If my 102/F11 refractor could be binoviewed from low power (about 20x). A linear binoviewer would be interesting to play with but it wouldn't go low enough. Some have experimented with focal reducers which may be hit and miss and then starts to involve a lot of fumbling with equipment in the dark rather than just observing. Those BT's do look good though, all shiny and exciting and would be better suited to what I want to do. Maybe a BT82 for it's better portability but then BT100's aren't much more and Christmas is coming.



#18 MT4

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Posted 21 November 2023 - 06:17 AM

You might as well go for gold, i.e. BT100.   Then you'd wonder no more "What if..."   


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#19 dnayakan

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Posted 21 November 2023 - 08:08 AM

If you enjoy binocular views (I.e., you have a neural system that provides good binocular summation), a BT is very likely to become your most used instrument - it certainly is for me, given my observing situation (medium transparency and seeing that limits the highest magnification I can use). At least in the OB lineup, the BT100 is in something of the Goldilocks zone - it’s not much heavier than the 82, which is the adjoining lower aperture and is considerably lighter than the 120 which is the adjoining higher aperture. At about the 150x highest magnification I use, I don’t need a tracking mount and can follow solar system objects reasonably well. This makes a pretty simple system - tripod, head and the BT. The only place the simplicity is lost is with the doubled eyepiece requirements. 
 

You could try a night or two of observing a variety of objects with a telescope and binoculars of about the same light grasp (I.e., a telescope with an aperture around 1.4 the aperture of the binoculars e.g., a pair of 7x50 binoculars and a telescope in the 3” aperture range). If you routinely find it easier to find objects in the binoculars or find yourself enjoying the binoculars more, you likely have neural processing that provides good binocular summation. This is not the most controlled of tests - e.g., the larger aperture of the telescope should provide better resolution - it is just one of many things you could do to see if your brain wiring produces binocular summation that would make BTs worthwhile. When I try this with the BT100 and the TOA130, I routinely find I can spot features at lower magnification in the BT (including seeing limitations - I can detect the wobbling atmosphere at lower magnifications as well). The TOA is undoubtedly the finer instrument but whatever my brain is doing with the dual inputs in the BT is clearly covering a lot of ground. I don’t think the BT shows me stuff the TOA does not but it is more immediate and definitely has more stereopsis (often a fiction, of course, since the objects are at infinity - but a rather enjoyable fiction). I prefer the telescope for some kinds of observation and on nights of excellent seeing (double star work comes immediately to mind, but also solar system features like shadow transits, lunar features etc. - basically nights focused on high magnification work) but the BT has become my more generalist instrument and the one I haul out on typical nights. 


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