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bino/quadroculars?

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#26 GlennLeDrew

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Posted 24 August 2017 - 10:37 AM

Immediately upon reading in that PDF that very brief flashes of dim light were used to assess detection *rates*, I intuitively recoiled, for this reason. A steady source near the detection threshold is perceived to 'wink' in and out of visibility already due to visual system noise. Making the source flash would only further reduce the detection rate, due to some appearances coinciding with moments when visual noise dominates. Thus the squaring of the detection rate ratios that would obtain for a steadily illuminated source, which in turn leads to the too-large equivalent aperture ratio of factor 1.4.

 

At least, that's my first assessment without any application of deeper thought.


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#27 SPastroneby

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Posted 24 August 2017 - 12:48 PM

@Jon Isaacs

No, I didn't. I don't own a binocular, and even then I would have to compare it with a mono of 1,19 and/or 1,42 bigger. I feel I otherwise wouldn't be sure how much 'better' it would be, certainly not if in practice it would be between the 1,19 and 1,42. Maybe you can gauge such a thing accurately, but clearly there is a high subjectivity in it, seen the descriptions and assertions of others with a binobsodian. I'm pretty sure those guys are seasoned sky-watchers too, but clearly they come to another appreciation of it. And...well... they DO have the most data to show for as of yet, which doesn't mean they couldn't be subject to some form of subjectivity themselves - I'm not blind to that possibility.

 

Maybe someone can make a paper about this, some day? It would be nice to have such an experiment repeaedt, but then with double-blind tests that are statistically relevant, and maybe with a measure apparatus that can note the differences more objectively.

 

 

"I take it by your silence that you are in agreement."

 

It's rather because I was at work, and also because I can't answer to everything. ;-)  Though Glennledrew made basically the same argument before you, and I answered him. In short, I think it's only partially because of the technical limitations, and for a bigger part, just about the cost... and the cost-benefit argument was one that I made as well.

 

There is something not quite right in the claim  that pro's use segmented mirrors because they can't do anything else because of *technical* limitations. Look at your and Glennledrew's examples. He said that the MMT used 6 x 1,8 mirrors, because it wasn't technical possible in that time (late 70ies) to make a mirror of 6,5 meter. First of all, even that is a bit doubtful, since the BTA of 6,05 meter was already built in the beginnings of the 70ies. But, let's, for arguments sake, say that was right, and it was impossible to make a 6,5meter one back then. Ok. But if one claims pro's want the biggest mono's they can get, and it was only because of technical limitations the 6,5 meter wasn't built... then why didn't they built 4 x 4m class mirrors? Because 4 meter was CERTAINLY "technical possible". And surely, 4 x 4meters would have been better - and being bigger mono's - then 6 x 1,8 meter. Using logic, this points to the fact it was not - at least not primarily - a *technical limitation*, but a cost limitation. They had a certain budget they couldn't pass over. And 3 or 4 meter telescopes were technical possible alright, but they were too expensive.  A single 1,8 meter would have been the cheapest of course, but would also meant the least scientific return. 

 

What they did, thus - and this was/is one of my arguments - is to make a cost benefit analysis. "What's the most we can get with the budget we have." And lo and behold, they used 6 small diameter mirrors in conjunction to get the equivalent of a 4,5 meter one. Why not directly a 4,5 meter one, then - that WAS technically possible, after all? Well, exactly because of what I said: cost. It was cheaper to have 6 mirrors of 1,8 meter than one of 4,5 meter. Which is exactly my point.  Think about it. If a 4,5 meter is better, is technical possible AND you have the budget for it, and you claim pro's always go for the bigger mono instead of the smaller segmented, than *there is no reason* why they would have chosen the 6 x 1,8 meters ones, now is there?

 

I'm just using logical sense here. It's a fact a 4,5 meter telescope was already technically possible before the 50ies, let alone late 70ies. So that CAN NOT have been the determining factor. This leaves cost/budget. Which means they thought 6 x 1,8meter telescopes would be a better cost-benefit approach than one 4,5meter one, contrary to what you claim.

 

Idem with your example of the Keck. I'm not sure what you try to show, in fact, since it seems rather speaking against your conclusion. Maybe you wanted to say 'well, the minimum is 72", so you see how big it is'...yes, but they also have big budgets. No-one is denying one monolithic mirror is better then several smaller for the same aperture, it's a matter of cost-benefit that is the point here. If they *really* wanted big monolithic mirrors at any cost, there is absolutely no reason to go for small (in their eyes and budget) mirrors like that. No, they would have gone for a handful of very large mirrors, like the GMT has as concept.

 

This, thus, in effect once again shows it's not about technical possibilities, or that pro's will always choose bigger mirrors if it's technically possible, but that there is a cost-benefit trade-off. Of course, that trade-off (the threshold I spoke of) lays far higher, financially, than that of an amateur astronomer, which is why 72" mirrors is 'small' for them, even when it's huge for us. That's relative to the budget one has, however, not relative to the merits of the underlying principle.

 

Now, are you and Glennledrew wrong in every instance? No. Your argument holds, imho, where there really IS a technical limitation.  You two would be right, for instance, with the example of the GMT. There, we see they use 7 x 8,4 meter mirrors, giving it the equivalent of (resolving power of) a 24.5 m (80.4 ft) primary mirror and collecting area equivalent to a 22.0 m one.

 

Why didn't they go for a 25m monolithic one? THERE you are right: because it's a technical limitation. For the longest time now, since the 90ies, the largest single mirrors that can and have been made, are between 8 and 8,5 meters. Even the future telescopes, like the GMT, which indeed goes for the largest mono's, has that limitation, and thus goes for 7 of them used in conjuction. It's currently impossible to create one large 25meter mirror; there is not a manufacturer that can make such a thing right now. You won't find any telescope mono-mirror in the world which is larger than 8,5 meter. All the ones that have bigger aperture, are segmented. So your argument holds there. Because the 8,4 meter limitation is there, and that's a technical limitation for the time being. This clearly isn't valid for all pro observatories that have a (total) aperture that is less than 8,4 meters, obviously - since one *can* make a mono of 8,4m. There, it's a matter of cost/benefit.

 

Am I saying something outrageous here? I don't think so. I'm only applying logic.

 

 

 

There are two separate problems.

 

First, suppose two telescopes of 100mm each combined into a single (monocular) eyepiece. Their light gathering ability would be equal to a single 142mm telescope (here is the area which matter).

 

Second, supposte two telescope of 100mm each used to make a binoscope (i.e. one eye--> one telescope). Here the binocular summation apply, and their light gathering (for visual use obviously) could be considered equivalent to a 119mm monocular telescope.

 

Third, suppose 4 telescopes, joined two by two in a binoscope. Then their light gathering ability would be equal to:

100mm*1.41*1.19=168mm.

 

A much more efficient way to increase the light ability is to concentrate the lights of all 4 telescopes into a single eyepiece: 100mm*1.41*1.41=200mm. If a kind of reverse beam-splitter could be made, with 4 100mm ED doublet you obtain a 200mm ED telescope - cost saving, yes! But the optics to merge the light of 4 telescopes intoa single eyepiece would be super difficult.

 

What they do in real life is a different application: 4 telescopes with 4 cameras (and many more).

http://www.dunlap.ut...tion/dragonfly/

 

Dragonfly is an innovative, multi-lens array designed for ultra-low surface brightness astronomy at visible wavelengths. Commissioned in 2013 with only three lenses, the array is growing in size and proving capable of detecting extremely faint, complex structure around galaxies. The most recent upgrade—completed in 2016—saw Dragonfly grow to 48 lenses in two clusters.

 

half-of-48_48-4.jpg

 

Wow! Riccardo, this is about exactly what I was looking for and envisaging how big such a thing could grow, like a fly's eye (though they apparently called it a dragonfly's eye). Thanks for the link, I'll be sure to check up on it! But cursory glancing the article already seem to indicate they definitely do it for the advantages it gives (though I'd have to delve deeper to see the economics of such a thing). Why would they make it, if it would be more advantaguous in price and/or capabilities, if they could use a big mono-lens or mirror, after all?

 

It's pretty interesting they got the same basic idea as I, but in effect manage to make one. If you have any other links/info/specs about it, feel free to give it! :-)

 

Also, thanks for explaining things further, your explanation was pretty clear and succinct.


Edited by SPastroneby, 24 August 2017 - 12:52 PM.


#28 Jon Isaacs

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Posted 24 August 2017 - 01:37 PM

dem with your example of the Keck. I'm not sure what you try to show, in fact, since it seems rather speaking against your conclusion. Maybe you wanted to say 'well, the minimum is 72", so you see how big it is'...yes, but they also have big budgets. No-one is denying one monolithic mirror is better then several smaller for the same aperture, it's a matter of cost-benefit that is the point here. If they *really* wanted big monolithic mirrors at any cost, there is absolutely no reason to go for small (in their eyes and budget) mirrors like that.

 

 

My point is simple:. The Keck designers felt that  are monolithic 72 inch mirror  was preferable to using smaller segments. That an indicator of the design trade off.  In sizes of telescopes that are practical for amateur astronomers, monolithic mirrors are a better choice for a number of reasons. 

 

When you mention "cost-benefit", you are discussing money that comes out of the amateur astronomers billfold.. How much are you thinking you'll invest in this scope?  

 

Have you ever looked through a 25 inch amateur  telescope or seen one? Have you considered the difficulty is the electro-mechanical system necessary to maintain alignment of the mirrors to a rather small fraction of a wave length of light.

 

A 25 inch Telescope is a big telescope.. a monothithic mirror is easier to make and can be supported with a simple flotation cell. No need for sensors, servo control systems that multiple mirrors require. Simple is good.. On a cold night with the wind blowing 20 knots, something simple and trouble free is good.

 

If you haven't spent time with a larger amateur scope, I invite you to join me and you can do some binocular comparisons at the same time.. the one on the left was sold..

 

4 Dobs plus Jon.jpg
 
Another thing to consider: Most large aperture amateur telescopes need to be easily transported and easy to setup. This is because most amateur astronomers do not live where the skies are dark..
 
Jon

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#29 SPastroneby

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Posted 24 August 2017 - 02:15 PM

Immediately upon reading in that PDF that very brief flashes of dim light were used to assess detection *rates*, I intuitively recoiled, for this reason. A steady source near the detection threshold is perceived to 'wink' in and out of visibility already due to visual system noise. Making the source flash would only further reduce the detection rate, due to some appearances coinciding with moments when visual noise dominates. Thus the squaring of the detection rate ratios that would obtain for a steadily illuminated source, which in turn leads to the too-large equivalent aperture ratio of factor 1.4.

 

At least, that's my first assessment without any application of deeper thought.

?

 

It says:

 

"So the question is: how does the size of two binoscope mirrors compare to one larger mirror. One way to test this is to determine limiting magnitudes of stars under equal observation conditions and compare these for both a binoscope and a comparably larger single mirror telescope. To determine the limiting magnitude one simply determines the faintest star one can still see with either the binoscope or the comparable one, larger mirror. The resulting differences in limiting magnitude are thereby an indirect measure for the value of the binocular summation factor. In a mail exchange withMel Bartels, he proposed that he would directly compare a binoscope and an equivalent mono-mirrored Dobsonian telescope to determine the limiting magnitudes of either instrument. I followed up his suggestion."

 

So it's clear they measured real-life stars and made a direct comparison with a mono of the equivalent aperture. Which makes perfect sense to do so.  It's all there in part 2, which goes deeper into the relevant part of the 1,19 vs 1,42 issue.

 

I'm not sure if you read it completely. They first went for measuring of the value of the binocular summation factor with stars as object, and then they did the same to determine the value of the binocular summation factor with extended objects as test objects. In both cases, the conclusion was that it was far above 1,19.


Edited by SPastroneby, 24 August 2017 - 03:22 PM.


#30 daquad

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Posted 24 August 2017 - 02:15 PM

I read the PDF in which the test protocol involved very brief flashes of light, comparing counts seen with one eye vs two. This *may* not be as relevant as detecting an otherwise steady source near the threshold of visibility. Our view of the world about us is in the main a 'steady' scene (discounting motion of objects), not a flickering one.

 

For us astronomer types, seeing faint stars is a common and relevant discriminator. Most observers report a gain of 0.3-0.35 magnitudes for two-eyed views, whereas theory suggests 0.37m, corresponding to a linear aperture increase of factor 1.189.

 

No one possessing reasonably similar eyes I've ever encountered claimed a gain of 0.75m, which would equate to an effective linear aperture increase of factor 1.414. If the improvement were to be that profound, I could hardly imagine anyone willingly squinting through a monoscope.

So Glenn, I thought I had this nailed, but are you saying that If one uses a binoviewer, vice a monocular eyepiece in the same instrument that one can expect a 0.3 magnitude gain in limiting magnitude?  That is, if I use a binoviewer, I can expect to see 0.3 magnitudes deeper than I could by viewing with just one eye?  That is what your second paragraph is suggesting.



#31 SPastroneby

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Posted 24 August 2017 - 03:02 PM

 

dem with your example of the Keck. I'm not sure what you try to show, in fact, since it seems rather speaking against your conclusion. Maybe you wanted to say 'well, the minimum is 72", so you see how big it is'...yes, but they also have big budgets. No-one is denying one monolithic mirror is better then several smaller for the same aperture, it's a matter of cost-benefit that is the point here. If they *really* wanted big monolithic mirrors at any cost, there is absolutely no reason to go for small (in their eyes and budget) mirrors like that.

 

 

My point is simple:. The Keck designers felt that  are monolithic 72 inch mirror  was preferable to using smaller segments. That an indicator of the design trade off.  In sizes of telescopes that are practical for amateur astronomers, monolithic mirrors are a better choice for a number of reasons.

Yes, but for the observatory and organizations that made them, those ARE small segments.

 

As I said, no-one doubts a monolithic mirror of the same aperture is better then a lot of small mirrors combining to the same aperture. They probably went for the 72" diameter because those were the most cheap for the biggest scientific bang, coupled with other considerations like ease of transport, etc. So of course there's a trade-of; no one who can make and afford 100 x 72" mirrors will say: ah, but we'll make 10000 x 0,72" mirrors. That wouldn't make sense at all, since it's not that a whole bunch of smaller ones are *better* in scientific return then bigger ones. If they're better, then they're better when they reach a cost-benefit threshold.

 

So maybe you misunderstood, but I wasn't saying "smaller is always better". One of THE reasons why almost all observatories using segmented mirrors have a 1,6 to 1,8 meter (your 72") diameter, is because they are (relatively) cheap and easy to  make (certainly compared to much bigger mirrors), and because they can be transported by normal vehicles. Since smaller doesn't have any ADDED value, why would they make it smaller? There is no reason for it, since you would augment the complexity, but would gain nothing - the cost-savings by going for thousands of 0,72" would be completely lost, and their budget is large enough to go for 1,8m segments.

 

In contrast, however, this does not explain why they wouldn't go for a far larger mirror, IF  larger monolithic mirrors are always preferred, and cost-benefit would always favor large monolithic mirrors. Clearly, they don't.

 

This points that not the *size* matters on itself, but the cost-benefit in regard to the available budget.

 

Since the budgets of astronomy-clubs are much smaller, if they want to go for the best bang for the buck, they will have to make the same cost-benefit analysis. But of course, seen the fact their budget is far more limited, the 'segments' they can afford is also much smaller. The principle of determining the most bang for the buck remains the same, however.

 

Therefore, if one has a limited budget, one could be better of with a two 22" bino, than one monolithic mirror of more then 31". The reason being, that it would be cheaper, for the same benefit. The same goes for the Keck, but since its budget is much bigger, the segments can be bigger as well. But there too, they will have looked at cost-benefit. They *could* have gone for 2 x 8m mirrors, and that would have given better results than what they have now, but the cost would have been far, far greater as well, so they didn't.

 

I agree with your last point, but I already indicated a large enough bino to be cost-effective in regard to a mono of the same aperture, would probably be too big to be very mobile, hence why I said in one of my posts it would be more suited for private observatories (many amateur astronomer have those), or for astronomy-clubs who want or have an observatory.

 

I want to stress once again I'm not talking about actually making one myself and going around star-parties with it. For now, I'm talking about the principle of it, and if it would be more economical to do so (going for a bino instead of a mono), and from which point/threshold on. For instance, I gave the example of (actual prices) for a 14" dobsonian I saw, and a 16". While the difference between a 12" and a 14" was only 300 dollars, the difference between a 14" and a 16" was already 1500 dollar.

 

It follows that, while it wouldn't be worthwhile to buy a bino of 2 x  12" to trump a 14", it *could* be worthwhile to buy a bino of 2 x 14" to trump a 16" mono. You see where I'm getting at? (And if the 1,42 number would be right, it would mean it almost equals a 20", which *definitely* would be WAY more costly than the two 14" telescopes). That's what I mean by the cost-benefit threshold.

 

So you're right and I  don't doubt that monolithic mirrors are better for amateur-astronomers, just as they are for pro's, but this doesn't change the principle that, given a certain budget, what is the best option in regard to cost-benefit. COST-benefit. Not only benefit, otherwise mono ALWAYS wins, obviously.

 

BTW, thanks for the invite! Much appreciated! Alas, I think we're on different continents. ;-)


Edited by SPastroneby, 24 August 2017 - 03:25 PM.


#32 GlennLeDrew

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Posted 24 August 2017 - 08:04 PM

daquad,

The (theoretical) 0.37m gain applies for a true binocular, where a separate instrument is devoted to each eye.

 

With a binoviewer on a monoscope, half the light goes to each eye, which makes for an equivalent aperture of factor 0.707 for each. Then binocular summation partially regains some of this loss, resulting in an aperture equivalent for each eye of 0.707 * 1.189 = 0.84. This is a net loss, but for subjects that are not so faint does not prove to be deleterious. 



#33 GlennLeDrew

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Posted 24 August 2017 - 08:14 PM

SPastroneby,

The PDF paper I was referring to is the one you linked to in your post #15. In that study faint point sources that were made to flash on very briefly. I read through the first ~1/3 or so, then skimmed the rest after ascertaining their experimental result.



#34 Jon Isaacs

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Posted 24 August 2017 - 09:16 PM

Therefore, if one has a limited budget, one could be better of with a two 22" bino, than one monolithic mirror of more then 31". The reason being, that it would be cheaper, for the same benefit. The same goes for the Keck, but since its budget is much bigger, the segments can be bigger as well. But there too, they will have looked at cost-benefit. They *could* have gone for 2 x 8m mirrors, and that would have given better results than what they have now, but the cost would have been far, far greater as well, so they didn't.

 

 

The stumbling point here is the the fact that a binocular telescope is not a segmented telescope.  A binocular telescope is two separate telescopes that rely on the brain for summation.  In terms of resolution,  a binocular telescope has no greater resolution than the single apertures it made up of.  That's a fact that cannot be disputed. 

 

 A segmented mirror telescope is a single telescope that produces a single image.  But to build a segmented mirror scope in these small sizes would be much more expensive and complicated than just building a monolithic mirror. The cost of the servo mechanical system to make the segmented mirror work..  That does not scale linearly..  

 

Binocular telescopes are available commercially. 

 

http://www.jimsmobile.com/buy_rb.htm

 

The 14.5 inch is $15,000.  They're not very popular. The last one that was sold on Astromart went for $3000.

 

$15,000 will buy a much larger scope..  The last price on the 25 inch Obsessions was $15,000.

 

The reason I invited you to join me is so you would have some hands on,  practical experience.  

 

Get out with some binoculars,  a few Scopes,  see what people are talking about..  

 

Jon



#35 havasman

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Posted 24 August 2017 - 10:20 PM

So you're right and I  don't doubt that monolithic mirrors are better for amateur-astronomers, just as they are for pro's, but this doesn't change the principle question that, given a certain budget, what is the best option in regard to cost-benefit? COST-benefit. 

The marketplace has already answered this question clearly, worldwide and very consistently.

 

If the economics favored another design such as the one you propose that incorporates existing technologies in uncommon configurations then that design would exist. Such is the nature and power of market driven innovation.

 

Regarding the general efficacy of the proposed bino/quad/hexa/octa... scope, I am reminded of Bill Cummins who often said, "Nothing is difficult for the man that won't have to do it."


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#36 SPastroneby

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Posted 25 August 2017 - 05:14 AM

 

So you're right and I  don't doubt that monolithic mirrors are better for amateur-astronomers, just as they are for pro's, but this doesn't change the principle question that, given a certain budget, what is the best option in regard to cost-benefit? COST-benefit. 

The marketplace has already answered this question clearly, worldwide and very consistently.

 

If the economics favored another design such as the one you propose that incorporates existing technologies in uncommon configurations then that design would exist. Such is the nature and power of market driven innovation.

 

Regarding the general efficacy of the proposed bino/quad/hexa/octa... scope, I am reminded of Bill Cummins who often said, "Nothing is difficult for the man that won't have to do it."

 

 

Haha, yes, I sort of concur with your last part, there. It's always easier to be a "Salon philosopher" than actually doing something. ;-) Note it was the guys with the dobsonian bino that said collimation etc. was fairly easy and could be done in 5 minutes after some training. And for me, it was to know why it wasn't more applied in amateur-astronomy, since it clearly is being applied by the pro's. That there are no advantages to it at all, not even in cost-benefit analysis for threshold apertures, I found unpalatable, otherwise the pro's won't and wouldn't use it neither.

 

As far as I've understood from the whole thread here, the arguments given can be grouped in a few points:

 

1) there is little advantage to it in a light-gathering context (this involves the disputed numbers 1,19 vs 1,42. Clearly that makes a huge difference)

2) there is a difference between a classical binocular and a segmented mirror, where the last has the 1,42 factor, but the former has 1,19 (could be wrong in this, but that seemed to be the implication of some comments)

3) it's not practical because the optics/computerized systems needed for a...segmented (?)... mirror telescope are too expensive (which could be true, but why doesn't it go down in price just like PC's did (and CMOS/CCD camera's)? Free market is certainly a good thing, but sometimes there is a chicken-or-the-egg problem, where manufacturers don't mass produce it because there's little demand for it, and consumers don't ask for it, because it's too expensive)

4)For amateur astronomers, it's only interesting to buy mono-mirrors since the diameter of the mirror is less then 3,5m (which would be considered the threshold, then). On itself, I find this last one the most doubtful, since it would deny there is a cost-benefit analysis to be made for anything smaller, which would deny the non-linear price-augmentation of mirrors according to their aperture long before they hit the 3,5m mark, nor would it explain why pro's wouldn't always go for the biggest mirror possible. Coupled with point 3 it could have some partial validity since it would rise the threshold where it becomes interesting, but on itself I think this is not correct.

5)varia (this covers all other side-arguments. Including quotes of Bill Cummins ;-))


Edited by SPastroneby, 25 August 2017 - 05:20 AM.


#37 SPastroneby

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Posted 25 August 2017 - 05:43 AM


Binocular telescopes are available commercially. 

 

http://www.jimsmobile.com/buy_rb.htm

 

The 14.5 inch is $15,000.  They're not very popular. The last one that was sold on Astromart went for $3000.

 

$15,000 will buy a much larger scope..  The last price on the 25 inch Obsessions was $15,000.

 

The reason I invited you to join me is so you would have some hands on,  practical experience.  

 

Get out with some binoculars,  a few Scopes,  see what people are talking about..  

 

Jon

 

 

Interesting. I find it telling that a bino of 14,5 inch costs $15000, while 2 normal 14" telescopes cost $4000. Is the $11000 extra really due to a more robust gear (which would cost more than two separate gear??) and the optics, I wonder. I have the feeling it's more due to the fact it's not in high demand, and thus is not mass-produced, and thus takes more workhours, just like a custom-made telescope would. There is no way a bino would cost $15000 if it were as mass-produced like a normal 14" is. Which would lead back to the economical principle that if the demand is there, production rises and prices will drop, but also that demand rises if prices drop.

 

For $3000 I would *certainly* buy a 14,5" bino, if I wanted to spend $3000 on a telescope... even a single one would almost be sold at that price! (assuming nothing was wrong with the telescope(s), of course)


Edited by SPastroneby, 25 August 2017 - 05:46 AM.


#38 GlennLeDrew

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Posted 25 August 2017 - 08:23 AM

1) I trust the 'quadocular' idea has been shoved off to the waste bin, if it involves the full merging of the two exit pupils of a pair of 'normal', stand-alone telescopes for each eye. The use of a combiner like a beamsplitter gains one nothing, really.

 

2) A binocular of the same aperture as a mono scope *does* afford a real gain in resolving power, by factor 19% when both eyes have similar capability. More correctly, shutting out one eye results in a loss of resolving power, it being 84% of two-eyed performance. With both eyes in operation one discriminates to a better degree what is present in the image. For instance, reading an eye chart is generally a bit easier with both eyes than for any any one of them alone.

 

3) To be sure that segmented mirrors are completely understood... A segmented mirror could in principle be made all at once, with each segment held in place and the entire thing ground/polished just like a monolithic chunk. Only the center segment (if actually from the center) could thereafter be removed and used individually just like any other axially symmetrical reflector. The other segments have their optical axis offset, perhaps even located *outside* their perimeter, requiring that they be suitably tilted and have their secondary mirror/eyepiece suitably offset as well. Used individually they would be off-axis reflectors, having either an offset secondary obstruction or none at all, and presenting an offset exit pupil. (The situation would be exactly as found if the full segmented primary had all other segments but the one in question masked off.)

 

In other words, a bunch of individual, axially symmetric Newt primaries *cannot* be arranged so as to make up a segmented primary. The aberrations induced by all other than the central element would be horrific.


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#39 PETER DREW

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Posted 25 August 2017 - 11:34 AM

Even if there was no increased resolution gained by the summation of two primary optics, the added ease by which borderline details are seen certainly gives a perception that there is. If nothing else, I think the OP has been valuable for initiating such an interesting discussion.  smile.gif 



#40 daquad

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Posted 25 August 2017 - 01:32 PM

daquad,

The (theoretical) 0.37m gain applies for a true binocular, where a separate instrument is devoted to each eye.

 

With a binoviewer on a monoscope, half the light goes to each eye, which makes for an equivalent aperture of factor 0.707 for each. Then binocular summation partially regains some of this loss, resulting in an aperture equivalent for each eye of 0.707 * 1.189 = 0.84. This is a net loss, but for subjects that are not so faint does not prove to be deleterious. 

Thanks, Glenn for the clarification.  I always thought the binoviewer would give an aperture equivalent of 0.707 for each eye.  I never considered the additional enhancement provided by using two eyes.  Nice to know.  Makes binoviewing even more attractive, for me at least. 

 

So to be clear, using my binoviewer with my 6" refractor, I should expect my limiting magnitude to be that of a 5" refractor (0.84*6 = 5.04)?

 

Edit:  Additionally, if I use a 6" binoscope (ie. two 6" refractors side by side)  I gather twice the light of one 6" refractor, which yields an increase of 0.753 magnitudes over the single refractor.  But, since I am using two eyes the actual, or perceived, aperture increase is 1.189*6*(sqtr2) = 10.0" , which means the actual gain in magnitude over a single 6" refractor is 5log(10/6) = 1.1 magnitude, instead of 0.753.  (1.1 - 0.753 = 0.347, close to the 0.37 you mentioned.)  Am I correct in this?

 

 Dom Q.


Edited by daquad, 25 August 2017 - 01:59 PM.


#41 GlennLeDrew

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Posted 25 August 2017 - 06:40 PM

Dom,

With a true binocular, the monoscope equivalent aperture is 1.189, for a limiting magnitude gain of 0.37.

 

I should clarify the matter of improved resolving power when using both eyes. This derives purely in the visual cortex, arising from the improved signal to noise. The instrument itself does not magically deliver a smaller Airy disk, for example.



#42 SPastroneby

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Posted 26 August 2017 - 04:32 AM

 

daquad,

The (theoretical) 0.37m gain applies for a true binocular, where a separate instrument is devoted to each eye.

 

With a binoviewer on a monoscope, half the light goes to each eye, which makes for an equivalent aperture of factor 0.707 for each. Then binocular summation partially regains some of this loss, resulting in an aperture equivalent for each eye of 0.707 * 1.189 = 0.84. This is a net loss, but for subjects that are not so faint does not prove to be deleterious. 

Thanks, Glenn for the clarification.  I always thought the binoviewer would give an aperture equivalent of 0.707 for each eye.  I never considered the additional enhancement provided by using two eyes.  Nice to know.  Makes binoviewing even more attractive, for me at least. 

 

So to be clear, using my binoviewer with my 6" refractor, I should expect my limiting magnitude to be that of a 5" refractor (0.84*6 = 5.04)?

 

Edit:  Additionally, if I use a 6" binoscope (ie. two 6" refractors side by side)  I gather twice the light of one 6" refractor, which yields an increase of 0.753 magnitudes over the single refractor.  But, since I am using two eyes the actual, or perceived, aperture increase is 1.189*6*(sqtr2) = 10.0" , which means the actual gain in magnitude over a single 6" refractor is 5log(10/6) = 1.1 magnitude, instead of 0.753.  (1.1 - 0.753 = 0.347, close to the 0.37 you mentioned.)  Am I correct in this?

 

 Dom Q.

 

 

What? Wait, I'm not following anymore.

 

Ok, let's keep this simple, with examples:

 

1)If I get  a bincocular where both lenses (well, the light in them) of, say, 10cm (4") reach one eye (in total); which light-gathering power and which resolution improvement does one get out of that? Is this the 1,9 or the 1,42 number? I get an equivalent aperture of which size?

 

2)If I get  a bincocular where both lenses (well, the light in them) of, say, 10cm (4") reach one eye EACH (thus, two eyes in total); which light-gathering power and which resolution improvement does one get out of that? Is this the 1,9 or the 1,42 number? I get an equivalent aperture of which size?

 

3)Is it actual, or perceived 'better', and in which of both cases? Meaning, if one would replace the eye(s) by a camera(s) with the same dimensions as an eye, (but obviously without the cortex doing anything), would one still see the same improvement (objectively, thus)? By that, I mean: would *the difference* remain the same?

 

4)Does a segmented mirror, like the pro's use, and where they DO use the 1.42 number obviously, look/is more akin to the first, or the second of my examples? Are the pro's correct in using the 1,42 number with their segmented mirrors? If so, what is inhibiting amateur-astronomers to do the same?

 

5)Is there a difference in light-gathering or resolution between a system like that of the old MMT or LBT, where each lens has it's on focal point and then get combined, or the GMT, which has one focal point for all mirrors? (Because what GlennLeDrew describes in point 3 in post nr 38, is exactly what the GMT is and does. *However* it's not what the MMT did; there were no axially asymmetric primaries there to compensate and get everything in one common focalpoint. Yet they still claimed to be the equivalent of one 4,5m telescope. EACH mirror on it's own, there, could be used as a proper working telescope, however. In contrast, as GlennLeDrew said, with the GMT, only the central one could be used on its own.)

 

6)In what way does the pdf here: http://arieotte-bino...tion Factor.pdf contradict what is being said here? Is it claiming 1,42 for my first example, while most guys here dispute that? Or for the second one? With what, exactly, does one not agree with the data in this pdf (especially the most relevant part 2 of it)?

 

Sorry if I seem to be a bit obtuse, but everytime I think I understood the nuances of what is being said, someone else comes along that seems to say a different thing. Going over this point per point with the 6 points above seems to me to be the most easy way to understand the differences (at least for me). Could someone go through these points one by one, with a clear answer, please? Much obliged!

 

PS. I just realized there might be some miscommunication about 'segmented'. Is a telescope like the LBT (https://en.wikipedia...cular_Telescope) considered segmented? Because that's what I mean when I use the LBT and old MMT as examples where they describe as being the equivalent of a larger mirror with the 1,42 number, I'm not talking about truly segmented ones with the hexagonal mirrors like the Keck, where all simulate one big parabolic mirror.


Edited by SPastroneby, 26 August 2017 - 04:50 AM.


#43 Riccardo_italy

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Posted 26 August 2017 - 08:29 AM

Let's forget the binocular part of the problem.

 

It might be interesting, IMHO, to explore the possibility of having two - say skywatcher 120mm ED - telescopes joined to deliver their photons into a single eyepiece. No compound mirrors (non sense in our aperture range).

 

Suppose we use (theoretical) 100% efficiency optical system (so, no light loss) to merge the images of the two telescopes into a single eyepiece. Then, you obtain the same power of a 170mm ED telescope. If this theoretical optical system is costless, the economics is here: a single 170mm ED costs way more than two 120mm ED.

 

Of course, first this kind of reversed beam splitter (i figure it as a kind of reversed binocular turrett) is not 100% efficient, but even with a 10% light loss it still makes sense.

But, most importantly, it should be quite difficult and expensive to precisely merge the image of the two telescopes, and that's the reason why we don't have it.

 

The only reasonable thing left is a photographic use: many telescopes, each with its camera, and then combine (via software) the image. That's what I have posted yesterday.


Edited by Riccardo_italy, 26 August 2017 - 08:30 AM.


#44 GlennLeDrew

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Posted 26 August 2017 - 11:02 AM

Getting the light from two 100mm telescopes into one eye can be done in two ways:

 

- Combining via a beamsplitter. This overlaps the two exit pupils into one. At best, this provides *almost* the brightness of a single scope. Resolving power is no better. A net loss, really, but we could say the result is as though just one objective was employed.

 

If you have a binoviewer, look through it in reverse. Cover any one eyepiece tube and the image is of roughly half brightness. With both eyepiece tubes uncovered the image is nearly of full brightness (less that 100% due to losses in transmission/reflection.)

 

- Making one offset so that its exit pupil is also offset. In other words, a two-element segmented objective. This doubles the light throughput, and hence image brightness; the effective linear aperture is root two, or 1.414 times larger. Resolving power along the axis defined by the objective centers is, I believe, improved; perhaps as the sum of the apertures and not the aperture separation. (I'd have to look into that.) But perpendicular to the objective-to-objective axis resolving power will not be improved.

 

One could investigate this using a large-ish scope and making a series of masks, all having the same hole diameters. A one-holer, a two-holer (opposite sides) and a four-holer (at equidistant separation, in a 'diamond' pattern) would supply much to cogitate on.



#45 GlennLeDrew

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Posted 26 August 2017 - 11:10 AM

A binocular of 100mm aperture which sends the light from each half into each eye results in an effective monoscope aperture of 119mm. Signal to noise is 1.414X better. Faint source detection is improved by factor 1.414, or 0.37 magnitude. Visual system *linear* resolving power is 1.19X better, which in the 2-D array of an image results in 1.414X more information being derived.



#46 GlennLeDrew

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Posted 26 August 2017 - 11:19 AM

Regarding real vs perceived gains in resolving power.

 

For a segmented objective where each segment contributes to its own part of the exit pupil, the resolution increase is real.

 

For the binocular whose two objectives each serve an eye, the resolving power increase is perceived.

 

For the 'binocular' whose two images are merged, as for long baseline interferometry, with due care in maintaining coherence in phase one does realize a gain in resolution along the axis joining objective centers, scaling as the separation between the objectives.



#47 Riccardo_italy

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Posted 26 August 2017 - 12:29 PM

Thanks Glenn, that's very interesting. I really had no idea how to properly "merge" the light of the two telescopes, and you just told us how to do in order to increase also resolution and exit pupil.

 

I guess an optical device like that is very complex...



#48 SPastroneby

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Posted 26 August 2017 - 03:59 PM

Thanks, Glenn, interesting read, and I think it covers most of what I asked.

 

Just one thing in your first response with the two 100mm telescopes: "- Combining via a beamsplitter. This overlaps the two exit pupils into one. At best, this provides *almost* the brightness of a single scope."

 

A single scope...of... what aperture? 142mm? 119mm? 100mm?

 

If it's the latter that would be, indeed, completely useless then, since you'd get the same benefit for double the price! Rather the reverse of what I was hoping? ;-)

 

 

Phew. I didn't think optics were this hard to get my head around. For instance, I can't understand how a monoscope aperture of 119mm can have a faint source detection improvement of factor 1.414. Isn't light-gathering directly related to detecting faint sources? I mean, the more you can gather light (thus the bigger the aperture), the more faint objects can be detected, no? I could understand it if both were 1,19, or both 1,42, but the difference between them seem to be non-linear. Does this mean that in a hypothetical perfectly working quadroscope it would be 100 x 1,19 x 1,19, but you could detect faint sources at 100 x 1,42 x 142 ?



#49 GlennLeDrew

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Posted 26 August 2017 - 06:14 PM

The end result of combining two telescope images via a beamsplitter so as to feed one eye would be a tad worse than what just one of those scopes would do by itself. Even if you could perfectly register the images to within a fraction of an Airy disk diameter.

 

The 1.414 gain in signal to noise via binocular summation arises from the square root of the aperture increase, here a doubling of the input from one eye to two, the square root of two being 1.414.

 

If you had four eyes, compared to using just one the contribution from all four would again be the square root of the aperture increase, this going from one to four, the square root of four being two. Four eyes vs one would result in a gain in signal to noise of factor two, the aperture equivalent being 1.414.

 

If you had 100 eyes, compared to one by itself the gain in signal to noise is the square root of 100, or 10. The aperture equivalent would be the square root of 10, or 3.16.



#50 PeterDob

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Posted 01 September 2017 - 08:17 AM

After having used my 18" bino-Dobson for over a year now and having compared it extensively with other instruments, including a 27" mono-Dob, I can confirm that a 18" bino is at least equal, if not superior to a 25" mono. On faint objects I was even able to discern more detail than in the 27". Not only because the binocular summation factor is 1,42 (and not 1,18 as stated above), the increased contrast, more relaxed observing experience and reduced signal to noise ratio increase the summation factor to even 1,8x on those objects. I just have to close one eye to see the difference with a 18" mono and that difference is HUGE. If it were only 1,18x that difference would hardly be visible.

Also I'd like to clarify a misunderstanding. Counting in shipping and customs I paid significantly LESS for my binos than I would have paid for a 25" Obsession. Then again, double eyepieces and filters add to the expense...

Peter


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