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Is a bigger clear aperture bino a better bino?

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#1 Eddgie



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Posted 25 November 2012 - 01:50 PM

Funny thing... For many applications where a GPC is required, a bino with a bigger prism may not offer any advantage at all in terms of true field.

I have been doing the math on my 6" f/8 APO and what I have come to realize is that the extra light path lengh of the Mark V binoviewer (shipping this week!!!!!) will require the use of a higher power GPC, and that when I do the math on it, the true field I will get in the 6" APO is going to be slightly smaller than the true field I could get using the Maxbright, even though I have to use a higher power eyepeice. I can just reach focus with the 1.25x GPC in front of the diagonal with the Maxbright/T2, but I do not have another 10mm of travel that will be necessary for the Mark Vs. This will force me to use a 1.7x (or maybe higher) with the Mark V in that scope.

I did the numbers on a lot of scopes, and what I found is that if the use of the bigger prism units always adds about 10mm to 12mm of light path.

If the scope has to bump up even one step in OCS, it means that in most cases, you can get a wider true field using a 21mm Hyperion in the Maxbrights than you can using a 24mm Hyperion in the Mark V....

People that use Binoviewers primarly with refractors should pay careful attention to this. Do the math to see if you can get by with a 1.25x GPC using the Maxbright/T2/21mm Hyperion vs going to the 1.7x big prism setup.

Again, I did the math on several scopes, and it almost always worked out with the Maxbright/T2/21mm coming in with at least as wide or a wider true field than with the Mark V or Denk (which would need an T2 Prism adapter to get short enough to even be in the game).

For the SCTs and MCTs, the slighly longer focal lenght is of course a slight factor, but since you don't need a GPC, it always comes out ahead for every scope that I did.

Anyone that is gong to require a higher power GPC to use the Mark V in a refractor should (if widest field is an important point) pay careful attention.

You could spend 4 times as much on a bino thinking you will be better off in this one area to find out that there is no benefit.

For me it really isn't an issue.. If I had to go to 1.7x, then I won't get a true field that is much bigger than my Edge HD 8" so there really would not be any point in Bino-viewing the 6" APO (I don't use it for planets, only for wider field viewing), and if I can't do with the 1.7x and have to step up to the next power, and this means that the EdgeHD 8" would actually be able to give a much bigger and brighter field of view.

Anyway, I though this was an interesting finding.

I have been saying that what makes the Maxbright/T2 (standard prism) so compelling is that it really does have a huge advantage in keeping the focal lenght as short as possible, and for scopes that require a GPC, this may in fact make it work at least as well and perhaps better than a larger prism unit.

Food for thought. Refractor ownwers should do careful measurments and spend a few minutes with a calculator before jumping on the larger prism (and far more expensive) binoviewer band wagon.

#2 MrGrytt



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Posted 25 November 2012 - 03:10 PM

What you're describing here is a situation in which a particular scope was built with no thought given to bino-viewing and a resultant lack of backfocus. The simple answer is to shorten the tube if it's practical to do so. It's smart to get the best bino-viewer and make the scope work with it if you prefer bino-viewing.

Perhaps even send yours to Astro-Physics and have them do it. Have them give it an absolute minimum of 170mm of backfocus. It may require the use of an extension for normal use but the bino-viewing will no longer be a problem.

My AP 130 f/8.35 has the least backfocus of any scope I own and it has 164mm of backfocus. It just barely requires the use of the 1.25x corrector. All my other scope can bino-view with no corrector but require an extension for normal use. I much prefer it that way.



#3 johnnyha


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Posted 25 November 2012 - 08:04 PM

Yes it's very scope-tube-length-dependent. :grin: One of the happy happiest results of installing the 3.5" Feathertouch focuser in my FS152, was the ability to reach focus at 1X with my MkVs and 24mm Pans (and with 1/2" still to spare). And with the 4.5" drawtube I can also rack it out and reach 2.6X without an extension, with 1/2" left over. :cool:

So get out the hacksaw Eddgie! I know it's an AP and you don't want to shorten the tube - but the ability to binoview at 1X will probably NOT reduce the value of the scope and will make you very, very happy.

You could also maybe get a shorter tube made and keep the original. :question:

#4 Eddgie



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Posted 25 November 2012 - 11:32 PM

In my case, shortening the tube is not an option. When racked in, the focuser tube would hit the baffle if the tube were even a half inch shorter.

I could put in a shorter focuser tube maybe, but I don't really use this scope much anymore so I am not sure what the point would be.

But my point was that for someone that likes small refractors but still wants a wide field of view, they may not get a wider true field using a 27mm prism than they could get from the Maxbright/T2 is the Maxbright/T2 requires one GPC lower, or no GPC than with the bigger aperture binoviewr.

Again, I don't really use my 6" APO much, so I don't want to cut the tube and change the focuser so for me it isn't at all important.

#5 thesubwaypusher


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Posted 02 December 2012 - 12:27 AM

You could spend 4 times as much on a bino thinking you will be better off in this one area to find out that there is no benefit.

It's interesting that you discovered this just before you are to receive your Mark V. It is probably not the only disappointment you will face with them. Realistically, I don't think they will be perceptibly brighter than your Maxbrights. I did a side by side with my Tele-Vue Bino-Vue and could not tell the difference. They will most likely be heavier than your Maxbrights also. The advantage could be solely psychological, as owning something so nice, and something we know can't be beat, gives us. As for myself, I still get a tiny bit of vignetting on all my scopes with the 24mm Pans, which I thought would be gone with the Mark V and its larger clear aperture. The biggest and best advantage to me are the eyepiece holders that you simply turn a bit and the oculars are locked in firmly. I only change two sets of eyepieces, but it still makes a difference. I also like that they oculars are always perfectly equal at all times without having to adjust them. But as mentioned, the hugest plus is psychological (for me). I am a firm believer in knowing that I have paid (and continue to pay) my dues, and deserve the best, even though it may all be a mind game. That may not be enough for some.
Oh, and I do like to use the gpc units also. They are fun to experiment with, and I think an ingenius way to keep the length down. AND I think the way the binos themselves attach to the diagonal is kool also- again, to keep the length down. HEY! Maybe these things were worth it!

Thanks, Chris

#6 Eddgie



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Posted 02 December 2012 - 09:27 AM

I think you have misunderstood the intent of my post.

I am in no way disappoited to have realized this after the purchase of the Mark Vs becuase I don't intend to use these in my refractor.

I use my C14 for 90% of my observing. It really doesn't matter to me that they might offer no wider true field than the Maxbrights in the refractor becuase they will likely never be used in that scope.

And I don't expect them to be brighther than the Maxbrights, which I find to be excellent.

But in the C14, the difference in prism size offers a considerable increase in true field.

My post was to alert refractor ownwers that might be contemplating a binoviewer purchase that it may save them some money by getting the Maxbrights in place of a large prism device if it can save them from having to use a GPC or a larger GPC. The shorter light path of the Maxbright could make all the difference in the world.

As for me, it is of no consequence at all. They will only be used in the C14 and the EdgeHD 8".

My Mark Vs are supposed to arrive this week and I can't wait to plug them into the C14.

#7 Bob S.

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Posted 02 December 2012 - 10:50 AM

Eddgie, OTOH, Chris' point about the TeleVue BinoVue's being pretty on par with Mark V's has been my experience. I sold a friend of mine a pristine pair of TV BV's and was surprised that they have not been significantly better in my 6.3" Apo refractor than the TV BV's or my former Siebert BN25 bv's. On the other other hand (I was born with a birth defect), my Seibert 45 Elites simply blow everything I have used off of the map! Yes, they are every bit as heavy as my Mark V's and then some with 2" ep's like my 20mm T5 Naglers but whoa, the views are absolutely worth it! I can't wait until I get the Seiberts in my 20" f/3 tracking Newtonian that will be ready in a couple of weeks. The tracking/drives will neutralize the weight issues and I will be good to go. Bob

#8 GlennLeDrew


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Posted 02 December 2012 - 12:15 PM

Without having examined other than the low cost binoviewers introduced some years ago and which carry/carried numerous brands, I can offer only general comments here.

If the beamsplitter and prisms are merely scaled in overall size, going bigger does not aid in the acceptance of a faster light cone. What is gained in upsizing for aperture is lost due to the now longer optical path length through glass.

A larger rear prism aperture allows to use eyepieces having larger field stops before edge-of-field darkening becomes noticeable.

The front aperture imposes a constraint on the fastest light cone which can be accepted, as well as the field illumination. For given entrant f/ratio, the size of the circle of full illumination is determined by the front aperture, if it has not already been made smaller by the scope or any other optics ahead of the BV.

The location of the focus with respect to the rear aperture has some impact on the acceptance of the fastest f/ratio permissible. The farther rearward of the rear aperture the focus lies, the more lesser must be the steepness of the entering light cone. And the size of the circle of full illumination diminishes, too. But only when operating near the fastest f/ratio permissible is this of any real concern.

As long as the central illumination us full, the image is as bright as it really needs to be. Fall-off in illumination off-axis will be slight enough to not be of concern. Except, of course, that near the field edge when the field stop is larger than the rear prism aperture.

An optic needs be only large enough to permit full on-axis illumination and an acceptable level of vignetting. If the latter is no worse than ~50%, all is fine for visual use. Going larger in a binoviewer may gain nothing more than the ability to use eyepieces having larger field stops. The circle of full illumination might still have about the same diameter, and with a larger field stop illumination at the field edge might be even lower.

The BV must be made so as to reduce the ratio of optical path length-to-aperture in order to realize a better field illumination pattern at the focal surface. High index glasses are one way to realize some gain here. At any rate, it would be handy if the makers provided this figure of path length-to-aperture ratio. And of course, the value must be the equivalent path length through *air*, as refraction through glass moves the focus rearward by an amount roughly equal to 1/3 the glass path length, this increasing with the glass's index of refraction.

#9 thesubwaypusher


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Posted 02 December 2012 - 01:53 PM

Okay, I understand now. I think the reader of this thread will learn a lot about their prospective purchase of the Mark V, and that's the important thing. ;)

That unit belongs with your 14" scope.

#10 Eddgie



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Posted 02 December 2012 - 05:16 PM

And that was my intention...

When I first came into the market for binoviewers, I quickly realized that as far as astro-gear goes, binoviewers present one of the most complex purchase decisions I have ever made.

I greatly underestimated the impact that back focus requirements can have on both refractors and SCTs.

What I have come to believe is that back focus is perhaps the most crucial element in the selection process, especially for refractors that may not or cannot reach focus without a GPC. The difference of one GPC step can completely negate the advantage of a larger prism, and the big prism units almost always have at least 10mm more back focus than the Maxbright/T2 (standard prism) which could be all the difference in the world.

Hence the question at the beginning of the thread. I a bigger clear aperture going to give a wider true field if it takes a step up in GPC than a Maxbright/T2?

And when I did the math, it turned out that for the models I ran the numbers on, there was no benefit in terms of true field by using a bigger aperture bino vs the Maxbright/T2.

Refractor users need to measure carefully and do the math and they might find that the larger aperture will not bring any benefit at all.

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