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Curious about an inexpensive refractor binoscope..

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#1 Simon Alderman

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Posted 26 October 2013 - 01:19 AM

Ever since I received my 15x70's a few years ago I've been curious about trying a larger binoscope. Other than the difficult issues of aligning two optical tubes and allowing for collimation as well as interpupillary adjustment, do you think something like this might work?
I was thinking of using a couple of cheap meade refractors from a popular ebayer along with 2 extra mirror diagonals. Please excuse my crude attempt at the design. Just some "cut and paste" in "paint". I would probably get it "dialed in" and leave a particular set of eyepieces in, maybe 15mm. What are your thoughts?

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#2 orlyandico

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Posted 26 October 2013 - 01:31 AM

collimation will be the big issue. you'd need a very robust tube holder to keep the two tubes rigidly aligned. so unless you have access to machining equipment and/or you can build the holders yourself to a good degree of precision, you'll be having headaches and crossed eyes from the miscollimation.

#3 Simon Alderman

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Posted 26 October 2013 - 02:12 AM

Or maybe more like this so I could set them up like regular binos with a hinged joint between the ota's.

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#4 Biff

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Posted 26 October 2013 - 02:27 AM

I doubt you'd be able to make the tube holders accurate enough as stated above... so just give your self a means to adjust the tube alignment. ;)

However I don't think your diagonal arrangement will work. IIRC using two diagonals like you have drawn will give two images 180° out. You need to use three diagonals. first two to offset the optical axis, then the third to make the 90°. There is a way to do it with two mirrors but it takes some fancy work using ~60° reflections if memory serves me right the last time I looked into this.

edit - I see you posted in the time I was writing this. The second drawing will work. Add the third diagonal if you want the 90° viewing

#5 GlennLeDrew

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Posted 26 October 2013 - 03:33 AM

A simpler arrangement would dispense with having the tubes side by side, and so widely separated. Using a combination of lateral and vertical offset, thectubes can be practically in contact. Mire importantly, you can get by with just *one* diagonal on each scope. Because one scope is lower than the other, it's tube might need a bit of shortening, and *maybe* a somewhat larger diagonal fixed to it.

If the scope f/ratio is about f/7 and longer, you can likely get by with the same size diagonal for both. In case the mental picture I'm trying to paint is not clear, realize that for the lower scope, the barrel on the diagonal into which the eyepiece inserts must be longer than the other, so that it extends up to the same height as the other.

#6 Simon Alderman

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Posted 26 October 2013 - 04:10 AM

Thanks for the insight and ideas guys! The scope is a 70mm f8.5 so the offset tubes should work. Though I've been thinking of doing some careful measuring and milling a heavy clamp for the objective end of both ota's and having another clamp with fine threaded srews for adjusting the alignment of the tubes in relation to each other. If I get the tube spacing correct then I can use some minor adjustment at the diagonal for io distance. It will probably just be my eyes looking through this so once it's set it shouldn't have to move again, except for bump correction.
Glenn, I love your bino chair! That may be one of the next things on my build list, even before this scope (if I get around to trying it).
Thanks' again!

#7 schang

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Posted 26 October 2013 - 07:53 AM

Glen:

I had been thinking a DIY binoscope from refractors for a while, but have not taken any action yet. One reason was that it appears to be more complex than building a refractor or a Newtonian, particularly at the back ends. In your opinion, is it worth it to go this DIY route from the aesthetic, ease of use, and performance standpoints?

#8 dan_h

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Posted 26 October 2013 - 08:49 AM

Thanks for the insight and ideas guys! The scope is a 70mm f8.5 so the offset tubes should work. Though I've been thinking of doing some careful measuring and milling a heavy clamp for the objective end of both ota's and having another clamp with fine threaded srews for adjusting the alignment of the tubes in relation to each other. If I get the tube spacing correct then I can use some minor adjustment at the diagonal for io distance. It will probably just be my eyes looking through this so once it's set it shouldn't have to move again, except for bump correction.
Glenn, I love your bino chair! That may be one of the next things on my build list, even before this scope (if I get around to trying it).
Thanks' again!


Here is a li\nk to a CN article that provides some details to what Glen has suggested.

http://www.cloudynig...hp?item_id=2439

dan

#9 schang

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Posted 26 October 2013 - 09:36 AM

Dan: Thanks for the link. Now it appears that building one is not quite daunting, especially for the interocular distance adjustment (I think it can be adjusted easily by rotating the binoscope and diagonal in opposite direction, without needing a complex parallelogram mechanism linkage. Though you will need to find a way to rotate the binoscope.

#10 GlennLeDrew

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Posted 26 October 2013 - 02:23 PM

Shien,
If one has two working eyes, why not use both? We binocularists are fond of saying that two eyes afford a gain in signal to noise of 41%. But really, it's the telescopists who are restricting themselves to a view only 71% as good as their two eyes can provide.

All you have to do is cover one eye for a time, day or night, to realize what an improvement it is to have that eye back. (Discounting completely the utility of 3-D vision on the nearby world.) Especially at night, something like the milky way is so very much better seen with two eyes. At low light levels is where the benefits are really felt.

#11 schang

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Posted 26 October 2013 - 04:16 PM

Glen: Thanks. Some here have developed a "skill" to open both eyes but only using image from the scope eye. I could not concentrate doing that, and closing one eye for a while causes eye strains. It feels so much better with two eyes, that I agree with what you have said. I am thinking about getting inexpensive 80 or 104mm refractors for the project if I can get the cost below $500 for low to medium powers observation. The other option would be to buy a binoviewer for my dob, though this approach will not get powers lower than 35X, which is what I can get without using a binoviewer. The binoviewer, though dims the light somewhat, does not require a special tripod mount like that of a binoscope at similar price range. In your opinion, which one would be a better approach? Or, go for a 4" to 6" Newtonian binoscope?

#12 Simon Alderman

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Posted 26 October 2013 - 08:46 PM

This is the scope I was thinking of using. Certainly of a lower quality than some but at this price, plus a couple of mirror diagonals that he offers for $8 each, it seems like a worthwhile project.

I might also add an 1.25" extension so that I could try both arrangements.

http://www.ebay.com/...idescope-new...

#13 schang

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Posted 26 October 2013 - 10:03 PM

Simon: This 70mm refractor should be a good start for a binoscope, though I'd think that its aperture is a little small, but acceptable (exit pupil about 3.7mm with a 32mm ep). F ratio of 7 or 8 would be better, or with an 80 mm f/6 or 7 for low to medium power binoscope. I may search around for something like that. If I can not find one, then I may use this one (at least it is very inexpensive).

#14 Simon Alderman

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Posted 26 October 2013 - 11:54 PM

Shien, you've brought up a great point. With a 32mm ep it would be an 18x instrument. 18x70. I own a pair of 15x70's already. I would really like to use it at 40x to 60x which would only illuminate a little over 1.5mm. I'm just not sure it's worth building a 70mm scope. Looking for an inexpensive 100mm seems like a better idea.

#15 GlennLeDrew

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Posted 27 October 2013 - 01:28 AM

I feel that up to about 6" aperture refractors are more elegant. In a Newtonian configuration the OTAs must be separated by a distance equal to one's head width, plus a margin, which makes for a relatively much wider/bulky instrument. This is fine for already-fat tubes on larger aperture scopes.

HOWEVER, the cost per unit aperture for a 4-6" Newt begins to tell, and may overcome the 'inelegance.' Moreover, one can employ Amici prism diagonals, oriented so that the observer faces the target, and enjoy an upright and non mirror-reversed view. Albeit with the discomfort of any straight-through bino, unfortunately. But a head-supporting, reclined seat would negate this.

#16 schang

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Posted 27 October 2013 - 09:08 AM

Thanks, Glenn. Look like I need to digest all the pros and cons in this subject before I can decide what will be the best approach to use both of my eyes for star gazing.

#17 Jon Isaacs

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Posted 27 October 2013 - 09:27 AM

Regarding the over/under design:

One issue I did not see addressed in the article is the importance of making sure that the drawtube does not cut into the light path. Because extension tubes are used to raise the eyepiece of the lower scope, this increases the effective length of the drawtube..

I am working on an over/under based on two St-80s. Tube length and drawtube length seem to be important issues..

Jon

#18 Chuck Hards

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Posted 27 October 2013 - 10:24 AM

Somebody actually did it the way Simon suggested, at Riverside many years ago with 6" objectives, IIRC, but the details escape me at the moment.

#19 schang

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Posted 27 October 2013 - 06:29 PM

Jon: Good point. I see that you are going to use the Orion ST80 for this design. I was eyeballing it as well ($120). It is a F/5 so there may be some CA for bright objects. Is it any worse than a regular binocular? Also, can you take the light shield off the tube so that you can choose a shorter offset of the down scope to avoid blocking the light path.

#20 dan_h

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Posted 28 October 2013 - 01:37 PM

Regarding the over/under design:

One issue I did not see addressed in the article is the importance of making sure that the drawtube does not cut into the light path. Because extension tubes are used to raise the eyepiece of the lower scope, this increases the effective length of the drawtube..

I am working on an over/under based on two St-80s. Tube length and drawtube length seem to be important issues..

Jon


Thinking about the over/under design, I have a question.

How much does the horizontal separation of the objectives contribute to the overall 3D effect in the view through a binocular? With the over/under the separation is pretty much limited to the IPD of the viewer. In a bino with the tubes mounted in the horizontal plane and the IPD adjusted via reflections, the objectives can be separated as much as the designer wishes to accomodate. Is there value lost by utilizing a small separation with large objectives? I would think that astro targets at infinity do not exhibit any 3D effect. Terrestial targets may be a different story.

dan

#21 GlennLeDrew

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Posted 28 October 2013 - 03:26 PM

Shien,
An f/5 achro will suffer less chromatic aberration than the common f/3.75 cemented achro bino objective. An ST-80's light shield can never clip the light path.

Dan,
Objective separation contributes to the 3-D effect linearly as the separation increases. But this works only when parallax is resolved at the eyepieces. For astro targets no amount of objective separation will deliver sensible parallax; 3-D here is always purely illusory.

#22 dan_h

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Posted 28 October 2013 - 05:58 PM

Dan,
Objective separation contributes to the 3-D effect linearly as the separation increases. But this works only when parallax is resolved at the eyepieces. For astro targets no amount of objective separation will deliver sensible parallax; 3-D here is always purely illusory.


Then the obvious solution is to use a binoviewer with a single objective of enough aperture to overcome any losses encountered in the viewer.

dan

#23 schang

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Posted 28 October 2013 - 07:51 PM

Dan: Due to the extra light path with the use of a binoviewer, most ends up with higher magnifications than desired (low power end), in addition to the dimmer light compared to the same scope without it. So if I'd like to view at lower mag of 20X and up, I could not use a binoviewer on my dob at all. Though it does offer the comfort of using both of your eyes.

#24 GlennLeDrew

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Posted 28 October 2013 - 09:28 PM

Furthermore, a binoviewer always introduces a halving of image brightness to each eye at given exit pupil. This is the real reason why a true binocular is advantageous, for astro or terrestrial use. Otherwise, if one is satisfied with half brightness views, a completely uncoated binocular (with some 12-14 air-glass surfaces per side) would be acceptable with its approx. 50% transmission.

No amount of aperture can increase image surface brightness, thereby 'compensating' for lower transmission. A 2" or a 20" instrument at given exit pupil delivers the same surface brightness (other things being equal.)

#25 schang

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Posted 29 October 2013 - 08:18 AM

Here is a link I thought may be informative about vignetting of a standard binoscope, which stated that the maximum focal ratio is F/6.2. For Orion ST80 F/5, it should be fine, though if we want to stack them in asymmetric (up and down) way, then the down tube offset position may be critical, as I understood from Jon's comment. If it is offset far behind the up tube, then then you will need longer draw tube to make it up, which it may block light path. I was asking if the light shield can be removed, for if you can, you can put these two tube closer together, and lessen the offset distance. This way, the two scope tubes will be less asymmetric (more horizontal). Am I correct ?


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