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# How to analyze a Binoviewer configuration on a refractor

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### #1 Eddgie

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Posted 13 October 2019 - 06:03 PM

Binoviewers almost always force some meaningful compromises on the user, and one of the most meaningful of those is the size of the fully illuminated field..

I just got a new to me telescope and I thought I would share the analysis so that others would know how to very accurately calculate their own fields size.  It is actually easy to do, and takes nothing more than a ruler and some simple math.

The first very simple formula needed is the reduction in image circle for distance traveled and this is pretty easy. One starts with the size of the fully illuminated circle at some point in the light path and subtracts 1mm of circle for every multiple of the focal ratio of the instrument.

Here is an example of this.  The telescope I am working with has a 2" focuser, and the focuser tube is 134mm long.  Since it is 51mm at the objective end, this would be the light cone size as it entered into the tube.  The scope I am working with has a focal ratio of f/6.5, so for every 6.5mm of length, the 51mm circle is reduced by 1mm, so doing the math, the fully illuinated circle shrinks from 51mm at the front of the focuser tube and loose 20.6mm of diameter, so when it comes out, it is (51-20.61) 30.39mm in size.

Now, my home made T2 diagonal has a 2" nose with a 12mm flange, so by the time the light gets to the end of the flange, it will have traveled 12mm and lost 1.85mm, so when it reaches the face of the diagonal, it is now 28.54mm.

Now what I find here, is that when it reaches the opening in the front of the mirror box on the diagonal, the illuminated field is actually .54mm bigger than the hole in the mirror box, so at this point, I have to star to use this new figure as the fully illuminated circle size.  Had the hold been larger (Baader T2 diagonal) then the light cone would have retained its full 28.5mm.

You might say "well Ed, get a T2 diagonal so you can use the full light cone!"  Well, this is why we are doing the analysis.

The light path through the mirror box of the diagonal is 58mm.  This means my newly sized 28mm light cone will be reduced by 8.9mm so the 28mm light cone is now 19.1mm.   Now the clear opening of the RAF adapter is 22mm, so here, the light cone passes without restriction and continues to converge.

Because the RAF adapter adds 3mm to the light path, my light path through the binovewer is 99mm and this means that in this distance, the light cone shrinks by 15.3mm!!!  The fully illuminated circle that was 19.1mm is reduced to 3.8mm in size!

In its journey, the light cone has been reduced from 51mm to a tiny 3.8mm circle!

Now this is not terrible, but it is not really all that good either.  The illumination falloff is not huge when using eyepieces with smaller field stops as is the case with short light path binoviewers, and because it is gradual and the field is narrow, this is probably OK, but at the same time, for best planetary performance, the planet should be kept very near the center of the field of view.  Outside of this circle, and in effect, you are looking at the planet with reduced aperture. The loss is small, but loss is loss.

Suppose I went to the Baader T2. Here, the light path is about 12mm shorter and this is due to the fact that I would not have a flange in front of the diagonal.  Once again, at the point where the cone leaves the focuser tube and would enter the T2 diagonal, the cone is 30.39mm, so it would pass though the 34mm opening at the front of the diagonal and travel 50mm before getting to the opening in the RAF adapter.  This would mean that it would be 22.44mm at the entry to the BV.

The BV though has the 22mm openning, so this now becomes the light cone size opens up to a whopping 6.77mm (which is actually not terrible. Some SCTs don't have a fully illuminated field much larger than this!).

So for the cost of the Baader T2 mirror diagonal, I would increase my fully illuminated circle size only a tiny amount. Would it be worth it?

Well, If I were doing white light solar and wanted a fully illuminated disk, then maybe it would be.  See, in this scope, the sun would be 6.33mm in diameter, and this means that with the T2 mirror diagonal, I could fully illuminate the suns disk.

But how about another approach?  For the same price, I could get the Televue 2x Amplifier. Now since the light path is shortened by the length of the binoviewer (the TV amplifier makes the BV parfocal) I would now get the 17mm illuminated circle provided by the Televue 2x Amplifier, and as a plus, I would still get a full disk sun, but I would not have to rely on very short focal length eyepeices to get high power.

So, what this analysis tells me is that there is little value for me to buy a T2 mirror diagional. The size of the fully illuminated field is not that much bigger for general observing, and while it would allow the suns disk to be fully illuminated, my zooms would not give enough magnfication to reach the high powers I would use for sunspots, so I would have to add some form of Barlow anyway.

Now, I already have a 2x Amplifier, so for me, there is no extra cost, where the T2 mirror would be a lot extra.

As the prisms get bigger, the light path gets longer, and in this particular case, I only have 3mm of focuser remaining so I would not be able to reach focus.   If I went to the T2 mirror I could gain myself 10mm of light path, but it would be very close.   If I could not reach focus, then I would have to use the 1.25X GPC, and now I negate the wide field advantage of the bigger prisms, but I do get bigger illuimated field because the GPC shortens the light path though the Binoviwer.   A BV with a 110mm light path (Maxbright) would now have a light path of only 92mm, and since the front aperture was larger, the fully illuminated field would be larger as well (and I would let you do that math on that now that you know how). As can be seen though if the goal is to increase the fully illuinated field size and preserve image sharpness over the largest circle possible, then a lot of amplification that shortens the light path of the binoviewer is the probably the most optimal path.  This is one of the many reasons that for viewing planets, lunar, or solar,  I always suggest people use a Barlow, GPC, or specially designed amplifier like the Televue. All of these will greatly increase the fully illuminate field size when working with modern, fast refractors, and amplifiers like the GPC or TV amplifier correct aromatherapist to boot.

I realize this has been a long post and if you slogged your way through it, I hope that it has given you some valuable insight in to how to analyze a binoveiwer configuration.   Is all it takes is some measuring, simple division, and simple subtraction to accurately calculate the fully illumined field size you will be presented with in your own configurations.

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### #2 Joe1950

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Posted 13 October 2019 - 06:52 PM

Excellent work, Ed, as usual. Thanks!

### #3 Eddgie

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Posted 13 October 2019 - 08:37 PM

Thank you.

As can be seen, with today's fast refractors, even a well configured system using T2 diagonals will not have a very large illuminated field, but it is important to remember that most reflectors and standard SCTs don't have much larger illuminated fields and the limit of the 1.25" eyepieces means that the amount of falloff is small enough that it will not be evident for deep sky work.  In some cases, like larger fast refractors being used for white light solar white light, it could lower contrast on the edge of a fully illuminated solar disk, but this kind of situation is rarely encountered.

Now a good example of where I did have a concern was with my Lunt 80ED Hydrogen Alpha scope.  Since the prominences of the sun can extend quite far from the disk and I enjoy seeing the full disk view, it was important to me that I have not only the 5.13mm to fully illuminate the disk, but also 100% illumination for an area that would be more than big enough to capture the largest and faintest prominences I might ever encounter.  I could have reached focus with a 1.25X GPC, but this would not have fully illuminated even the full disk, so I went to the 1.7x GPC, which I calculated would give me a bit over 6.25mm of fully illuminated circle, so more than enough that I would be sure of getting 100% performance over the area of the sun and surrounding sky.

For slower scopes, if you can reach focus with no amplification most of this is simply not important.   When we get into the sub-f/7 arena, it can start to be more of a concern and it seems like refractors are getting ever faster, so when binovieweing these very fast systems, doing some analysis may help in deciding the best configuration.

Edited by Eddgie, 13 October 2019 - 08:41 PM.

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### #4 De Lorme

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Posted 14 October 2019 - 05:36 AM

Thanks Ed! Question for ya; I just recently bought a Baader/Celestron bino that doesn't use a diagonal which means I will be able to come to focus without a ocs.  I'm assuming I would get full illumination in my 5" 7.5 fr refractor; Am I correct?

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### #5 Eddgie

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Posted 14 October 2019 - 08:38 AM

doesn't use a diagonal

I am not sure what you mean when you say it does not use a diagonal.   You don't have to use a diagonal with a binoviewer, but that means you would be viewing straight through?

Or are you talking about a one of the new linear binoviewers that don't use prisms? If this is the case, I can only guess.

If you meant a bionviewer that does not use prisms and reaches focus in any telescope without a Barlow, then it would be my guess (and only a guess, and maybe not a very good one) that these use some kind of relay lens to extend the light path.

A relay lens is an achromatic doublet.   If you place an achromatic doublet  two times its focal length from the focal plane, it will form an image two times its focal length behind that lens (at least that is what I recall).    So, if I used a relay lens with a focal length of 30mm and placed it 60mm behind the focal plane, it would form an image 60mm past this.   The mirrors inside the binoviewer would be used to bring the image to both eyes but would also provide the necessary 60mm of light path before and after the relay lens.  If a beam splitter was used as the first element, then there would need to be two collimating lenses, one for each eyepiece light path.

I am guessing the "no light path length binoviewers" use something like this. If it is indeed some form or relay lens, then the size of the fully illuminated circle at the focal plane of the telescope would be transferred through the optics and restored to it's original size at the focal plane.

But to be absolutely honest about it is only a guess that it is using some kind of relay lens system and this means that whatever would be present at the focal plane would be replicated at the eyepiece.

Now one thing we know though is that in these, the focal plane is inside the binoviewer or at the front aperture of the binoviewer, and we know that in the case of the Linear binoviewers, the clear aperture is 17mm, so the fully illuminated circle could never be bigger than this but this is a bigger diameter than the fully illuminated circle produced by most SCTs and reflectors, and even refractors using a standard 1.25" diagonal.

So, it was not clear to me from your post what you were talking about, but if it is a linear binoviewer, then it should preserve the size of the fully illuminated field or a 17mm circle, whichever is the larger of the two. If you used an eyepiece with a field stop of bigger than 17mm it would vignette outside of this circle, but I don't know if that vignetting would be enough to easily see.  If the source of the vignetting is quite far away (as it is in a regular binoviewer), the effects of vignetting can be impossible to see without specifically testing for it.

Edited by Eddgie, 14 October 2019 - 08:46 AM.

### #6 rob.0919

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Posted 14 October 2019 - 02:27 PM

Thanks for posting this Ed. Interesting topic.

What mounting are you using there for your refractor ?

It looks similar to the Swiss AYO Vamo Traveller.

I'm looking at grab / go options for a similar set up myself.

### #7 Eddgie

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Posted 14 October 2019 - 03:43 PM

The mount in the picture is was branded as "Astro-tech Voyager" and today is generally called the Voyager 1. There is also a Voyager 2 and I they are substantially different.  I believe the Voyager had a bit high weight rating than the Voyager 2.

The scope, rings, and dovetail come in at 5.5kg, so with binoviewer, diagonal, and light eyepieces, the total load is more like 6.2kg, and while the original suggested payload was something like 9kg, I would say that this scope is either right at the max if you can tolerate a 4-5 second settle time, or over it if you can't.

I think a 100mm f/7 doublet would be much more comfortable on this mount.

I make things work on this mount because this mount lives outside 24/7.  Sometimes I bring it under my covered patio, most of the time, I don't.   That might sound abusive, but this mount was inexpensive, and if I had to store the mount inside and carry it out as a unit, I would have to have a very light telescope, or make two trips, and I choose to not do either of those things.

The HAL 110 legs are only OK.  I think a larger steel tripod would be a bit better for stability, but I do move the mount and scope as a unit outside during the day, so I put up with the slow damping time to keep it manageable.

So, it has basically been badly abused for many years, and it keeps on trucking so I would say if you need a robust mount that can stand a lot of neglect, I highly recommend it.

(The mount does solar white light more than dark work. I do solar every day, and again, this is why the mount simply lives outside. If I had to carry it in and out every time I did some form of observing, I would observe less.  It is also why I like to use zooms in the BV.  I have dedicated BVs and zooms for each scope, so when I carry out the scope, it is 100% complete and ready to go... As Bruno Mars would say "It's what I like.")

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### #8 De Lorme

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Posted 15 October 2019 - 01:07 AM

It's this one with a 1.25" nosepiece.

### #9 Eddgie

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Posted 15 October 2019 - 01:42 PM

Ok, that is not really zero light path so that was my bad.

It will though behave the same way as any standard binoviewer.

First, you have to measure the light path length. There are various ways to do this, but unless you know the actual light path length, you won't be able to really do the calculation.

The the calcuation is the same as any other standard binoviewer.

For example, if the light path is 90mm, and the front aperture is 25mm, and the scope you want to use it in is f/7.5, you would divide 90mm by 7.5, you get 12mm circle reduction.   Since the front aperture is 25mm, then the fully illuminated circle would be 13mm.  I made up those numbers.  You would have to measure them to really know what the are, or maybe someone on the forum has that figure for you.

There is a complication though.  Inspect the BV to see if the nose removes, and if so, check to see if the aperture of the body is the same as the aperture of the nose.  If it is, then you have to add the lenght of the nose to the light path.  For example, if the path of the body is 90mm, and the nose is 20mm, if the aperture in the BV is the same size as the opening in the nose, the nose becomes your restriction.  If the aperture is much smaller than the 1.25" opening, then the aperture in the BV body is the restriction.

I know this last part is confusing, but this is why you have to know the distance and size to every restriction.

### #10 Eddgie

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Posted 15 October 2019 - 02:00 PM

I know this last part is confusing, but this is why you have to know the distance and size to every restriction.

And here is an example of that.. Suppose you are using a 1.25" binoviewer and you can reach focus using a standard 1.25" diagonal.

Because you are using a standard 1.25" diagonal, the biggest restriction is not going to be in the binovewer, it is going to be the front of the diagonal.

For example, say you used a standard 1.25" diaognal with a light path of 75mm. Since the nose piece of the binoviewer is inside the eyepeice holder, we don't normally count it because we would be double counting the light path of the nose due to the fact that you already included it in the diagonal.

Now one would think that at this point you would use the front of the mirror box, but because we would be double counting the light path where the nose of the binoviewer went into the visual back.

The problem though is that the entry side of a 1.25" diagonal is usually only about 28mm and this is the same as the opening in front of the diagonal body, but since it is further up in the light cone, then you have to account for that..

So in fact, the light path of a 1.25" diagonal has to be considered as being 100mm, becuase you need to account for the nose. You would subtract that nose length from the amount of focuser tube though so you don't double count.

So, if you had an f/6.5 scope and used a standard 1.25" diagonal, the 28mm hole at the front of the diagonal would reduce the fully illuminated circle to about 12.7mm.  Now if you passed this though a BV with a 96mm light path, the cone would converge another 14.7mm, so in fact, this setup would result in aperture reduction.

This is why for faster scopes, the T2 system becomes highly desirable. Using a 1.25" diagonal with a 2" nose ensures that the diagonal body becomes the vignetting source and eliminating the eyepiece holder saves another 25mm to 30mm.

So, a regular 1.25" diagonal has a light path of almost 100mm (counting the nose, which you have to count on a regular 1.25" diagonal) when used with a binoviewer like the new TS binoviewer (light path of 118.5mm) would only be able to give full aperture with a light cone of no faster than about f/7.8.

Now since only a very very few models will reach focus with even the 118mm light path of the new TS binoviwer and a standard 1.25" diagonal, I see a lot of T2 diagonal sales on the horizon.  LOL.

Edited by Eddgie, 15 October 2019 - 02:00 PM.

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### #11 vineyard

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Posted 22 October 2019 - 06:27 PM

Thanks for this.  Very helpful.  If I'm understanding it right, would my calcs for my scope be correct?  Its a TV102iis - 102mm aperture, 880mm focal length, f/8.6, 2.4inch (ie 61mm) focuser, 2 inch diagonal (although I also use a hi-hat adapter to 1.25inch), say 11cm Zeiss binoviewers.

So, the light beam would be 61mm at the front of the focuser. I can't remember how long the focuser is.  But if we work with the 134mm in your example, then for every 8.6mm it shrinks 1mm, so at the end of the focuser, it is 134/8.6=16mm smaller - ie down to 45mm.  I don't know the length of the path in the 2" diagonal, since it sits nose in, perhaps its just the path from the diagonal mirror up through the hi-hat (to avoid duplication).  If that is say 25mm long, that's a further reduction of 25/8.6=3mm, so the image is down to 42mm?

But if I'm using the 1.25inch adapter, then that censors the image width to 32mm.  The binos have 24mm clear aperture on the telescope side, so that censors the image further.  An 11cm light path in the binos then reduces the image by a further 110/8.6=13mm, meaning that when the image gets to the eyepiece its 11mm wide?

Sorry for any stupid mistakes - I'm a newbie on binoviewers.

Thank you!

### #12 Eddgie

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Posted 24 October 2019 - 09:19 AM

Yes, you are very close....

The light path though a 2" mirror is usually about 100mm, but in this case, the exact figure will depend on where the nose of the binoviewer would be when it is used with the High Hat.

For example, let's say that with the high hat, the nose of the binoviewer is 10mm inside the top of the diagonal.

If your focuser tube was 134mm and the distance from the front of the mirror box of the diagonal (this is where the light path is measured for the diagonal, you you would be counting it twice because it would be inside the focuser tube, and we have already counted that once) and we have another 90 degrees inside the diagonal to the front opening of the BV, then that would be 224mm of travel.  At f/8.6, a 61mm cone would have been reduced in diameter by 26mm, so when the cone gets to the nose of the binoviewer, it is 35mm in size.  Now since the opening in the binoviewer is only 27mm, this means that the nose of the binoviewer now becomes the limiting size for the cone and the off axis rays are just shaved off and the cone starts as 27mm in size from this point to the focal plane.  (You have to use the inside diameter of the 1.25" nose because this is what the light cone passes through)

Let's say that the nose of the BV is 30mm long and the light path is 110mm.  This means that the starting 27mm cone will be reduced in size by 1mm for every 8.6mm of path, so the cone will be reduced in size by 16.3mm (I am rounding) so when it arrives at the focal plane, it will have reduced in diameter from 27mm to (27 - 16.3 = ) 10.7mm.

It is important to remember that 10mm is the typical figure for the fully illuminated field of most reflectors and many standard SCTs, and at f8.6 with a vignetting source that is 140mm ahead of the focal plane, you would not actually see the vignetting.

So, don't panic.  In the telescope world, a fully illuminated field of 10mm is very common.

Now if it was of big concern to you, you could perhaps improve the situation by going to a 2" nose on the binoviewer, but now the front aperture of the binoviewer is going to be your limit. Let's say the front aperture is 25mm and the light path is 110mm.  If an f/8.6 light cone enters the BV, it will travel 110mm, and in that distance it will be reduced by 12.79mm in size so it will hit the focal plane 12.21mm in diameter, so you would not gain all that much by going to a 2" nose because at this point, the binoviewer itself becomes the limit, and an increase of 1.5mm in fully illuminated circle size is not going to be something you would see.

Now in this case, the exact position of the nose of the diagonal is of little concern because the moving it 10mm or 20mm to the rear is not going to change things due to the fact that the fully cone would still be to big to fit into the nose of the BV and in the end, you are stuck with the front aperture of the Binoviewer which sets the absolute limit on the fully illuminated  field size to 12.21mm.  The only ways to make the fully illuminate field size bigger would be to use a Barlow or other amplifier to shorten the light path, or use a scope with a slower focal ratio.

In your case though, it is the 1.25" nose that is your choke point and at f/8.6, there would be little benefit to going to a T2 diagonal or anything else, because the difference between the configuration with the nose and without the nose is only going to be 1.5mm and 10.7mm is already a very generous size fully illuminated field for a binoviewer.  If you were using a Televue 101 and a big prism binoviewer, then the advantage of T2 diagonal with direct connection would be much more important.   For you though it would not be worth it from a fully illuminated field perspective.

Edited by Eddgie, 24 October 2019 - 09:24 AM.

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### #13 Eddgie

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Posted 24 October 2019 - 09:32 AM

And I missed that your front aperture was 24mm, but in this case, it is the 27mm opening in the nose that seems to be the restriction point at f/8.6. That is why you have to work down from the focuser end.  If you come to a point where the there is a restriction smaller than the light cone, you use that new diameter from that point until you get to the next restriction, or the focal plane.

But your answer was pretty close and at this point we are splitting hairs, but again, the post was to illustrate how to analyze where different choke points might be and my numbers were not from accurate measurements, so in the end, your figure might be closer than mine, but we know that the front aperture in the binoviewer will almost always set the absolute limit when not using any kind of amplifier.

Edited by Eddgie, 24 October 2019 - 09:35 AM.

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### #14 vineyard

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Posted 24 October 2019 - 02:26 PM

Wow that is really helpful.  I would not have appreciated that Barlowing would add more value than trying a 2" binoviewer nose, so thank you for that (that probably also shows how little I understand how Barlows work!).  Would it be better to use a 2" barlow before the high-hat, or a 1.25" barlow after it (or would it not make any difference)?

I'm going to start taking telescope making classes so it will be fun wrapping my head around light paths & light properties - I'm sure I will need a wet towel on head many a time!  Saw Foucault's knife test in practice yesterday - v cool (esp seeing the heat waves radiating off fingers in front of the mirror).

Anyway thank you again - v helpful.  I think I will pull the trigger on a binoviewer soon...gulp.

### #15 Eddgie

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Posted 24 October 2019 - 03:32 PM

A Barlow has the effect of shortening the light path through the binoviewer but that also means that you now have a much smaller true field.  It also makes you move the focuser out more, so the light cone is probably not being clipped by the front of the binoviewer nose. You do get a bigger illumined field, but 10.7 is already pretty big and there is little value in making it bigger just for the sake of making it bigger.

10.7mm Is really more than enough field for planetary observing.  Suppose you are using a pair of 5mm Naglers for planetary observing (and I just picked this represent an extreme example). In this case, the field stop is 7mm, so you could never get the planet outside of the fully illuminated field of the eyepiece using focal length eyepieces you would typically use.

Now in the case of my 106LE, it is f/6.5, and my fully illuminated field is only about 4mm (with no Barlow).  For me then, I would want to avoid letting the planet get outside of the 4mm circle but when you think about it, that is not likely to happen.. Suppose I use a Nagler zoom at the 3.5mm setting.  The field stop would be about 4mm, so here, the planet would still be in the fully illuminated circle even though it is only 4mm.

So, this is why 10mm is the pretty standard fully illuminated field size on most visual only reflectors.  It is big enough to allow a planet to drift across the field of a high power eyepiece without going into the vignetted area.

Now I recommend Barlows for planets because it will reduce the eye strain that can come from tiny amounts of miscollimatoin in the binoviewer and in SCT because of the improved mirror spacing and in your case, because it would allow you to get by with longer focal lenght eyepieces that generally have better eye relief. The field stops are much bigger, but so is the fully illuminated circle.   So you would probably use a Barlow anyway for planets, but for deep sky, I think most would prefer the much wider field they can get by running without a Barlow (if they can that is.).

Edited by Eddgie, 24 October 2019 - 03:43 PM.

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