7 replies to this topic

[HenkSB]

I own Celestron 20x80 SkyMasters and WO binoviewers.  I enjoy them both very much.  I am fantasizing about buying 25 x 100 ED binos someday.  However, I find it difficult to judge the quality. 

 

Specifically, I wonder how much light gets lost along the way by obstructions such as prisms and binoviewer light paths.  Is there a mathematical model, preferably common for binoviewers and binoculars, that explain this? Is there a common blockage model for prisms and clear aperture? Otherwise, how can I tell that when I buy a pair of 90 degrees 25x100 ED binoculars that cost $1K to $2K, the illumination is not getting overly obstructed by poor choice of prisms or mechanical construction?  Or if I bought the wrong kind of binoviewers?  The WOs are fine for the price but don't use 30 mm eyepieces, as I found out.  I would like to be able to calculate these losses and limitations myself.  Wikipedia also doesn't discuss the type of model that I am looking for.

 

I have spent some hours reading the "Best of..." thread and found interesting things like "exit pupil illumination" but I can't find any exit pupil illumination specs on vendor's pages.  When I compare Celestron SkyMasters 20x80 and SkyMasters 20x80 Pro ED pages, the specs are hardly different - well maybe the coatings and diopter tolerance, but nothing related to the ED part.  Based on those specs the regular 20x80 have the best value but maybe the 20x80 Pro ED have the better value if more information was displayed.  The "The best of..." thread is not bad but the number of links is overwhelming, and sometimes it ends in discussion threads that are not always to the point, so it is difficult to find the specific things that I am looking for.

 

[sevenofnine]

You might start here:...and then call Kevin at Oberwerks for more technical information. He loves to talk about binoculars  

 

https://oberwerk.com...ce-information/.

[HenkSB]

You might start here:...and then call Kevin at Oberwerks for more technical information. He loves to talk about binoculars  

https://oberwerk.com...ce-information/.

Nice website.  It convinced me to look for 45 degrees not 90 degrees.  Their tripods look nice but are pricey.  I hope my Tiltall that I got for $35 at a swap meet can handle it, it has a 40 lbs. capacity and should be tall enough with 45 degree binos, even for me at 6'2".

[sevenofnine]

The Tiltall tripod legs might work but in forum reports a proper mount for heavy binocular telescopes often costs as much as the binos. Sometimes more   The legs have to be very solid to prevent vibrations and the head has to have a strong damping system or the binos will sag in operation. The head must also pan very smoothly too or it will be a pain to use. BT's in the 100mm class have a lot of caveats. Good luck!   

[TOMDEY]

The total integrated throughput of an optical viewing system is (to very good accuracy) the product of its axial transmittance and {unity minus vignetting losses}. Most people ignore/discount that second affective, provided that the center field luminance is decently preserved (90% typically deemed OK). This may be a mistake, especially for ~cheap~ binos.

 

"how much light gets lost along the way by obstructions such as prisms and binoviewer light paths.  Is there a mathematical model, preferably common for binoviewers and binoculars, that explain this?"

 

Yes, most lens design programs do this computation with a drop-down selection. But that's most always on the lens designer's side of the fence, not the customer. An alternative (more reliable) way to assess is to actually measure the axial spectral luminance throughput. I did this on lots of optical components and systems, but had a metrology lab at my disposal to build the test sets and take measurements. You can also do a vignetting check (as a function of field)... but same comment regarding quantitative metrology --- need lab equipment etc.

 

But you can (if you really are obsessing over the numbers) sort of casually use a good camera, a "Lambertian light box" and some RGB filters to compare the spectral luminance of the system exit pupil with that of the naked light box. The ratio is the sought axial throughput. This is assured by Emmy Noether's Theorem, also called the Lagrange Invariant. Similar comment regarding checking for vignetting --- use the light box while examining the exit pupil as you tilt the instrument to field angles from zero to max. Also confirm that the axial exit pupil diameter is the stated aperture divided by the stated magnification. (e.g. a 25x100 bino should have a 4.0mm diam exit pupil... not smaller.)

 

It does rather surprise me that manufacturers rarely provide this information to the customers. The reasons are obvious. And if you ask... at best you are most likely going to get a wishy-washy arm-waiving statement that it's ~very good~.

 

Couple related images from my archives >>>

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[John A Roberts]

Hi Tom,

 

While the third image is an excellent illustration of the concept of IPD, in terms of measuring IPD . . .

 

It's often more convenient to just measure from the left outside edge of the left eyecup, to the left outside edge of the right one.

It is less fiddly than having to position the zero point of a rule in the exact centre of one exit pupil,

and then working out which marking is closest to the centre of the other EP.

 

 

Also when measuring to a millimetre or less, the markings on a plastic rule often make it less precise in use compared to a steel rule.
With plastic ones the unit markings are often thicker. And plastic rules also typically lack the convenience of a section of 1/2 mm markings at the start.
(Also with two examples that I have at hand: over the distance 10 cm, one measures over 1 mm longer, and the other 1/2 mm longer.)

 

 

John

Edited by John A Roberts, 21 June 2025 - 08:59 PM.

[Rich V.]

Nice website.  It convinced me to look for 45 degrees not 90 degrees.  Their tripods look nice but are pricey.  I hope my Tiltall that I got for $35 at a swap meet can handle it, it has a 40 lbs. capacity and should be tall enough with 45 degree binos, even for me at 6'2".

While your Tiltall tripod's legs may handle the weight of a 100mm BT or big binos, that tripod's head will likely be very awkward to use with them.  It is a simple 3-way photo head best used with still cameras as it offers no resistance to movement. Unlock, move and re-lock, but it will be a pain for making smooth, fluid motions with a bino. You don't want your binos flopping around out of control.  I used a Tiltall tripod with 4x5 film view cameras years ago; they worked OK together for that purpose. I wouldn't use it with a BT, though.

 

The better video heads have viscous drag that can be applied to the alt and az axes to keep the BT under control.  They have an added counterbalance feature keeps the BT from tipping backward under its weight as altitude is increased.  As mentioned above in another post, the cost of a good geared center column tripod and video head can be substantial, but you could find a good used sample for much less if you shop around.  A BT supported on a shaky, hard to control tripod/head could be a "hobby killer" and I wouldn't recommend it.

 

The Oberwerk website has videos of this equipment in action to help you understand what you're up against.  

[TOMDEY]

Hi Tom,

While the third image is an excellent illustration of the concept of IPD, in terms of measuring IPD . . .

It's often more convenient to just measure from the left outside edge of the left eyecup, to the left outside edge of the right one.

It is less fiddly than having to position the zero point of a rule in the exact centre of one exit pupil,

and then working out which marking is closest to the centre of the other EP.

Also when measuring to a millimetre or less, the markings on a plastic rule often make it less precise in use compared to a steel rule.

With plastic ones the unit markings are often thicker. And plastic rules also typically lack the convenience of a section of 1/2 mm markings at the start.
(Also with two examples that I have at hand: over the distance 10 cm, one measures over 1 mm longer, and the other 1/2 mm longer.)

John

Yep to those various hints... I used the clear ruler so that all the concepts show up in the one image... and also to capture that the luminance of the exit pupil to that of the light box is the axial throughput, provided the box is both uniform and Lambertian. [My work was building and certifying imaging satellites. We typically measured things to microns or better.]    Tom