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Transmission in 90° / 45° BT vs. straight through Binos

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#1 Andeas72202

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Posted 04 December 2020 - 05:56 AM

Hi all,

 

can anyone tell, how big the difference in light transmission between 90 / 45° BTs is compared to straight through binos? Would be interesting to know, how much the added glasspath and reflections in BTs change the light gathering capabilities of those instruments. Let´s assume, that we only take prism-BTs into account, not mirror based (like those with EMS). 

 

Does anyone have experiences or even data?

 

Regards

Andreas



#2 Allan Wade

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Posted 04 December 2020 - 06:03 AM

Not the answer you are looking for, but. I've done 6 hour sessions with my APM 120-90 degree binoscope without issue. I briefly owned a pair of 15x70 straight through binoculars on a parallelogram. They were simultaneously a pain in the neck and a PITA, so I sold them. Therefore I conclude from my experience that my 90 degree bino's saw much more light than my straight through bino's. 


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#3 TOMDEY

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Posted 04 December 2020 - 08:30 AM

Properly executed, it should make no difference. Modern premium-grade optical glass suffers very little, nearly negligible absorption, and modern premium-grade coatings same comment regarding absorption, scatter, R/T inefficiency losses. The operative term here is premium-grade. Allan's comment above is consistent with my assertion.

 

PS: I've designed, built, and executed spectral throughput measurements on telescopes, prisms, windows, eyepieces, Star Diagonals, and complete systems... and found that the most consistent predictor of throughput performance is... cost. The very best are almost all outstandingly superior; the mid-grades include great performers but with some clunkers; the economy models are most often deficient... sometimes terribly-so. You most often get what you pay for; no cheap lunch.   Tom

 

[Decades or more ago, this wasn't necessarily the case. Sophisticated MARC coatings were still in the developmental stages, only occasionally appearing in a few top premium gear. I was both a coating engineer/scientist and optical metrology specialist.]

 

Few related pictures showing measurement set-ups and eg results how such metrologies are done >>>

 

~click on~ >>>

Attached Thumbnails

  • 154 SLIDE 8 OF 32 Throughput paper.jpg
  • 153.1 eg eyepiece throughput from Tom Dey's nother white paper.jpg

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#4 TOMDEY

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Posted 04 December 2020 - 08:31 AM

couple more pics >>>    Tom

 

~click on~ >>>

Attached Thumbnails

  • 155 Star Diagonal Spectral Throughput Measurement 72.jpg
  • 152.1 eg star diagonal report page from Tom Dey's white paper.jpg

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#5 PEterW

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Posted 04 December 2020 - 02:47 PM

“Progressively smaller irises shear away ghosts”... ?!
Not using an integrating sphere on the output? Definitely good coatings. Pretty small estimate of uncertainty, is that at 95% confidence, is it purely from random effects or consider systematic effects. You probably got better data than Televue!

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#6 TOMDEY

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Posted 04 December 2020 - 04:38 PM

“Progressively smaller irises shear away ghosts”... ?!
Not using an integrating sphere on the output? Definitely good coatings. Pretty small estimate of uncertainty, is that at 95% confidence, is it purely from random effects or consider systematic effects. You probably got better data than Televue!

Peter

Hi, Peter!

 

Yep!

Specular-only throughput.

Yep!

Three sigma.

Systematics calibrated out and known references confirmed to be in agreement.

 

The full reports/white-papers have the description of the test sets and their calibrations in the (typically) first ten pages or so.    Tom, guru optique extraordinaire humilibus


Edited by TOMDEY, 04 December 2020 - 04:39 PM.


#7 PEterW

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Posted 04 December 2020 - 05:52 PM

Three sigma... 50% better than NIST, half way to 6-sigma ;-) Very impressive!

returnikg to the original question the only difference between theb45/80 degree is likely the angle ok one prism... you can get 45/90 degree amici prisms that are just different sizes, so a little bit more path length would be the only difference, so essentially the same transmission.

 

Peter



#8 TOMDEY

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Posted 05 December 2020 - 12:30 AM

Three sigma... 50% better than NIST, half way to 6-sigma ;-) Very impressive!

returnikg to the original question the only difference between theb45/80 degree is likely the angle ok one prism... you can get 45/90 degree amici prisms that are just different sizes, so a little bit more path length would be the only difference, so essentially the same transmission.

 

Peter

Ehhh... that Six-Sigma baloney that the Business 101 geniuses foisted on us a couple decades ago. What bunko babble-speak! I still can't believe that those morons actually believed what they were "teaching". Somehow we survived and still produced good stuff... mostly by ignoring them. I'll bet a lota guys here suffered through that malarkey back in the day.   Tom


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#9 Andeas72202

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Posted 07 December 2020 - 02:39 PM

Not the answer you are looking for, but. I've done 6 hour sessions with my APM 120-90 degree binoscope without issue. I briefly owned a pair of 15x70 straight through binoculars on a parallelogram. They were simultaneously a pain in the neck and a PITA, so I sold them. Therefore I conclude from my experience that my 90 degree bino's saw much more light than my straight through bino's. 

Alan, that's what I can fully agree upon. I have the same BT and it's just no comparison to straight throughs. I was just curious, whether transmisson suffers significantly from the added glass in the BTs. As we have some experienced Black Belts here... I am sure you have checked measurement system capability to be ok and suitable for the task (just kidding) bow.gif

 

All in all, I get the impression, that you all say, that multiple reflections with prisms do no harm to the overall transmission, right? 

 

Andreas



#10 Rich V.

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Posted 07 December 2020 - 03:17 PM

Though I've never seen actual data on transmission, I've never heard anyone say they have seen a noticeable difference between a four reflection 90° BT or a six reflection 45° BT vs. a four reflection Porro or six reflection roof straight-through bino.

The number of coated air to glass surfaces in the prism assys is similar between the bino designs as well.

Rich

#11 GamesForOne

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Posted 07 December 2020 - 04:23 PM

I briefly had both the 45° and 90° versions of the APM 100mm ED-APO binocular telescopes and I could not tell any difference in the transmission between the two in side-by-side tests under the night sky.

 

The main difference was viewing comfort. The 90° offers superior comfort above about 60 degree elevation viewing. However, the 45° is superior in comfort at viewing angles below that and especially so for horizon or near-horizon objects -- in my opinion.

 

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Michael Mc



#12 TOMDEY

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Posted 07 December 2020 - 04:34 PM

I briefly had both the 45° and 90° versions of the APM 100mm ED-APO binocular telescopes and I could not tell any difference in the transmission between the two in side-by-side tests under the night sky.

 

The main difference was viewing comfort. The 90° offers superior comfort above about 60 degree elevation viewing. However, the 45° is superior in comfort at viewing angles below that and especially so for horizon or near-horizon objects -- in my opinion.

 

---

Michael Mc

Exactly! I had and enjoyed both of those exact same binos. Found them to be functionally identical... but eventually sold off the 45o in favor of the 90o. The Manfrotto MK161-2 crank elevator tripod sure makes the ergonomics luxurious.    Tom



#13 GamesForOne

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Posted 07 December 2020 - 04:57 PM

Exactly! I had and enjoyed both of those exact same binos. Found them to be functionally identical... but eventually sold off the 45o in favor of the 90o. The Manfrotto MK161-2 crank elevator tripod sure makes the ergonomics luxurious.    Tom

Yes, I sold the 45's as well. Sometimes I regret it when I am searching for late evening comets, low planetary targets, or enjoying the Summer Milky Way at southern USA latitudes, however, I could not justify keeping both.

 

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#14 sonny.barile

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Posted 07 December 2020 - 07:06 PM

Ehhh... that Six-Sigma baloney that the Business 101 geniuses foisted on us a couple decades ago. What bunko babble-speak! I still can't believe that those morons actually believed what they were "teaching". Somehow we survived and still produced good stuff... mostly by ignoring them. I'll bet a lota guys here suffered through that malarkey back in the day.  

I remember those training courses.....sleepy.gif I was well rested.....


Edited by sonny.barile, 07 December 2020 - 07:19 PM.


#15 Mad Matt

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Posted 14 December 2020 - 05:26 AM

Internal reflection above the critical angle of the glass/air surfaces is 100% so the the number of prism reflecting surfaces does not have a noticeable effect on brightness. The number of transmitting glass/air surface has a much more substantial and potential noticeable impact. The 45° and 90° both have the same number of glass/air surfaces so there should be no noticeable difference.

 

Don't confuse the 45°-90° differences with Schmidt-Pechan to Abbe-König prism transmission differences. Schmidt-Pechan prisms have two additional glass/air surfaces which must both transmit and reflect simultaneously which requires very special coatings on those surfaces in order to reach the same transmission of an Abbe-König system.

The 45° and 90° APM's use large "non splitting" variations of Schmidt and Amici prisms respectively. 

It should be noted that every surface does introduce aberrations (or wavefront errors) which is were you may see differences.


Edited by Mad Matt, 14 December 2020 - 05:28 AM.

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