Erik D
Post Laureate
Reged: 04/28/03
Posts: 3196
Loc: Central New Jersey, USA
|
|
Quote:
Maybe we should insist on binoculars being advertised stating effective aperture.
I would like that. But,I am not sure how to go about listing the "EXACT" effective aperture unless we can first agree on a test methodology for the American Telescope Inspection & Testing Institute.
Let's take the BT 100 /45 deg and straight models discussed in this thread. I recall seeing figures ranging from ~ 91 mm to 95 mm depending on the configuration, EP and the person doing the measurement. Mr. Bill's own measurement indicates 93-95 mm. Would it be deceiving if the importer changes the figure to 94 mm but another member measures 91 mm?
We'll have to endow a fund to clone EdZ first.... ;-))
ERik D
|
Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3149
Loc: Just passing through.....
|
|
Notice signature change to reflect reality....
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
|
Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3149
Loc: Just passing through.....
|
|
I nominate edz for Binocular Czar....in charge of honest aperture numbers.
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
|
|
You know, actually we have documented in these forums at least five different methods of measuring the effective aperture. Only two methods have been proposed by me.
Measuring exit pupil has been around for decades (although first you must measure magnification to make that one reliable),; reading a scale by using a loupe over the eyepieces was proposed here I think originally by Henry Link (or perhaps Kimmo Absetz), and this easy method proposed here by Glenn (although we see care needs to be exercised for all three methods).
The other two methods are using a crosshairs target laser over the objectives as it is moved over a window, measuring extent across the window, and measuring changing exit pupil thru use of repetatively smaller objective masks (this has the advantage of also giving a close estimate of magnification).
The best advice is to confirm measurements obtained by one method with a result obtained by a second method. That's what I do. In fact, I compare results obtained by three methods, so there is really no question.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
|
|
Quote:
Glenn,
I have just taken apart a 20x60 for cleaning and guess what I found? Large prisms mated to smaller prisms on each side. The large one is 1 1/16" wide and the small one is 3/4" wide. The shelf has 2 small apertures for the small prism and 1 large aperture for the entrance to the large prism.
I guess this isn't as unusual as I thought.
I have the same experience.
I have taken apart a Meade/Bresser 10x50 (20 euros Lidl binoculars), and to my surprise the prisms near the objective are 2 or 3 mm wider than the prisms near the eyepiece.
So it is probably a very common design. Naturally, that shortens the total optical path inside the prisms.
Jean-Charles
|
Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3149
Loc: Just passing through.....
|
|
Quote:
You know, actually we have documented in these forums at least five different methods of measuring the effective aperture. Only two methods have been proposed by me.
edz
Never thought much about differences in effective aperture and stated aperture until this thread. I always naively assumed that they were pretty much the same thing.
Frankly, I don't spend a lot of time on this stuff except when an obvious problem shows up.
A six percent difference in diameter works out to about 12% difference in light gathering which IMO, is significent.
Also, the nagging feeling that I've been had. I hope my arriving (tomorrow) TMB is an honest 130mm clear aperture when you consider it costs (new) $800 an inch!!
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
Never thought much about differences in effective aperture and stated aperture until this thread.
Hmmm. I've been publishing the effective aperture of binoculars that I've tested perhaps for about 5-6 years now. There are whole detailed threads, some with picture essays, and "Best Of" links pointing to all the methods of measuring effective aperture, including this one.
Perhaps you remember this thread from 2 years ago.
Small binoculars - Exit Pupil & Effectve Aperture
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3149
Loc: Just passing through.....
|
|
Quote:
Quote:
Never thought much about differences in effective aperture and stated aperture until this thread.
Hmmm. I've been publishing the effective aperture of binoculars that I've tested perhaps for about 5-6 years now. There are whole detailed threads, some with picture essays, and "Best Of" links pointing to all the methods of measuring effective aperture, including this one.
edz
Got lots of other stuff to read.... 
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
|
Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3469
Loc: Cambridge, MA, USA
|
|
Quote:
A six percent difference in diameter works out to about 12% difference in light gathering which IMO, is significant.
Barely. Yes, you could detect the difference if you looked really carefully, but you'd never notice it if you weren't looking for it.
Here's my rule of thumb, based on experiments with masks:
- 5% increase in aperture -- barely noticeable, outweighed by considerations of absorption, scatter, etc.
- 10% increase -- readily visible comparing two otherwise identical instruments side by side
- 20% increase -- quite significant, qualitative change in views over a significant range of targets
- 50% increase -- very different, a whole new world
Still, significant or not, it's always reassuring when a vendor or manufacturer gives accurate specifications.
OTOH, vignetting from the focuser is a very common problem in small, fast refractors. In this case, the vignetting will vary depending on where the eyepiece's focal plane lies and how nearsighted the observer is. That makes it hard to give an accurate specification!
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
|
Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3149
Loc: Just passing through.....
|
|
Quote:
Quote:
A six percent difference in diameter works out to about 12% difference in light gathering which IMO, is significant.
Barely. Yes, you could detect the difference if you looked really carefully, but you'd never notice it if you weren't looking for it.
Here's my rule of thumb, based on experiments with masks:
- 5% increase in aperture -- barely noticeable, outweighed by considerations of absorption, scatter, etc.
- 10% increase -- readily visible comparing two otherwise identical instruments side by side
- 20% increase -- quite significant, qualitative change in views over a significant range of targets
- 50% increase -- very different, a whole new world
Still, significant or not, it's always reassuring when a vendor or manufacturer gives accurate specifications.
OTOH, vignetting from the focuser is a very common problem in small, fast refractors. In this case, the vignetting will vary depending on where the eyepiece's focal plane lies and how nearsighted the observer is. That makes it hard to give an accurate specification!
Caught that one on my Antares 6 inch f/6.5 (did thread and review of the problem) http://www.cloudynights.com/item.php?item_id=1274 Raytracing confirmed the problem; hacksaw solved it.
Still, I expected more out of a $1500 binocular sold by a reputable dealer.....lessons learned.
Every time I think I'm too cynical, reality wacks a 2x4 upside my head.
The Who...."Won't get fooled again" (and again and again, etc. etc.)
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
|
beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 509
|
|
GLD,
What do you think is my true apature for the GBT 100/45 with the info that I provided?
I know that Rich V. took a stab at it, however, he was hoping that either you or EDZ would add your comments as well.
Thanks,
Beachchairbill
|
GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
|
|
BcB,
The figures you supplied covered a range of values--for each barrel, and I wasn't sure what was the source of variation. Did you indicate that for a particular barrel the entrance pupil is out-of-round? Other than that, I'd have to assume your measurements were as accurate as you could obtain, and have no reason to doubt them.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 509
|
|
Glen,
The figures that I supplied were from the right barrel. This happens to be the undamaged barrel. Also, the out of round exists for both barrels and it only happens when you hold the flash light at 8" away and 16" away.
Beachchairbill
|
GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
|
|
Celestron 25X100, purchased in 2004: 100mm! The circle is perfectly round and sharp-edged.
This photo shows why.
In a nutshell, it's because of the stepped prism which first fields the light cone from the fast, approx. f/3.5 objective.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 509
|
|
Could it be that the 25x100 are fixed ep's and the GBT's are interchangeable?
BB
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
Celestron 25X100, purchased in 2004: 100mm! The circle is perfectly round and sharp-edged.
This photo shows why.
In a nutshell, it's because of the stepped prism which first fields the light cone from the fast, approx. f/3.5 objective.
Interesting that yours are full aperture. Two others of us recorded the Celeston Skymasters at 90-92mm aperture. I wonder if they changed somewhere along the line?
edz
(I see you have the same question in your mind as well)
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 971
|
|
The entry and the exit prisms of the Zeiss 7 x 50B/7 x 50BGA are tapered, and of different sizes. I believe that is the way several other postwar Zeiss West Porro prism binoculars were/are made. The prewar 8 x 40 Deltar/Deltarem wide angle, with asphere on one eyepiece surface, also used tapered prisms. The Carl Zeiss Jena 8 x 50B and the 12 x 50B(?),from about the 1980's, used tapered Porro prisms. I have been into my Deltar, but do not recall if the prisms are of different size. I have not been into a DDR Carl Zeiss Jena 8 x 50B. The specimen which I examined was impressive. I am sorry that I did not buy one.
|
RichD
professor emeritus
Reged: 11/08/07
Posts: 567
Loc: Derbyshire, UK
|
|
That 25x100 of Glenn's seems to be of pretty good internal construction quality.
--------------------
Clear skies
Rich
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
Let's take the BT 100 /45 deg and straight models discussed in this thread. I recall seeing figures ranging from ~ 91 mm to 95 mm depending on the configuration, EP and the person doing the measurement. Mr. Bill's own measurement indicates 93-95 mm.
The second half of the equation, and the implications, not really discussed here in detail, is the cause of the aperture reduction.
In some cases it is a too small prism. Not only does that reduce aperture, but also, if the prism aperture gets to be considerably smaller than the eyepiece field stop, reduces the illumination of the field.
However, it may be a baffle and the prism might not be small at all. While the baffle will reduce the aperture, if the prism is still quite large it may provide excellent illumination to the eyepiece.
The Oberwerk BT100 and the BT100/45 have 32mm prisms. The prism aperture is actually larger than any possible field stop in any 1.25" eyepiece that can be used in the binocular. About 3-4 years ago I posted some illumination profiles for various eyepieces used in the BT100. There are very few binoculars that match that degree of illumination.
For example, the TV 14mm Radian, the Orion 17mm Sirius plossl and the TV 20mm plossl are all still illuminated 100% from every point approx 60-70% out on BT100 aperture. It varies a bit with each eyepiece. Just as significant, both the Radian and the Sirius plossl are illuminated 60% even from points 90% out on the aperture. The 20mmTV 50%.
Likewise exact same type of illumination measurments taken on the Celestron 25x100 show the eyepiece drops below 100% illumination from all points out beyond 25% of the aperture and is 50% illuminate from points 90% out on the aperture. Perhaps the tapered prisms, which result in a smaller prism aperture just ahead of the eyepiece field stop, cuts the illumination.
The upshot of this:
if you are going tto have to put up with a reduction in aperture, much better to have that occur due to a baffle rather than too small prisms. Too small prism aperture affects more than just the effective aperture.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3220
Loc: 51°N 4°E
|
|
Here are my measurements:
Swift Satellite 20x80 : 70 mm
Orion MiniGiant 15x63 : 57 mm (some prism clipping)
Docter Nobilem 15x60 : 60 mm
Swift Audubon 8,5x44 : 39 mm (some prism clipping)(this is the HR/5 model)
Vixen Apex Pro 8x42 : 40 mm
Nikon Action VII 7x35 : 32 mm (very slight prism clipping)
--------------------
Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD + Baader 90° T2 Amici
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
