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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 8431
Loc: Hanover, Ohio
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My 15X70's only go 1.2 magnitude deeper than my 8X42's? That's bizarre, kinda counter intuitive.
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Pete
6" Apogee/LXD55
Starhopper 6" Dob
Spaceprobe 130EQ
Black C8 OTA
WO Zenith Star 66 Patriot Edition
Sun Pak Pro 7500 Platinum Edition
8X42 Bushnell H2O Porro
7X35 Tasco
10X50 Nikon Actions (Type 7)
15X70 Skymasters
Dell Inspiron Dual Core 531s
"Science without Religion is lame, Religion without Science is blind" A.Einstein.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Based upon hundreds, if not thousands of observations, I developed this rule of thumb for determining the differences in Limiting Magnitude in Binoculars, referred to as BLM. This was originally documented in my 2004 article on BLM, and was further confirmed with several years more of similar tests
Each 10% increase in magnification will contribute a gain of 0.12 to 0.15 magnitude
Each 10% increase in area of aperture will contribute a gain of 0.03 to 0.05 magnitude
So in your example
My 15X70's only go 1.2 magnitude deeper than my 8X42's?
we would get:
magnification contributes 6.5x the mag factor = 6.5 x 0.12 = a gain of 0.8
aperture contributes 10.5x the aper factor = 10.5 x 0.03 = a gain of 0.3
for a total gain of 1.1 magnitudes
let's use the high end of the range
magnification contributes 6.5x the mag factor = 6.5 x 0.15 = a gain of 1.0
aperture contributes 10.5x the aper factor = 10.5 x 0.05 = a gain of 0.5
for a total gain of 1.5 magnitudes
The averages would put my calculations within 1/10th magnitude of what you have actually found.
Magnification has much greater influence on BLM at the low powers at which binoculars operate.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Andresin150
sage
Reged: 08/14/07
Posts: 305
Loc: Bogotá - La Calera / Colombia
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the numbers may not look as much, but 1.2 mag extra.... a lot more stars and objects to resolve....
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25x150 MT
30-50x120's
Ultra 15x70's
GOTO NEX Planetarium
1 Macaw, 1 Toucan, 1 parrot and many other little ones...
And the nicest greenhouse!
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3469
Loc: Cambridge, MA, USA
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Quote:
My 15X70's only go 1.2 magnitude deeper than my 8X42's? That's bizarre, kinda counter intuitive.
Like Ed, I find this matches my intuition very well.
The two binoculars have fairly similar exit pupils -- 4.7 mm for the 15x70s compared to 5.3 for the 8x42s. So to a first approximation, they will scale pretty well going either by magnification or by aperture.
Aperture is simpler. The areas of the objectives are in the ratio 70/42 squared, or approximately 2.78. So if the exit pupils were identical, the faintest star visible in the smaller binos would be 2.78 times brighter than the faintest star visible in the bigger ones.
In magnitudes, that's 2.5*log(2.78) = 1.1 magnitude -- somewhat *less* than you've observed.
But if you go by magnification, the same reasoning gives you a ratio of 15/8 squared, or 3.52, which equals 1.4 magnitude.
Your own result is neatly bracketed between 1.1 and 1.4. As Ed says, at typical binocular exit pupils, limiting stellar magnitude depends more on magnification than on aperture, so I would expect closer to 1.3 magnitude rather than 1.2. But that's almost certainly within experimental error. Moreover, your 70-mm binos may well have smaller than advertised aperture.
1.2 magnitudes is a lot! That almost quadruples the number of stars you can see.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 990
Loc: South of the Mason-Dixon Line
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It's not funny, it's  !!
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Tools that make objects very far away appear much closer than they actually are.
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KennyJ
Reged: 04/27/03
Posts: 12909
Loc: Lancashire UK
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I'll be surprised if the 15 x 70 involved really does have a clear aperture of 70mm.
I don't know which 8 x 42 model Pete was refering to .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I'll be surprised if the 15 x 70 involved really does have a clear aperture of 70mm.
Kenny
the Skymasters?
expecting that they are the typical 63mm that we often see, that would reduce the aperture calc by only 0.06 to 0.1 magnitude. So, rather than 1.1 to 1.5, for a 15x63 it would be 1.04 to 1.4
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 8431
Loc: Hanover, Ohio
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When I ran the calcs, I ran the Bushnell as 40mm.
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Pete
6" Apogee/LXD55
Starhopper 6" Dob
Spaceprobe 130EQ
Black C8 OTA
WO Zenith Star 66 Patriot Edition
Sun Pak Pro 7500 Platinum Edition
8X42 Bushnell H2O Porro
7X35 Tasco
10X50 Nikon Actions (Type 7)
15X70 Skymasters
Dell Inspiron Dual Core 531s
"Science without Religion is lame, Religion without Science is blind" A.Einstein.
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KennyJ
Reged: 04/27/03
Posts: 12909
Loc: Lancashire UK
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Hi Pete ,
The only Bushnell on you sig. list is a 25mm .
I know specs can be innacurate , but a stated 25mm showing up as a true 40mm would be a first for sure :-)
Actually -- I noticed your mention of the 42mm H20 in the Bushnell thread !
Kenny
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 8431
Loc: Hanover, Ohio
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I didleave'em out!! I need to edit my sig.
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Pete
6" Apogee/LXD55
Starhopper 6" Dob
Spaceprobe 130EQ
Black C8 OTA
WO Zenith Star 66 Patriot Edition
Sun Pak Pro 7500 Platinum Edition
8X42 Bushnell H2O Porro
7X35 Tasco
10X50 Nikon Actions (Type 7)
15X70 Skymasters
Dell Inspiron Dual Core 531s
"Science without Religion is lame, Religion without Science is blind" A.Einstein.
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