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John K
scholastic sledgehammer
Reged: 12/26/05
Posts: 799
Loc: Vernon BC Canada
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Is a exit pupil of 6mm in a 10 scope exhibiting the same amount of light that a 15" scope is doing at 6mm? Is the only difference magnification? 
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RASC Member (Okanagan)
Home built 15"F5 Obsession clone
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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Need to know the focal length of the scopes in question. Exit s calculated most easily by taking the focal length of the EP and dividing it by the f/# of the scope. Another way is to divide the aperture of the scope by the power with a given EP. Exit pupil determines the apparent surface brightness of the object. For example:
If the 10" is f/6, and the 15" is f/4.5, using a 12mm EP in the 10" will produce 127x and a 9mm EP in the 15" will produce 190x, while both will have a 2mm exit pupil. The object will have the same surface brightness, but the object will appear 1.5 time larger in the 15" scope.
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John K
scholastic sledgehammer
Reged: 12/26/05
Posts: 799
Loc: Vernon BC Canada
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That's kind of what I am asking.here is an example if I use a 32mm plossl in my 10 scope and then us a 31mm Nagler in the same scope would the nagler be the same exit pupil size and just more magnification and a larger field of view.
Another question If I where to use a O-III filter for looking at the Veil Neb would I be better off with a 31mm Nagler with a exit pupil of 6.2 or would I be better of with a 35mmPan with a exit pupil of 7mm?What would yield the best results?
I am asking because I finally have the cash to afford to buy one or the other but I am nervous about dropping that kind of coin and regretting it down the road.I would be buying a 31mm Nagler blind.I don't want to spend that much just find that it sits in my case and becomes my $600 Veil eyepiece
Thanks John
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RASC Member (Okanagan)
Home built 15"F5 Obsession clone
Bushnell 76mm Modified Dob (Little Richie)
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Astronomik filters OIII UHC
Televue Eyepieces 31mmNagler,32mmPL,15mmPL,13mm Ethos,11mm PL,2X Barlow
10mm Speers Waler
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MitchAlsup
super member
Reged: 08/31/09
Posts: 185
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Quote:
if I use a 32mm plossl in my 10 inch scope and then us a 31mm Nagler in the same scope would the nagler be the same exit pupil size and just more magnification and a larger field of view.
The magnification is essentially identical (~3%), the Nagler will have a bigger field to view.
Quote:
If I where to use a O-III filter for looking at the Veil Neb would I be better off with a 31mm Nagler with a exit pupil of 6.2 or would I be better of with a 35mmPan with a exit pupil of 7mm?What would yield the best results?
If you eye opens to 7mm the Pan might be ever so slightly better, if not the Nagler will slightly be better. But in reality, here, you are splitting hairs.
Quote:
I am asking because I finally have the cash to afford to buy one or the other but I am nervous about dropping that kind of coin and regretting it down the road.I would be buying a 31mm Nagler blind.I don't want to spend that much just find that it sits in my case and becomes my $600 Veil eyepiece
Trust me (and everyone else) you 31mm Nagler will not sit in your case. The only issue with buying your first Nagler is the damage that might occur to your bank account when you want more.
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Darenwh
Pooh-Bah
Reged: 05/11/06
Posts: 1224
Loc: Covington, GA
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Exit Pupil = Apperature in MM's divided by Magnification
So, if you have a 6mm Exit pupil in a 10" (254mm) scope then you have 254/6=42.3x magnification.
Ok, now lets look at the 15" scope. A 15" scope is 381mm. A 6mm exit pupil used with the 15" scope would have a magnification of: 381/6=63.5x magnification.
So, if both 10" and 15" scopes are giving a 6mm exit pupil then those two scopes are 42.3x and 63.5x magnification respectively.
What else can we tell from this? Well, if the 10" (254mm) scope is an F5 (1270mm fl) then the eyepiece used would be 30mm.
If the 15" (381mm) scope is also F5 (1905mm) then the eyepiece used would be 30mm also.
This is because to find the Focal Length Eyepiece to use for a given exit pupil you can take the desired exit pupil x the F ratio to get the eyepiece size.
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Daren
Covington, GA
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turtle86
professor emeritus
Reged: 10/09/06
Posts: 710
Loc: Red Sox Nation
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Very true!
Quote:
Trust me (and everyone else) you 31mm Nagler will not sit in your case. The only issue with buying your first Nagler is the damage that might occur to your bank account when you want more.
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1296
Loc: Ottawa, Ontario, Canada
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John,
It appears your original question may not have been fully addressed. Here goes...
No matter the scope's aperture, an exit pupil of given size delivers a basically equal image *surface* brightness for such things as the sky glow, a fuzzy nebula or any other non-point like object. (This presupposes similar efficiency in transmission between instruments.) And yes, the main difference is the magnification. But a secondary--and also important difference in astronomy--is stellar limiting magnitude. Because stars are essentially point sources (disregarding for the moment the diffraction effects dictated by the laws of optics, and optical aberrations as well), the larger aperture scope will reveal fainter stars, even when the overall image surface brightness is the same as seen in a smaller scope.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 733
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Quote:
John,
It appears your original question may not have been fully addressed. Here goes...
No matter the scope's aperture, an exit pupil of given size delivers a basically equal image *surface* brightness for such things as the sky glow, a fuzzy nebula or any other non-point like object. (This presupposes similar efficiency in transmission between instruments.) And yes, the main difference is the magnification. But a secondary--and also important difference in astronomy--is stellar limiting magnitude. Because stars are essentially point sources (disregarding for the moment the diffraction effects dictated by the laws of optics, and optical aberrations as well), the larger aperture scope will reveal fainter stars, even when the overall image surface brightness is the same as seen in a smaller scope.
Glenn since your knowledge of this stuff far exceeds mine I would like to cut to the chase with an extreme example. Suppose I have a five inch scope and a fifteen inch scope pointed at the sun, and each has a heat proof eyepiece yielding 6mm exit pupil.
If I hold matches at the focal plane will the five inch scope light the match after or at the same time as the fifteen inch scope?
It seems to me this is a brutal test of energy throughput.
Since it is in each case a question of gathering energy, merely of one being close and the other at a distance, I then would like to know, how the ability to perceive an object relates to crude energy throughput.
In other words. One could say that a four inch scope will show a barely detectable 11th mag galaxy with the same brightness as a fourteen inch scope will show a 15th mag galaxy. But in the four inch scope the 15th mag galaxy is invisible.
Perhaps I'm not being clear, but what I'm getting at is, you haven't cleared up (to me) at any rate the relationship of brightness, exit pupil, and visibility of faint objects. It's something I'd like to know more about, sort of walking through the steps.
It does seem to me possible that my 4 inch refractor with a 10mm (66x) is giving the same surface brighness as my C14 (390x) but where I begin to wonder is if we go back to the sun example, seems to me the big guy would cook the eyeball faster. And that therefore there is more energy coming though the C14 at 390x than the 102mm at 66x.
...if that makes sense.
Greg N
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John K
scholastic sledgehammer
Reged: 12/26/05
Posts: 799
Loc: Vernon BC Canada
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So A four inch scope @60x on a galaxy(You Choose) and a fifteen inch scope @60x on the same galaxy,The fifteen will be brighter because of the exit pupil.but lowering the magnification in the four inch scope to achieve the same exit pupil there will be no difference in surface brightness just loss of detail due to magnification. Correct? 
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RASC Member (Okanagan)
Home built 15"F5 Obsession clone
Bushnell 76mm Modified Dob (Little Richie)
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Astronomik filters OIII UHC
Televue Eyepieces 31mmNagler,32mmPL,15mmPL,13mm Ethos,11mm PL,2X Barlow
10mm Speers Waler
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