First trip with the 12"

[Paul73]

Re: the brighter issue. 

Pure semantics chaps. Things LOOK brighter in my big scope compared to my little scope. This allows me to view in at higher magnification (surprisingly high mags). As an observer, how it appears is important.

Paul

PS. How come the stars look brighter. They have no discernible scale.

 

[estwing]

30 minutes ago, Littleguy80 said:

I once started a thread asking what people’s ideal scope would be. @estwing responded “any scope under dark skies”. That really stuck with me.  

Glad to be of help....get out there and enjoy!

[furrysocks2]

My understanding is "exit pupil = aperture / magnification", so for a given scope of fixed aperture, "exit pupil" implies "magnification" and vice-versa. "Larger aperture simply allows more magnification for a given exit pupil size" - I get that.

Why are the CN folk talking about exit pupil? Magnification compares across scopes in terms of field of view, but what's so important about exit pupil? I don't understand "aperture cannot increase the surface brightness of the object, as the surface brightness is determined by the exit pupil". For a given exit pupil size, aperture/magnification can move around provided the equation above is satisfied, so is there something more intrinsic about exit pupil that makes it something to measure performance by? Or does it nevertheless depend on the scope?

 

[Stu]

2 hours ago, furrysocks2 said:

My understanding is "exit pupil = aperture / magnification", so for a given scope of fixed aperture, "exit pupil" implies "magnification" and vice-versa. "Larger aperture simply allows more magnification for a given exit pupil size" - I get that.

Why are the CN folk talking about exit pupil? Magnification compares across scopes in terms of field of view, but what's so important about exit pupil? I don't understand "aperture cannot increase the surface brightness of the object, as the surface brightness is determined by the exit pupil". For a given exit pupil size, aperture/magnification can move around provided the equation above is satisfied, so is there something more intrinsic about exit pupil that makes it something to measure performance by? Or does it nevertheless depend on the scope?

 

You are totally correct of course, for the same aperture of scope, exit pupil and Magnification are absolutely linked, but Magnification itself does not explain the effects of what you see. At very small exit pupils, the views become dim and prone to floaters if you suffer from them. Very large EPs (>7mm) and you are effectively wasting light because it overfills a fully dilated pupil, whilst in between there are varying uses. Around 2mm is excellent for showing maximum contrast on deep sky objects unfiltered, whilst 3 to 5mm can be excellent for filtered, specifically narrowband or line filters like OIII or Hb.

If I want to view a small faint galaxy at x200 in my 4" frac, I end up with an exit pupil of 0.5mm. The result is a faint (sorry, invisible) object as the light has been spread over too large an area and the contrast will not be detectable.

If Steve (Swampthing) views the same Galaxy in his 20" dob at x200, the exit pupil is 2.5mm which is pretty much slap bang on the optimum for contrast on DSOs. The benefit is the larger image scale whilst maintaining the surface brightness of the object. Larger objects are easier for our eye to detect contrast boundaries, hence the use of large dobs under dark skies for this kind of observing. At 2.5mm exit pupil, my 4" frac is only at x40 and the object is likely invisible still because although the surface brightness is the same as in the 20", it is way too small for my eye to detect it.

One other example I can think of where exit pupil trumps things like field of view are when observing with an OIII filter with a Mak. You may think that (in 1.25" format) a 24mm 68 degree or 32mm 50 degree Plossl would be the optimum low power, giving max field of view. But, given the long focal ratio of maks, say f12, the exit pupil with a 24mm eyepiece is 2mm, and with a 32mm it is 2.6mm. Using filters the view can still be a little dim, so moving to a 40mm 43 degree will up this to 3.3mm which will help brighten the view a little despite the same fov and a bit of a 'looking down a straw' effect.

Whilst surface brightness remains the same, my understanding is that the object appears brighter in a larger scope at the same exit pupil because that brightness is now maintained over a larger surface area, meaning more 'total brightness' reaching your eye. This is a point which causes some discussion but I believe the discussion is more around how it is explained rather than the point being wrong!

Hope that is of some relevance to your questions.

[furrysocks2]

Thanks for taking the time, Stu - yes very relevant.

There are a few new concepts in there for me, I guess mainly that there is an exit pupil for optimum contrast, also re: size, contrast boundaries and the eye's ability to pick things out - I expected that the smaller (angular size) object in comparison would be more easily seen, as it would "peak" rather than "patch" on the retina - the idea that "the light has been spread over too large an area and the contrast will not be detectable".

I guess much depends on how much background light there is relative to an object, hence "dark skies".

I'll hopefully find some time over the coming cloud to understand exit pupils and the eye a bit more. I've given up my 4" frac, so can no longer compare to that.

 

[furrysocks2]

There looks to be an explanation of the various factors here - https://www.astronomics.com/eyepiece-exit-pupils_t.aspx.

And of course, much fodder at http://www.telescope-optics.net/.

[Stu]

No problem. It is a highly complex subject area, which I only scratch the surface of. Your eye's response to contrast depends upon a number of factors and the answers are not always intuitive  

I have referenced a number of sites to help me understand it. Mel Bartels is a bit of a guru when it comes to this, so reading up on some of his stuff will help. I've linked to a number of useful calculators but there are many more interesting articles on his site.

http://www.users.on.net/~dbenn/ECMAScript/surface_brightness.html

http://unihedron.com/projects/darksky/NELM2BCalc.html

http://www.bbastrodesigns.com/tm.html

http://www.bbastrodesigns.com/VisualDetectionCalculator.htm

http://www.bbastrodesigns.com/apertureCalc.html

http://www.bbastrodesigns.com/Calculators.html