RE: ATM Performance with Spherical Aberration

["Arjan te Marvelde (EMN)" <Arjan.te.Marvelde@emn.ericsson.se>]

Dominic,

The exit pupil is proportional to the primary size, the magnification being equal.
I would say that it can be compared to a diafragm, in other words, the light yield would 
be proportional to the square of the pupil size (or: of the primary size). As you 
probably know there is a usuful maximum to the exit pupil size, which is when it reaches 
that of the eye pupil (~6 or 7 mm). If it becomes larger, the eye pupil will block the 
surplus light.

Within the useful range and for a specific scope however, contrast of point- against an 
extended source will benefit from larger magnification (or: smaller exit pupil). This is 
because the magnified point is still the same point, but the extended object (like light 
pollution, nebulosity) is 'stretched' by the magnification. 

: by some experts to be severely affected by 
: exit pupil size. However, I thought it 
: would go the other way around and an exit 
: pupil around 3 mm vs 1.5 would favor a 
: brighter image in the 36" scope. Perhaps 
: someone knows which of 3 or 1.5 mm exit pupils 
: is superior for the present object (M13)?

So, to get back to your question, i'd say that the smaller exit pupil yields higher 
contrast with the same mirror, but the larger mirror should still give more light with 
the same exit pupil. So i'd expect that the bigger scope should give higher brightness 
with comparable contrast in the questioned case .

I'm not an expert though, so maybe i'm wrong. I'd sure like to hear the truth about this 
then...

/Arjan