Re: [ATM] Betr.: 48" Dob at OMI

[Alexis Cousein <al@sgi.com>]

Scott Ewart wrote:
> 	Ok, call me dense, call me stubborn.  I just don't get it.  50x was
> a bad example.  Let's say 200x.

 > Put two scopes side by side, and 8" and a
> 48", both at 200x, both with the same true field.  Both are easily capable
> of delivering all the light into the exit pupil and thus the eye, with a
> generous AFV.

My original point was that not all objects can be viewed in their entirety
at that magnification - certainly the whole of M42 cannot. That's why I
said that M51 would have been a better example.

But in the interest of understanding, let's look at the (changed) problem.

For a 7mm exit pupil at 200x, the aperture cannot be larger than 1400mm, so
you're OK. If your eye dilates that far, because if your pupil only dilates
to 5mm, 200x is not enough.

Yes, at *that* constant magnification the larger scope gives you a brighter
image, and this is why we have them. Keeping the magnification constant,
the scope with an exit pupil closest to 7mm will deliver the brightest
image, though not necessarily that with the largest contrast perception
(in super dark skies and for the smaller contrast deatures visible at
that magnification, yes, but for larger features in a light polluted sky,
it's possible that a 3mm or 2mm exit pupil gives you a better view - that's
a *VERY* complex subject).

QED: the larger scope doesn't let us look better at an object because the
surface brightness is larger in an absolute sense, but because we can view
an extended object with something close enough to the naked eye surface
brightness *at a much larger image scale*, which renders contrast features
more visible.

In a smaller scope, you have to trade off magnification and surface
brightness: in a 8", if you have a feature with low contrast against
the background, you'd need to make it bigger to see it but that *would*
decrease surface brightness; and it would then possibly no longer be
visible at 200x with that surface brightness (at that surface
brightness you could probably make it visible again at even higher
magnification, but sadly a higher magnification would mean even
less surface brightness in that 8" scope, etc.; some contrast
features are thus simply out of reach of the scope).

I also wanted to spell out that even at 200x in the 48" (close to its
minimum effective magnification), you *won't* need sunglasses to look
at M42; even though the trapezium itself will be quite bright, the
difference in apparent magnitude through the 8" and 48" will be just 2 
magnitudes, and M42 itself won't have a higher surface brightness than
in a 16" at 66x, for which I don't really need sunglasses.

That sunglasses thing is probably what prompted me to respond in the first
place. The surface brightness of the Sun as seen from earth is *a lot*
larger than that of M42.


-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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