Re: Expensive Small Aperture Refractor?
- From: tony_flanders@xxxxxxxxx
- Date: 28 Sep 2005 08:03:55 -0700
Shneor wrote:
> Visually speaking, small, high quality refractors are great if you
> don't mind not being able to view 80% of what you can see ...
> with an 18".
Actually, I compute that as closer to 99%.
An 18" scope has 4.5x the aperture of a 4" scope, gathering 20.25x
as much light, and allowing you to see equivalent objects 4.5x
farther away. So for any given absolute magnitude of star or DSO,
the 18" scope covers 4.5^3 = 91x times the volume. In other words,
only one out of every 91 stars or objects visible in the 18" is
visible in the 4" scope.
Obviously, that's overstating things quite a lot, for several
reasons. First, instellar extinction limits how far you can see
in the galactic plane, and you quickly run out of stars, nebulae,
and clusters when you look at right angles to that plane. Second,
seeing allows a 4-inch scope to performs to its theoretical limit
vastly more often than it allows an 18-inch scope to perform
to its limit. Finally, the secondary obstruction doesn't count
for much, but it does count for something, as do reflective
losses and so on.
Anyway, as it happens, of all the objects accessible to the 18-inch
scope, the 1% that *are* visible in the 4-inch scope are the cream
of the crop, as it were. Even people with 18-inch scopes spend much
or most of their time looking at them. Sure, you may get some
entertainment out of chasing down a 15th-magnitude UGC galaxy,
but it you want to see structure in a spiral, I bet you'll be
viewing a galaxy that's quite easy to see in a 4-inch scope.
> People with small refractors spend time splitting double
> stars, and view planets a lot.
Actually, to my taste -- and my eyes -- I much prefer a bigger scope
for viewing planets. No 0.5mm exit pupils for me, thanks!
But yes, small refractors are indeed wonderful for splitting
many double stars.
In addition, people with small scopes tend to know a lot more about
open clusters than people with big scopes. And they get better views
of large diffuse bright nebulae as well as large dark nebulae. Every
size of scope has some job to which it's ideally suited. For viewing
fine structure in M33, I use my 12.5-inch Dob. For viewing the Pleiades
or the North America Nebula, I prefer my 70-mm refractor. For viewing
the Coma Star Cluster, I use 10x binoculars. For viewing the Milky
Way, I use my eyeglasses.
- Tony Flanders
.
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