Expert: Tom Whiting - 6/25/2009

Question
Hi

People say aperture is everything, on the contrary they say, most of the small premium refractors can outclass larger telescopes, if the later is true, could you tell , for example, what kind and size of refractor (please name the company if possible for clarification)can produce equivalent effect as a typical 8" SCT  (like nextar 8 or lx 90 8" Sct's)?

Thanks
Syed

Answer
Hi Syed,
The contrary view depends on what you are viewing, but usually,
aperture is everything. That's why we prefer that you first determine
what you want to view and specialize in.
If one is going out to just view the moon, planets in high detail,
and split close double stars, then they grab their long focal length
high quality 4 or 5 inch refractor for the high resolution.
(But you won't get the light grasp of say, a 15 inch Newtonian
Reflector truss-DOB because of the small size, so deep sky objects
will just be dim little fuzzy balls with no definition).
But if I'm going after deep sky objects....Andromeda Galaxy, Ring
Nebula, etc...then I grab the high quality 15 inch scope above.
But if I'm going out comet hunting and want a very wide field of
view, say 3 degrees, I grab the tripod and my 15 x 100 binoculars
and leave the scopes at home.
If I'm going out to measure different variable stars, then I only
need a standard 6 or 8 inch inexpensive tube-type Newtonian
reflector telescope.
If I own a small Volkswagen Beetle-bug car, and don't have any room,
then I grab an 8 inch SCT, for the comfort of transportation, realizing that I'm suffering a trade off (with that 30% secondary
obstruction) for both Deep sky objects and resolution with a fine
4 or 5 inch long focal length refractor type scope...but my car
prohibits any larger scope.
So see, it depends on what you want to view, and what you want to
do.  So yes, aperture is everything...PLUS, with the large aperture
of say a 15 inch, I can always place an aperture stop (also called
an aperture mask) at the opening, cut it down from say a 15 inch f4 to a 3 inch f20, improving resolution 200%...to view the planets
and close double stars. That's why I prefer a large aperture reflector type scope which I can cut down to refractor size.
The guys with the 4 or 5 inch refractor, they can't increase their
size to view the deep sky objects. You can always cut to smaller,
but you can't "increase" a telescope's diameter.
I don't really like SCT's except for their portability as a last
resort...the sacrifice from "perfection" using the above mentioned
scopes....is too much for me. Why suffer a 30% central obstruction
when you don't have to with a different type scope?
Hope this helps,
(And I'm sure there are other opinions out there on this subject)
Clear Skies,
Tom Whiting
Erie, PA