From: chaloux@mandolin.mitre.org (Dave Chaloux)
Subject: Re: obstructed view?
Keywords: telescope refractor reflector
Reply-To: chaloux@mandolin.mitre.org (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
Date: Tue, 12 Jan 1993 22:44:43 GMT

Oh boy, another chance to get in trouble on another non-controversial topic.

Paul Butler writes:

>It seems that people prefer refractors for viewing the planets, but are
>willing to use a telescope with an obstruction (e.g. Newtonians) in
>order to gain aperture when viewing far away objects.

>1.  Why is this?

An images of a star in focus consists of an Airy Disk and diffraction rings
around that disk. An unobstructed telescope like a refractor can, if it is
perfectly made put about 84% of the light into the central disk. Any obstruction
(or under or over correction of the optics) puts more of the light into the
diffraction rings. Since an image of something like a planet is made up of
a bunch of these overlapped, the more light that goes into the central disk
the better off you are. This will let you see more subtle detail. To give
a musical analogy, the light that goes into the diffraction disks is like noise.
Would you rather listen to a record with lots of hisses and pops or a clean one?

The answer isn't this simple however. Obstructed reflectors of larger diameter
can be gotten for much less money than equivalent refractors. If the reflector
is well made, the larger diameter of the mirror can more than make up for the
loss of the light due to the obstruction. Doug McDonald has said on numerous
occasions on the Net that given perfect optics, a Reflector of size a and
with an obstruction of size b will perform as well as a refractor of size
a - b/2. This still isn't the whole story since the reflector is going to have
spider vanes that spread light as well in rather pretty but nasty ways for
planet viewing.

However, a well made reflector with a small obstruction can be very good on
planets. I would point out that Don Parker who is one of the big wigs in the
amateur community for planets uses a 16" reflector and used a 12.5" for years.

I have compared my 20" Obsession head to head with a Astrophysics 6" which
I think cost somewhat more than my 20" and the 20" held its own. This despite
the fact I know the mirror is not perfect. (Exactly how good/bad is yet to be
determined since when I took it to be tested it was to stinking cold and
never warmed up.)

On the other hand for planets a clock drive is really nice and a rock steady
mount is also a bonus. It is easier to achieve both criteria at the same
time with the smaller refractor.

Why do people use Newtonians for DeepSky? Aperture. However, more concern should
be given resolution even for Deep Sky in my opinion. This is more important
for some kinds of objects (Planetary Nebula). Than for others. All other
things equal, the telescope with the better resolution will show more.

>2.  What do people prefer to look through when viewing large far away
>    objects (nebulae)?  Why?

For the most part, I prefer the big Newtonian for this. However, I have seen
very nice views of things like the Merope Nebula in the Pleiades with
refractors. Sometimes smaller aperture is better because you can get a
bigger field of view.

Try looking at the Beehive M44 through a 20". Believe me it is much better in
a smaller scope (although you can see more stars in the part you are looking
at.)

>3.  Could a reflecting telescope without an obstruction be as good as a
>    refractor?

In theory yes. Especially if care was given to proper baffling and high
reflection (96% or better) coatings were used and the optics were top
notch.

>4.  Is it practical (possible?) to build a parabolic mirror
>    that is not centered around the trough of the parabola (so that the
>    primary's focal point is not directly over the mirror)?

Yes.

>5.  How is this idea related to the idea of using an aperture stop?

Very. However an aperture stop is often used on large reflectors with
so-so mirrors in an attempt to get better performance. This often leads
to less gain than you might hope for. I have argued before that sometimes
an on axis mask is actually superior.

>I don't know much about optics or telescopes, so keep it simple.

>Thanks,

Your welcome.

For your benefit you should know that I think that both reflectors and
refractors have their place and in an ideal world, Bill Clinton would
solve the deficit and still deliver a refractor and a large reflector
to every amateur in the USA.

Lets see. Say 50,000 amateurs who actually observe (as opposed to reading
about it.) Say 12,000$ to get a 20" and a 7" refractor. This is just
$600,000,000. That's just a few less strategic bombers.