From: chaloux@mandolin.mitre.org (Dave Chaloux)
Subject: Re: Purchasing a portable telescope
Reply-To: chaloux@mandolin.mitre.org (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
Date: Wed, 16 Dec 1992 17:44:00 GMT

Michael J. Edelman writes:

>The nice thing about small refractors is that they can often stand a lot
>more magnification than can fast reflectors- especially cheap ones. My 8"
>Coulter is not useable over 40x on planets without masking down to 3", but
>my old 3" Celestron SS-80 was a treat on Saturn at 60x- and it was an f/5
>scope, too. It was also wonderful at 20x.

Ouch! No wonder you're getting rid of it. This is the 2nd post about Coulters in
not to much time that has made me wince. The other one was about at 13" that
was bad at much above 100x. Their own literature says they are intended as
low power scopes between 5x-10x / inch.

My own view is that any 8" mirror not usable on the planets without masking
above 40x ought to be used as a dinner plate. Even the original optics on
a Meade 8" SCT I owned were sharp at 100x and I made Meade replace them. The
fact the mirror is F 4.5 is no excuse either. I have a 20" F5 which is more
difficult to manufacture by far than a 8" F4.5 that is absolutely crisp at
100x (proportionately the same) and if you have been following my Mars Observations is used on the planets at 360x.

I have looked through optics that were tested as almost 1 wave out at the
wavefront that are better than what you are describing.

This is not said to make Mike feel bad. After all he does have some
appreciation of Good Optics since he bought a Questar 3.5" for a good
deal more than he paid for the oulter. It is saying beware of bad mirrors.
Personally, I wouldn't use a 8" Scope that wasn't sharp above 40x on
Deep Sky let alone planets.

I wrote yesterday that not everyone needs the same quality of optics and that
it true. Still some minimum level ought to be maintained and this clearly
ain't it. 

Some reasons you want sharp optics for even Deep Sky are:

1) Small Planetaries require good resolution to distinquish them from stars. You
are going to want to use at least 100x on a Planetary 10" in size.

2) You can see fainter stars with good optics. Bad optics turn stars into
something other than a point source and make it difficult to pick up the
faint ones. You can easily lose 1 Mag in ability to see faint stars.

3) Resolving Globular clusters is much easier with sharp optics both because of
resolving power and the ability to pick up faint stars.

4) Seeing detail such as the nucleas of a galaxy is very possible in an
8" scope but will be difficult/impossible with poor optics.

I could go on.

I like aperture but like Mike says, it isn't everything.

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Some thoughts about off axis masks: I find it interesting that Mike has mentioned
using a 3" off axis mask to help. If a mirror is a figure of revolution with
a 1/2 mirror masking the mirror it is still 1/2 wave. Maximum Longitudinal and
transverse aberrations will be just as bad.

I think that a mask can help in two ways.

1) You can get better seeing.

2) The part of the mirror that is worst is almost certainly the outer rim. The
outer rim also collects the most light. By using an offaxis mask you are still
using a small portion of the outer rim but since the mask is circular it is
contributing less light compared to the middle of the mask. You'll still get
garbage from that part of the outer rim you use but the amount of garbage is
much less. It stinks like an outhouse instead of like a Sewage Treatment Plant.

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You don't always get what you pay for with optics but if you pay next to
nothing, next to nothing is almost certainly what you will get.

Clear Skies.