From: chaloux@mandolin.mitre.org (Dave Chaloux)
Subject: Purchasing Amateur Telescopes: Spherical Aberrations
Reply-To: chaloux@mandolin.mitre.org (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
Date: Tue, 15 Dec 1992 17:37:42 GMT

The following occurs in the FAQ.

>spherical aberration
>     A problem where a spherical lens or mirror in a telescope is  not
>     shaped  correctly,  so  the light from the center is focused at a
>     different location than the light from  the  edges.   You  should
>     never  have  to  worry  about this.  This only shows up in really
>     cheap telescopes, like the Hubble Space Paperweight.

Personally, I wish this was true. Certainly I agree that you should
never have to worry about this.

Unfortunately when it says this only shows up in really cheap telescopes
I can't agree if "Cheap" means "InExpensive".

All telescopes suffer from this defect to one extent or another and only
the user can really tell what is acceptable for his/her application.

I think the manufacturers should publish test results giving the amount
of spherical aberration. As a user, I would be more than happy to pay a
little more for optics certified in this way.

For some uses 1/2 wave optics may in fact be good enough. For example if
you were buying a 13" F4.5 mirror and knew you were only going to be
using it at a maximum of 130x and you also knew you were the kind of
person that won't bother to accurately collimate the optics anyway maybe
you would settle for this (and maybe not. I can think of reasons not to).

On the otherhand, another person might be into planets and want a 12"
F7 mirror good to 1/8 wave. Right now getting such a mirror without
grinding your own is little better than a crap-shoot with the odds very
much against you. Just because a manufacturer says their mirrors are
good to 1/8 wave on the wavefront doesn't mean they are. All it means
is that if you take the time to test it and it doesn't match the claims
you can insist they give you another mirror.

Since some of us do not want to take the time and effort to learn to
grind and polish our own, this leaves us in a spot if really accurate
optics are necessary for what we want to do.

Paying more for your optics is no guarantee either. When Sky & Telescope
tested 10" mirrors, the most expensive mirror was the worst!
(Edmund Scientific).

Nor is buying from the outfit with the best reputation (although it helps).
Galaxy Optics has a good reputation but their 10" mirror was still not
1/4 wave on the wavefront.

Bottom line is if you want/require really good optics you have only a few
limited choices right now.

1) Buy a scope you have looked through and know performs to the level you
require. This can be either new or used. If new it implies not using
mail-order.

2) Buy a new scope with a solid guarantee. Reconcile yourself to the fact you
are going to have to test and retest the optics until they get them to the
point you are happy. Be prepared to be unpopular with the manufacturer. Also
be prepared to lose the use of the scope for awhile and possibly depending
on the manufacturer or place you bought it having to pay the shipping charges.
This is my approach.

3) Get a trusted amateur who makes mirrors in your area to make one for you.
Be prepared to put out money for it. It takes a lot of work/time.

4) Make your own.
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