On Thu, 10 Jul 2003 09:54:33 +0100, "Roger Hamlett" ...reflected:


"Alan W. Craft" wrote in message
.. .
On Mon, 7 Jul 2003 23:42:37 +0000 (UTC), William Mc Hale
...reflected:

Shawn Grant wrote:
Anyone who buys a Questar is stupid plain and simple. Meade ETX is just
fine. In fact its optics are better.

The optics in the ETX are good, not sure I would say better than a
Questar, the Questars I have looked through have been more or less
perfect.

In any case, optics are only part of the reason to buy a Questar, and in
my opinion, in these days of beautiful Apos in the 3.5-4", the optics are
the least compelling reason. A Questar represents a beautiful precision
instrument whose mechanical components are as well thought out as the
optical components and will, with some care last a lifetime or more of
use, not sure anyone would say that about the ETX.

Have fast apochromatic refractors rendered the Maksutov's
redundant, especially considering the latter's seconday obstruction?

Wasn't their primary purpose to achieve slow-refractor-esque performance
in a tube half or even less in length?

After all, a secondary obstruction is ALWAYS just that, and never a boon
to performance; a necessary evil, if you will.
In one part, you are answering yourself.

I fancy myself a master at rhetorical questioning; just ask the denizens
throughout Usenet, particularly within the religious groups...

....and the not-so-religious...

Generally, an APO, will be quite
'fast' (perhaps f/10 or lower, especially on designs with larger
objectives). A Mak with a small CO, will be designed to give a long focal
length, and be relatively 'slow' (f/15 or higher). There are some people who
have built/used such APO's with focal ratios like this, since long focal
length can make them excellent for some types of planetary observation, but
once you have tried handling a f/20 6" APO (120" long...), you soon realise
just where the Maksutov wins!...
Unfortunately, many of the commonest Maksutov designs sold, are at the 'low
end', of the sort of focal ratios which were originally built (Maksutov did
designs up to about f/35), and hence lose out by needing slightly larger
CO's.
It is also worth realising, that the last line, that a 'secondary
obstruction is ALWAYS just that, and never a boon to performance; a
necessary evil', can at very small obstruction levels be argued about!. At
lower obstruction levels than are common for most Cassegrain designs
(perhaps 18 - 20% or less), the degradation is tiny, and can result in a
sharper fall off at the edge of the Airy disk.

Oh, but there's no denying an utterly obstruction-free, even if it's a
pinhead in size, REFRACTOR...

Newbie: "Uh...what's ~THAT~ in the center?"

Dealer: "Oh, that's just the secondary...O-B-S-T-R-U-C-T-I-O-N!!! O-B-S-T-R-U-C-T-I-O-N!!! O-B-S-T-R-U-C-T-I-O-N!!!

(Is there an echo in here?)

Newbie: "Ooooooo...what's that over there, glistening?

Dealer: "Uh...a refractor."

Newbie: "Sold."


Lastly, just who exactly would turn his back on a nudie show yet watch
it nonetheless with his wife's borrowed compact in hand?

And no, I did not type from personal experience.

When I want to see stars, I look DIRECTLY at them. When I want to look
at myself, which is often admittedly, I employ a mirror.

With any obstruction, energy
is shifted from the centre of the Airy disk outwards, but at low obstruction
levels, little energy actually shifts outside the Airy disk, instead moving
just towards the edge of the disk, and resulting in a slightly sharper edge.
Historically, some observers have even tried adding small obstructions to
APO's, to produce this effect!. With the larger CO's, that are common, the
effects are negative, but if you observe through a design like a f/20 8"
Mak, with a small CO, you will find just what Maksutov was trying to
achieve!.
Now the Questar, has a low enough focal ratio, that the CO, will be having a
noticeable effect. However against this, you have the very small amount of
chromatic aberration from a basically reflecting scope, against the best
that an APO can achieve, which is by no means 'perfection'. In a sense, both
designs are compromises. If however you look at the price of their 7" Mak,
and then try to find a 7" Flourite APO, their products may well start to
look quite a 'bargain'. The 3.5" scope, is in a sense, a 'niche market'
product for somebody who wants the best small scope, in a very portable
package. It is worth remembering also, that the current 'crop' of 4" APO's,
didn't really exist, only a very few years ago, with the units that were on
the market being relatively a lot more expensive.

Best Wishes

Not if you're advising a Mak over a Tak, you're not, you're not, you're not.

Toodles!

Alan