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Re: [ATM] Diagonal Size
I suggest you read William Zmek's articles in July and
Sept '93 S&T. What counts is the effective aperature, not
weather there is an obstruction or not. There also are other
twists such as the eye's contrast threshold factored in.
Have you considered this?
Yes you are right, the improvement is real, if you compare
equal apertures. But, a 25" with a 35% obstruction will KILL
any, say 8", APO on planetary detail as long as the 25" has high
quality optics, and given equal seeing conditions.
I hear over and over again that the views of the planets through
the large dobs are the best people have ever seen. Funny, I never
hear anybody raving about "breathtaking" views through that refractor.
A few issues back S&T has a section on planetary masters (CCD
imaging). The pictures of the planets are INCREDIBLE, and every
scope has a central obstruction, some LARGE. Ed Grafton has an
incredible shot of mars using a 38% obstructed C14. Other shots
are taken with a C8, a Dall-Kirkham, a Maksutov-Cass, and a C9-1/4.
Take a look at Brian Lula's page. Some of the best deep sky shots
ever taken by an amateur. His scopes have 30% and 38% obstruction
ratios. Serious imagers are buying RC's like hotcakes because the
field is fairly wide and well corrected and the large central obstruction
they have is NOT a factor. If it was, they would be using only
AP refractors. On and on and on again, the best images are coming
from scopes with large CO's
Guys like R.F. Royce question why someone would
spend many thousands on a APO when a Newtonian will perform
as well, and in larger sizes, much better. Two years ago Mel Bartels
made the statement "It is proven every day that large scopes with a large
central obstruction take the best high resolution images".
And don't think that the CCD overcomes the effects of a CO. Fact is
they do not. Take some terresterial test shots with a scope and then
increase the CO. You will see the diffraction effects of the increased CO.
Here is one such test:
http://www.digital-flight.com/thebigeye/Obstruction/obstruction%20test.htm
Do not spin my above statements to make it sound like I am saying that
a CO has no effect. I certainly know it does. But scopes need to be
talked
about in terms of "effective aperture". And because a scope has a CO
does NOT make it in any way inferior. Again, it is "effective aperture"
In the March 1992 S&T issue, 2 very experienced observers tested well
made 6" newtonians, independently, and were suprised that the
newtonians performance approched that of a 5-1/2" and a 7"
AstroPhysics APO refractor respectively.
The quality of the optics seems to be a much bigger performance
issue than the central obstruction.
So keep calculating how much better a smaller obstruction will be,
without considering how the human eye responds to this decrease
of contrast. The MTF is toilet paper without this consideration.
And keep making those big ugly awkward unobstructed
reflectors, or million dollar APO's. After all, what do I know.
Scott
----- Original Message -----
From: "Jerry" <wa4guu@verizon.net>
To: "'Polaraligned'" <polaraligned@optonline.net>; <atm@atmlist.net>
Sent: Tuesday, December 13, 2005 7:58 PM
Subject: RE: [ATM] Diagonal Size
> No. The improvement is real.
>
> If psychological phenomena comes into play it would be the observer
> viewing
> through a larger obstructed telescope thinking he is seeing all the detail
> the observer using a smaller un-obstructed scope is seeing. "I know my
> scope
> is bigger so my view must be better."
>
> I think it might be possible that physiological conditions of the observer
> might cause some not to see the benefits of un-obstructed over obstructed.
>
> Jerry
>
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of
> Polaraligned
>
>
>
> The differences people see looking through an unobstructed scope
> in many cases are just a psychological phenomenom. "I know there
> is no obstruction, therefore I am seeing finer detail".
> The biggest keys to high resolution is great optics, great collimation,
> and great seeing.
>
> Scott
>
>
>
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