SCT CO and Aperture question

"Stephen Paul" wrote in message
...
Given an SCT has a 33% obstruction that will have some deleterious effects
on contrast compared to a scope with a smaller CO, but of the same
aperture,
and, given that by comparison, a larger aperture provides the second
benefit
of a smaller Airy disk, and better (tighter) resolution, ... consider an
8"
aperture of undetermined CO, and an 11" SCT with 33% obstruction.

What would the obstruction of the 8" scope have to be, in order to perform
similarly to the 11" 33% obstructed scope on planets?

IOW, what is the relationship between CO by Area and aperture, as aperture
increases, and CO by Area remains constant?

Or, for those who wish to give a quick and less detailed response...,
would
the contrast ever/always be better by increasing aperture, given a
constant
ratio of CO by area?
Get youself a copy of aberrator, and you can play with aperture, and CO, and
see what the effect is on an image.
The normal constraint on CO, is the focal ratio, and the fully illuminated
field. If you want to have an 8" scope with a 20% CO, this can be built, but
to have a reasonably useable field, the focal ratio has to rise to perhaps
f/20. Hence normally the relative diameter of the CO, rises linearly with
the scopes diameter, and the percentage CO, remains fairly constant.
The traditional 'rule of thumb', puts an unobstructed 8" scope, and an 11"
scope on pretty much the same terms.
That being said though, the atmosphere will normally lower the difference to
practically nothing... :-(

Best Wishes

"Brian Tung" wrote in message
...
Stephen Paul wrote:
Given an SCT has a 33% obstruction that will have some deleterious
effects

Only for mid-frequency detail (about 0.75 to 3 arcseconds) is the dropoff
in MTF both present and significant. I won't kid you, though--much of
the good stuff, maybe even most, is in that range. But you can overcome
the contrast dropoff if you are willing to shoulder the burden of extra
aperture.

Or, for those who wish to give a quick and less detailed response...,
would
the contrast ever/always be better by increasing aperture, given a
constant
ratio of CO by area?

Always assuming equal optical quality and zero atmospheric turbulence,
then yes. However, such considerations are often academic, since optical
quality is not equal, and I haven't yet seen a night of literally zero
atmospheric turbulence.

Thanks. Answers = 1, Solutions = 0. g

I live in an area that spends an aweful lot of time under the influence of
the Jet Stream. Seeing here isn't usually too good, and planet observing,
even in unobstructed scopes, is a real labor of love, with plenty of
dissapointment. However, given that on nights of good seeing, the C8 does a
fair job on planets, and given that under the same conditions (I assume)
contrast improves with aperture, as %CO remains constant, I think I'll
forego the 5" apochromat for the time being, and move up to the 11" or 14"
SCT.

-Stephen

Check out Thierry Legault's discussion of this:
http://perso.club-internet.fr/legault/obstruction.html

Basically, it depends on the level of contrast. On high contrast
images, ie. lunar features, the 11-inch can take full advantage of its
aperture and cream any unobstructed 8" scope of similar quality.

However, on low contrast planetary detail, the central obstruction
becomes somewhat of a problem. With the values you gave, there are a
few features that can be seen with the 8" unobstructed vs. the 11
inch. Generally, a 11 inch scope with 33% obstruction will perform
like an unobstructed scope anywhere from about 8" to 11", depending on
what type of detail you are looking for.

Hope this helps.

-George

"Stephen Paul" wrote in message ...
Given an SCT has a 33% obstruction that will have some deleterious effects
on contrast compared to a scope with a smaller CO, but of the same aperture,
and, given that by comparison, a larger aperture provides the second benefit
of a smaller Airy disk, and better (tighter) resolution, ... consider an 8"
aperture of undetermined CO, and an 11" SCT with 33% obstruction.

What would the obstruction of the 8" scope have to be, in order to perform
similarly to the 11" 33% obstructed scope on planets?

IOW, what is the relationship between CO by Area and aperture, as aperture
increases, and CO by Area remains constant?

Or, for those who wish to give a quick and less detailed response..., would
the contrast ever/always be better by increasing aperture, given a constant
ratio of CO by area?

George, I think your points are *very* well taken, precisely
because we often focus on otpics and what optics can or cannot
do, or should or should not do on some theoretical level, with complete disregard of the object and
its inherent traits as viewed under atmosphere.

Your pointing out hi-contrast (lunar) features vs lo-contrast
(planetary) features etc... as an inherent issue ... any optical set
must deal with, is very well taken and needs to be kept at the forefront of any discussion of this
kind...

Thanks for the reminder!
-Jerry




George Wilkie wrote:

Check out Thierry Legault's discussion of this:
http://perso.club-internet.fr/legault/obstruction.html

Basically, it depends on the level of contrast. On high contrast
images, ie. lunar features, the 11-inch can take full advantage of its
aperture and cream any unobstructed 8" scope of similar quality.

However, on low contrast planetary detail, the central obstruction
becomes somewhat of a problem. With the values you gave, there are a
few features that can be seen with the 8" unobstructed vs. the 11
inch. Generally, a 11 inch scope with 33% obstruction will perform
like an unobstructed scope anywhere from about 8" to 11", depending on
what type of detail you are looking for.

Hope this helps.

-George

"Stephen Paul" wrote in message ...
Given an SCT has a 33% obstruction that will have some deleterious effects
on contrast compared to a scope with a smaller CO, but of the same aperture,
and, given that by comparison, a larger aperture provides the second benefit
of a smaller Airy disk, and better (tighter) resolution, ... consider an 8"
aperture of undetermined CO, and an 11" SCT with 33% obstruction.

What would the obstruction of the 8" scope have to be, in order to perform
similarly to the 11" 33% obstructed scope on planets?

IOW, what is the relationship between CO by Area and aperture, as aperture
increases, and CO by Area remains constant?

Or, for those who wish to give a quick and less detailed response..., would
the contrast ever/always be better by increasing aperture, given a constant
ratio of CO by area?