Hello all,

How would you rate the Contrast verses the Resolution of telescopes having
the same total light gathering capacity and are either REFRACTORS or
REFLECTORS.

Clear Skies to all.

Mohib..

I see you are making progress, at least you are asking an Astronomical
question Sir.
I will let others try to make a stab at it.

Clear Skies

P. Edward Murray

In sci.physics Dr.Mohib.N.Durrani wrote:
Hello all,

How would you rate the Contrast verses the Resolution of telescopes having
the same total light gathering capacity and are either REFRACTORS or
REFLECTORS.

Clear Skies to all.

Mohib..

I wouldn't unless you introduce something else, i.e. a camera, into the
system.

--
Jim Pennino

Remove .spam.sux to reply.

Apochromatic refractors clearly have better contrast than reflectors of
the same aperature. You specify light gathering which doesn't
correlate exactly since reflectors have the secondary mirror blocking
some light and both have the primary absorbing some.

But, introduce price into the equation, and reflectors win easily.
$1000 will buy a 100mm apo refractor or a 254mm reflector. The larger
aperature of the reflector gives it an enormous advantage in both
contrast and resolution.

Greg

How would you rate the Contrast verses the Resolution of telescopes having
the same total light gathering capacity and are either REFRACTORS or
REFLECTORS.

I'm guessing this is for the purpose of spotting a new moon?
If that is the case, you might also want to consider the
usefulness of a goto that can be aligned the night before and
left "parked" to goto the moon and aid in locating it.

That said, the main issue in spotting a new moon will be
contrast rather than resolution. Resolution will be a product
of the aperture.

Contrast is impacted by a number of factors and is not simply
a matter of refractor vs. reflector. Refractors include apos
and achros. Apos are very expensive, but the good apos have
excellent contrast. In an achro, the colors are smeared,
dramatically reducing contrast for the purpose of hunting down
a young moon.

Reflectors typically have less contrast than a good apo. The
surfaces are aspherical, which are harder to produce with the
same smoothness. However a mirror made by Carl Zambuto or
similarly skilled optician will be a delight to use as regards
contrast.

So the short answer is it still depends on how much money you
want to put into it. An expensive apo on a goto mount would be
the ideal young moon hunting scope. A nice newt on goto would
probably be next, followed by a MCT or SCT on goto and an
achro next.

But the skill of the observer will be the main factor.

Clear Skies

Chuck Taylor
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

Dear Mr.Taylor,

Thank you very much for your very informative detailed reply. I appreciate
the following and your summary:
"Apos are very expensive, but the good apos have excellent contrast." ... ..
..
"An expensive apo on a goto mount would be the ideal young moon hunting
scope."

Mohib.

"Chuck Taylor" wrote in message
...
How would you rate the Contrast verses the Resolution of telescopes
having
the same total light gathering capacity and are either REFRACTORS or
REFLECTORS.

I'm guessing this is for the purpose of spotting a new moon? If that is
the case, you might also want to consider the usefulness of a goto that
can be aligned the night before and left "parked" to goto the moon and aid
in locating it.

That said, the main issue in spotting a new moon will be contrast rather
than resolution. Resolution will be a product of the aperture.

Contrast is impacted by a number of factors and is not simply a matter of
refractor vs. reflector. Refractors include apos and achros. Apos are very
expensive, but the good apos have excellent contrast. In an achro, the
colors are smeared, dramatically reducing contrast for the purpose of
hunting down a young moon.

Reflectors typically have less contrast than a good apo. The surfaces are
aspherical, which are harder to produce with the same smoothness. However
a mirror made by Carl Zambuto or similarly skilled optician will be a
delight to use as regards contrast.

So the short answer is it still depends on how much money you want to put
into it. An expensive apo on a goto mount would be the ideal young moon
hunting scope. A nice newt on goto would probably be next, followed by a
MCT or SCT on goto and an achro next.

But the skill of the observer will be the main factor.

Clear Skies

Chuck Taylor
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

Mohib.N.Durrani wrote:

"An expensive apo on a goto mount would be the ideal young moon hunting
scope."


Mohib,

A wide-field go-to refractor would be great, but I don't think it would
have to be an "expensive apo" if it's only young-moons hunting you're
after. You might want to look into a much less expensive "semi-apo"
(or whatever they call scopes like the Pronto/Ranger these days).

If I didn't know better on s.a.a., I would ask why you are interested
in hunting for the young moon. I won't ask, then.

Tom

Dr.Mohib.N.Durrani wrote:
How would you rate the Contrast verses the Resolution of telescopes having
the same total light gathering capacity and are either REFRACTORS or
REFLECTORS[?]

See -

Thierry Legault's outstanding central obstruction page
http://legault.club.fr/obstruction.html
"It appears that in these frequencies a reflector with an obstruction
of 33 % is equivalent to an unobstructed instrument whose diameter is
33 % less (170 mm for 250 mm). A reflector with an obstruction of 20 %
is equivalent to an unobstructed instrument whose diameter is 15 % less
(210 mm for 250 mm)."

Legault's page will provide you with diagrams, astrophoto examples and
discussion regarding "MTF curves" referenced by the experts in this
thread.

SEDS Central Obstruction Webpage:
http://www.seds.org/~spider/scopes/obstruct.html

Search this newsgroup for prior posts on keywords "central obstruction"
narrowing with "resolution aperture effect"

Regards - Canopus56

Dr.Mohib.N.Durrani wrote:
Hello all,

How would you rate the Contrast verses the Resolution of telescopes having
the same total light gathering capacity and are either REFRACTORS or
REFLECTORS.

Clear Skies to all.

Mohib..



If you subtract the diameter of the secondary obstruction in a
reflector, you will arrive at the aperture of the refractor that matches
it in contrast. Resolution is another issue and in general the larger
apertures, even obstructed, will have better resolution. All this
assumes equal optical quality and the absence of severe figuring defects
or bad coatings.

In my experience, refractors always present a more aesthetically
pleasing image than reflectors.

Forgive me if this has already been posted for haven't read everything
in this thread.

It seems that most people here are trying to place a hard definition,
using MTF, on whether a refractor or reflector is superior.
Ubnfortunately, throughout the years, the value of MTF measurement has
been greatly overstated. It certainly is a wonderful parameter during
design and often during quality control but it has no more validity
than other parameters such as wavefront aberrations.

Having said all this, a useful concept that was suggested in the1970s
was MTFA. This is the area under the MTF curve. There appears to be a
fairly good correlation between perceived image quality (whatever that
is) and this simple parameter. How does this parameter compare with
the other discussions in this thread?