*** CONTRAST vs RESOLUTION for Telescopes REFRACTORS vs REFLECTORS ***

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

Chuck Taylor wrote:
I'd rate IT as A troll.

Are you sure Greg? That name sounds familiar but I can't quite place it.
Has he trolled here before? I was going to answer it until I saw the
comments labeling it a troll. But then, I've been had before.

No, I'm not SURE it's a troll at all. In fact I DOUBT it. But it is a
troll TOPIC here on SAA, which is sure to create a long nasty thread.

Just having a little FUN with CAPS, that's all.

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://www.skyhound.com/sh/comets.html

To reply have a physician remove your spleen

On Wed, 26 Oct 2005 22:20:43 -0700, Chuck Taylor
wrote, in part:

I'd rate IT as A troll.

Are you sure Greg? That name sounds familiar but I can't quite
place it. Has he trolled here before?

I think the post may be a forgery.

Dr. Durrani has *posted* here before. Although his posts are not
intended as trolls, they get the same kinds of replies a troll would
get.

They're the ones that announce that the crescent moon has been sighted,
and so it's time to turn the calendar over to the next month...

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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On Thu, 27 Oct 2005 08:17:45 -0600, Greg Crinklaw
wrote, in part:

No, I'm not SURE it's a troll at all. In fact I DOUBT it. But it is a
troll TOPIC here on SAA, which is sure to create a long nasty thread.

Just having a little FUN with CAPS, that's all.

I have to admit, this is reminiscent of a certain document which
announces that if you see a sailor with strange blue flames around him,
and appearing to fade in and out, you need to lay hands on him, because
he is a Philadelpha Experiment survivor and needs our help.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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On Thu, 27 Oct 2005 06:22:50 +0000 (UTC), (Brian Tung)
wrote:

If both telescopes are the same aperture, then a premium refractor will
have better contrast than a premium reflector. The difference will be
noticeable, though not earth-shattering (in my opinion). Both will have
about the same resolution (in terms of, say, ability to split tight
doubles). If you're comparing run-of-the-mill telescopes, the
differences will be somewhat smaller.

I would also add that it depends quite a lot on the particular reflector
design. A Newtonian will show so little contrast loss with respect to a
refractor that many observers will be unable to detect it. As the
central obstruction increases in size, the contrast loss will become
more obvious. Thus, Cassegrainians and SCTs (the latter not a pure
reflector, of course) will show more contrast loss than Newts, and RC
designs will show even more.

Also, this discussion is really only relevant to visual observing, not
imaging.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Thu, 27 Oct 2005 07:04:52 -0400, Davoud wrote, in
part:

Indubitably. Dr. Durrani is required to preface his writings and his
formal speeches with that benediction. If a few bigots have a problem
with that, that's the bigots' problem, not Dr. Durrani's problem.

Required?

On the other hand, since I'm _not_ a Muslim, it would be dishonest of me
to preface my writings with such a benediction. This makes it easy to
tell authors apart.

Such a habit could be a problem where non-Muslims, rather than Muslims,
are the target of bigotry.

Also, I'm sure there are many people here who have problems with
*anyone* posting here, or speaking in public in any forum, who...

a) has a belief in some form of revealed religion, and

b) expects others to acknowledge the empirical validity of this belief.

I don't find Dr. Duranni's real posts to be particularly offensive in
this particular respect. Much worse are the occasional posts that tell
us that EVOLUTION is a FRAUD and DARWIN is probably in HELL!

But then, the United States is the home of most of the loony
Fundamentalists, and it's also home to the First Amendment. The
homelands of Muslim extremists do not so restrain them. For that, and
certain other reasons, Muslims are a bit scary these days.

The Muslim world knows what it can do to dispel the clouds of bigotry
heading their way...

- full equal rights for non-Muslims in all majority Muslim countries;

- a clear distancing of public opinion from all forms of terrorism;

- an end to belligerency against any portion of the democratic Western
world, specifically including Israel.

As to the original topic of this thread (I'm pretty sure here Dr.
Duranni is the victim of a forger, though)...

Resolution in a telescope depends strictly on aperture, and is the same
for reflectors and refractors.

Contrast depends on many factors; construction quality is important.
However, central obstruction, not found in refractors, but found in most
reflecting designs, does decrease contrast.

However, refractors happen to be cheaper per inch. For the same dollars,
then, you can get a bigger reflector, and get your contrast back by
using a lower magnification.

Quick, easy answer. How could it possibly be a troll to start a long
thread about what everyone already knows?

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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On Thu, 27 Oct 2005 05:10:24 GMT, "George"
wrote, in part:
"Davoud" wrote in message
...

I further believe that the question means "Given the same light
gathering capacity, which has higher contrast and resolution -- a
refractor or a reflector?"

Well? Don't leave us in the dark. which is it?

For the same light-gathering capacity, since the reflector has a central
obstruction, it must have a slightly greater diameter to make up for it.

Hence, the reflector has more resolution, and the refractor has more
contrast.

For the same aperture, which is how most people would take the question,
both have the same resolution, and the refractor has more contrast.

John Savard
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http://www.quadibloc.com/index.html
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Dr. Durrani is the same fellow who posts young Moon sightings, chiefly
to mark the beginning of the next month in the Islamic calendar, which
is purely lunar.

That's it! I knew I knew the name. I just didn't know where.

I suspect some of the vitriol is drawn by the benediction to Allah at
the beginning of each post.

Agreed.

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/
*********************************************

One application that requires the highest contrast possible
is the solar coronagraph. Until SOHO, I may be wrong here, coronagraphs
were refractors. The SOHO space based coronagraph uses a super polished
off axis mirror. I agree the difference between a good refractor and a
off axis super polished mirror might not be visually noticeable.

Altitude of the site would be more important IMHO.


d.


Chris L Peterson wrote:
On Thu, 27 Oct 2005 06:22:50 +0000 (UTC), (Brian Tung)
wrote:


If both telescopes are the same aperture, then a premium refractor will
have better contrast than a premium reflector. The difference will be
noticeable, though not earth-shattering (in my opinion). Both will have
about the same resolution (in terms of, say, ability to split tight
doubles). If you're comparing run-of-the-mill telescopes, the
differences will be somewhat smaller.


I would also add that it depends quite a lot on the particular reflector
design. A Newtonian will show so little contrast loss with respect to a
refractor that many observers will be unable to detect it. As the
central obstruction increases in size, the contrast loss will become
more obvious. Thus, Cassegrainians and SCTs (the latter not a pure
reflector, of course) will show more contrast loss than Newts, and RC
designs will show even more.

Also, this discussion is really only relevant to visual observing, not
imaging.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:
On Thu, 27 Oct 2005 06:22:50 +0000 (UTC), (Brian Tung)
wrote:


If both telescopes are the same aperture, then a premium refractor will
have better contrast than a premium reflector. The difference will be
noticeable, though not earth-shattering (in my opinion). Both will have
about the same resolution (in terms of, say, ability to split tight
doubles). If you're comparing run-of-the-mill telescopes, the
differences will be somewhat smaller.


I would also add that it depends quite a lot on the particular reflector
design. A Newtonian will show so little contrast loss with respect to a
refractor that many observers will be unable to detect it. As the
central obstruction increases in size, the contrast loss will become
more obvious. Thus, Cassegrainians and SCTs (the latter not a pure
reflector, of course) will show more contrast loss than Newts, and RC
designs will show even more.

Also, this discussion is really only relevant to visual observing, not
imaging.

PotStirringOn
If you look at the MTF curves for an obstructed design the resolution
generally falls below the theoretical diffraction limit except at very
low contrast ratios where the obstruction seems to enhance the
resolution beyond the diffraction limit. If one's purpose were only to
observe low contrast details, such as detecting the young moon, could a
specific instrument be designed with the central obstruction tailoring
the MTF?
/PotStirringOn

--
jeff