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

Dr.Mohib.N.Durrani wrote:
If possible, I would like to extend the discussion to the sighting of very
thin Crescent Moons on the Twilight Sky (after Sunset). This will help in
determining the range of frequencies of the light from the Moon (reflected
by the Sun and absorbed/scattered/polarized by the atmosphere) and the
twilight sky (some averaged value for a particular depression of the Sun).

On the chance that you've confused two uses of the term "frequency" in
this discussion: As used with respect to the MTF, it refers to the
spatial frequency--that is, the characteristic size of detail. It does
not, in particular, refer to the frequency of the light.

In any case, the central obstruction will have a small effect on the
visibility of a thin crescent Moon. More important are scattering in
the optics and in the air, and proper baffling.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

"decaf" wrote in message
oups.com...
This is an interesting technical discussion, but it's 100% removed from
the real world of amateur optics.

Well, I'm definitely into amateur optics and would say it's 100%
included in my "real"world. MTF provides useful guidelines for one's
expectations of various types of telescopes and what information can be
ascertained regarding the optical quality of a telescope as an entire
system. If amateurs wanted to be a little more sophisticated in their
understanding of the effects of different aberrations with or without
obstructions, dirt, seeing,etc, then investigating MTF is at least a
well placed step in the right direction.

A simple webcam, two snapshots, and in an hour or so we can have a
detailed wavefront analysis for the entire optical train.

A cursory glance at this program suggests it is quite similar to
"Aberrator" with its fresnel ring pattern-aberration diagnostic
interface. The biggest difficulty
with using star test images to evaluate optical quality to the extent
of quantifying errors is the likelyhood of multiple types of
aberrations stacked on one another and misleading one to assorted
diagnosis and subsequent conclusions. WHile this program
may be of some use to obtain a very loose estimate of the instrument's
RMS wavefront error, (as is Aberrator) I doubt if it's smart enough to
sort out multiple errors of differeing
types that can be stacked and mixed in the patterns to a level of
accuracy one ought to take seriously.

Dan C.
It is not really like Aberrator. The 'point' is that the actually tries to
calculate the errors from the images, while Aberrator, gives you a
simulation of what the errors should do. In fact you could presumably
'play' with the pair, and use Roddier to work out what errors are present,
and then use Aberrator to see if these give results similar to the real
images. It has a similar 'rebuild' ability from the calculated errors
present though.
I'd suspect that the biggest errors with Roddier, will be the the large
amounts of atmospheric distortion in an individual image (presumably a
combined 'set' for each would give a better guide to the actual optics),
and how well it can distinguish a simple positioning error in the to focus
'delta' positions, from an error like spherical aberration. They do
recommend using fairly long exposures (20 or 30 seconds), to integrate the
atmospheric effects for good results.
How 'smart' it is, is the big question, but it is well worth realising
just how much maths can be performed by a modern PC, and I'd not be
suprised if it can do a better job than might be expected. The software
attempts to actually derive the Zerninke polynomials from the two images,
and should be able to give good results, but as the authors themselves
say, it needs to be accredited bytesting on a scope of known optical
quality, before being taken too seriously. However the tests they have
posted give quite hopeful looking results.

Best Wishes

Helpful person wrote:
... 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?

This is pretty much what the Strehl shows: average contrast loss over
the range of MTF frequencies (Mahajan, Aberration Theory Made Simple,
p95). Strictly talking, it is only valid when the OTF reduces to MTF
(i.e. when the PTF - phase transfer function - iz zero), or for
symmetrical aberrations. However, I'd guess that it is a good
approximation - in the form of an average for the contrast varying
with the angle that the bars are positioned at - for asymmetrical
aberrations as well.

Vlad

On Tue, 01 Nov 2005 22:57:12 GMT, Zane wrote:

I don't think that the quality of the telescope you use, assuming any
pretty good one, is the limiting factor, or even a very important
factor, in what you are trying to do. I think it will be deciding
what level of moon edge illumination constitutes a "Crescent Moon".

Under Islamic rules, a New Moon occurs when it is first visible-
regardless of the amount of illumination (and I would think visibility
is to the naked eye, not through a telescope).


The edge of the moon can be seen through a telescope with very good
clarity before any direct sunlight hits any visible part of the
surface.

Have you observed this yourself? Keep in mind that unless the New Moon
is marked by a solar eclipse, it always has some degree of illumination
along an edge. What distinguishes first visibility isn't so much the
degree of illumination, but the distance from the Sun. A fine crescent
Moon simply isn't visible until it gets a certain distance from the Sun
because the sky around it is too bright. Trying to observe a New Moon
via telescope can actually be quite dangerous. The New Moon that occurs
in just a couple of hours will be 2.5° from the Sun, and will show a
0.04% phase. I'm not about to aim an unfiltered telescope that close to
the Sun to see if this fine crescent is visible, though!

_________________________________________________

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

On Tue, 01 Nov 2005 23:20:35 GMT, Zane wrote:

Under Islamic rules, a New Moon occurs when it is first visible-
regardless of the amount of illumination (and I would think visibility
is to the naked eye, not through a telescope).

That would make the OP questions a little puzzling, I guess.

Well, he can always chip in. I'd guess he is generally interested in
studying New Moon visibility, and a telescope can be a part of that,
regardless of the requirements of Islamic law.


Given your comments above, it would seem to say even more that
standard telescope aberrations or what we normally consider "contrast"
factors wouldn't be of dominant interest in picking an instrument to
determine when a New Moon occurs.

The situation is a bit like observing the solar corona. You need very
high contrast optics, defined by complex baffling and very low scatter
materials. This is not the sort of contrast that depends on the MTF or
factors such as a central obstruction. It is easier to manage scattered
light with a refractor.

The results will also be much better at high altitude locations.

_________________________________________________

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

Hello Zane and Chris,

There are, at present, two valid Islmic opinions on sighting the Crescent
Moon (Hilal).

1. Use of unaided eyesight only (no binoculars nor telescopes). Optical aids
can be used to locate a Hilal but actual sighting has to be with the unaided
eye. Eyeglasses to correct for vision to be brought back to "normal" vision
is ofcourse allowed.

1. Aided sighting (binocular and telescopic) of the Hilal is possible. This
is the later (Islamic history) development since there we no optical aids
near the time of Prophet Muhammad (from Arabia).

Astronomically speaking, sighting in the "human visible range of
frequencies" is the criteria, not enhanced CCD/film recorded images. Of
course Radar imaging will always "show" the Moon

Mohib.


"Zane" wrote in message
...
On Tue, 01 Nov 2005 22:42:41 GMT, Chris L Peterson
wrote:

(snip)

Under Islamic rules, a New Moon occurs when it is first visible-
regardless of the amount of illumination (and I would think visibility
is to the naked eye, not through a telescope).

That would make the OP questions a little puzzling, I guess.

The edge of the moon can be seen through a telescope with very good
clarity before any direct sunlight hits any visible part of the
surface.

Have you observed this yourself? Keep in mind that unless the New Moon
is marked by a solar eclipse, it always has some degree of illumination
along an edge. What distinguishes first visibility isn't so much the
degree of illumination, but the distance from the Sun. A fine crescent
Moon simply isn't visible until it gets a certain distance from the Sun
because the sky around it is too bright. Trying to observe a New Moon
via telescope can actually be quite dangerous. The New Moon that occurs
in just a couple of hours will be 2.5° from the Sun, and will show a
0.04% phase. I'm not about to aim an unfiltered telescope that close to
the Sun to see if this fine crescent is visible, though!

Good points.

Given your comments above, it would seem to say even more that
standard telescope aberrations or what we normally consider "contrast"
factors wouldn't be of dominant interest in picking an instrument to
determine when a New Moon occurs.

Zane