Re: Testing of optics through magnification

James Fusco said:
Testing a lens with high magnification until the image breaks down, this limit is it a test? Does this indication a quality of optics or design.
Jim

I think this "test" can reveal bad optics. But I feel it is less useful
for distinguishing amongst a range of good to perfect optics. Once
you reach enough power to resolve the Airy disk, and the Strehl ratio is
well within the range of being diffraction limited (say >90% Strehl), I
think this test tells you much less.

This test is very closely tied to the eye's resolution.
The human eye can resolve around 1 - 2 arcminutes, and allowing for
some minimal sampling by the eye (say Nyquist type of sampling or
2 samples across the Airy disk), the Airy disk will become well-resolved
for most people around 30x to 60x per inch. Once the Airy disk is
resolved to the eye, differences between good and great optics will be
very difficult to judge. On some lunar or planetary scene, great optics
will give slightly higher contrast and visibility of fine features
than merely good optics. But even testing side-by-side it will be difficult
to judge subtle differences with seeing effects present, etc.

Note that this connection to the eye's resolution also makes it
difficult to compare results from different people, or even the
same person on different nights. People with poorer eyesight,
or having a day with blurry vision, will say the image looked
"sharp to them" at more power per inch. In effect they could
make the Airy disk bigger before it was fatter than their eye's
resolution.

As already pointed out, this test is also very sensitive to seeing
effects, making it difficult to compare scopes evaluated on
different sites, different nights, or even different moments of
variable seeing.

A better test, I think, is to look at the out-of-focus
patterns of rings inside and outside focus -- the so-called
"star test." The patterns are much larger than the eye's resolution,
and mostly removes the eye's performance from the test. A perfect optic
would give identical patterns on either side of focus. But the test is
also extremely sensitive and can reveal small optical errors of little
consequence. This test can also be made quantitative by comparing what you
see with software programs like Aberrator. Critics of this test (mainly a
few makers of high-end scopes) will point out that most people observe with
images "in focus" or that their particular type of scope can't be tested
with this method.

You are magnifying everything, including atmospheric disturbance, heat plumes in the scope as well as roughness or uneven areas in the optics that causes scatter and distortion.

James Fusco said:
John, I do alot of projection through a eps into the camera.So with me it's no eye resolution but more with the camera. So what I see i'm seeing on the computer. So say when looking at the moon at high/ higher/highest then image begins to break down. What are we seeing here?
Jim

A couple things will happen as you increase the image size
on the detector. For faint objects the best image usually occurs
when the Airy disk is about 2 pixels across. The images is big
enough to sample most of the available detail, and there is not
too much noise. As the image size gets bigger beyond this point, it
will be spread over more pixels. Each pixel contributes a certain amount
of read noise and dark current noise, so the noise will start to go up
as the number of pixels per Airy disk increases.

For bright targets you can make the image larger before it
hits the noise problems. But this will provide almost no additional
astronomical information. Your brain will tell you the
imaging is getting fuzzy, and any defects in the optics, seeing,
etc. will become more apparent. So I guess this is the point where
you might say the image is "breaking down" although there is
not really any less information than at a lower power (for
a bright target).