Question about false color

Why does chromatic aberration appear (apparently) only at the edge of
targets? In other words, I took the 2 new big achros out tonight to compare
results looking at the moon. The Celestron shows more false color than the
Antares ---so far, Im very, very pleased with the Antares and can't wait for
the moon to get out of my way.

Question is, why the limb of the moon with all the false color? I suppose a
real crappy scope would show false color on craters too (as they have an
edge) but why isn't everything either yellow or purple when slightly in or
out of focus? Thing do get pretty on the Celestron AT focus---though again,
the Antares is superior.

Doink

On Sun, 16 Oct 2005 22:24:18 -0700, "Doink" wrote:

Why does chromatic aberration appear (apparently) only at the edge of
targets? In other words, I took the 2 new big achros out tonight to compare
results looking at the moon. The Celestron shows more false color than the
Antares ---so far, Im very, very pleased with the Antares and can't wait for
the moon to get out of my way.

Question is, why the limb of the moon with all the false color? I suppose a
real crappy scope would show false color on craters too (as they have an
edge) but why isn't everything either yellow or purple when slightly in or
out of focus? Thing do get pretty on the Celestron AT focus---though again,
the Antares is superior.

Inside the target, you do still have CA, but it doesn't produce an
overall color shift because at any one point you still basically have
the same mix of wavelengths, even though they aren't all at the same
focus. What happens is you end up with a loss of resolution and
contrast. The fringe effect requires a high contrast edge. In fact, if
you look closely you might be able to see fringing between the lit and
shadowed portions of some craters.

_________________________________________________

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

On or about Sun, 16 Oct 2005 22:24:18 -0700 did "Doink"
dribble thusly:

Why does chromatic aberration appear (apparently) only at the edge of
targets? In other words, I took the 2 new big achros out tonight to compare
results looking at the moon. The Celestron shows more false color than the
Antares ---so far, Im very, very pleased with the Antares and can't wait for
the moon to get out of my way.

Question is, why the limb of the moon with all the false color? I suppose a
real crappy scope would show false color on craters too (as they have an
edge) but why isn't everything either yellow or purple when slightly in or
out of focus? Thing do get pretty on the Celestron AT focus---though again,
the Antares is superior.

With bad enough correction, you probably would notice it all over the
place. But with a typical achromat, the correction is good enough
that the out-of-focus colors blend into the background image. You can
only see them near the limb, where the image is bright and contrasted
against a black background.

Or something like that.
--
- Mike

Ignore the Python in me to send e-mail.

Doink wrote:
Why does chromatic aberration appear (apparently) only at the edge of
targets?

As others have said, it's there all over the place, but if the target
is a uniform grey, as the Moon generally is, you can't see it in the
interior. You can only see it in the edge.

That's because the mix of colors is still right. Remember that CA
basically makes the red and blue fuzzier than the green at best focus.
Imagine you have a RGB image of the Moon, but the R and B images are
a little fuzzed out. This fuzzing does not change the R and B average
intensities; it only smooths out the peaks and valleys, so to speak.

As a result, most of the image, which is *not* peaks and valleys*,
doesn't have a different mix of R, G, and B than it would if the focus
were perfect, so it still looks the same. Only in those peaks and
valleys--that is, where there are intense contrast differences--does
the CA reveal itself. The place where the contrast changes the most
is the limb of the Moon itself, although, as Chris says, some craters
may show enough contrast themselves to show some CA, too.

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

On 2005-10-17, Doink wrote:
Why does chromatic aberration appear (apparently) only at the edge of
targets? In other words, I took the 2 new big achros out tonight to compare
results looking at the moon. The Celestron shows more false color than the
Antares ---so far, Im very, very pleased with the Antares and can't wait for
the moon to get out of my way.

Imagine a set of overlapping colored images of the moon. The yellow image
is in focus and therefore the smallest. The red and blue images are
slightly out of focus and therefore slightly larger than the yellow image.
In the center the moon appears more-or-less colorless, but the edge will
be surrounded by a purple haze. The out of focus light in the center will
reduce image contrast slightly and highlights and shadows will show a litle
color at the edges when you look carefully.

--
The night is just the shadow of the Earth.

OK, then if I'm using a filter, say blue, am I getting a sharper image by
eliminationg the color which is not at its focal point (assuming blue is the
color)? Sorry if I didn't express that correctly.

Doink
"William Hamblen" wrote in message
...
On 2005-10-17, Doink wrote:
Why does chromatic aberration appear (apparently) only at the edge of
targets? In other words, I took the 2 new big achros out tonight to
compare
results looking at the moon. The Celestron shows more false color than
the
Antares ---so far, Im very, very pleased with the Antares and can't wait
for
the moon to get out of my way.

Imagine a set of overlapping colored images of the moon. The yellow image
is in focus and therefore the smallest. The red and blue images are
slightly out of focus and therefore slightly larger than the yellow image.
In the center the moon appears more-or-less colorless, but the edge will
be surrounded by a purple haze. The out of focus light in the center will
reduce image contrast slightly and highlights and shadows will show a
litle
color at the edges when you look carefully.

--
The night is just the shadow of the Earth.

On Mon, 17 Oct 2005 09:21:31 -0700, "Doink" wrote:

OK, then if I'm using a filter, say blue, am I getting a sharper image by
eliminationg the color which is not at its focal point (assuming blue is the
color)? Sorry if I didn't express that correctly.

Exactly. The effects of CA are commonly reduced by using a filter to
block the shortest wavelengths (a yellowish filter often called "minus
violet").

Related to this is the fact that ordinary achromats are very good for
tri-band color imagers. Each color channel is imaged over a narrow
enough wavelength range that CA doesn't cause any problems. The optics
are refocused after filter changes. Of course, this doesn't work for
wide band imagers (B&W), so for that you want an apo or primarily
reflective system to minimize CA.

_________________________________________________

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

On 2005-10-17, Doink wrote:

OK, then if I'm using a filter, say blue, am I getting a sharper image by
eliminationg the color which is not at its focal point (assuming blue is the
color)? Sorry if I didn't express that correctly.

A deep yellow filter will sharpen things up a little. Red or blue filters
will not increase the apparent sharpness as much because the eye sees
those colors less sharply and because of the effect of spherochromatism
- the change in the correction of spherical aberration with color.
Visual objectives are designed to have minimum spherical aberration in
yellow wavelengths.

--
The night is just the shadow of the Earth.