Chromatic aberration

So I have these eyepieces that are presumably good quality
(they cost me approx. 100 US$). I think the brand is TeleVue,
but I'm not sure (I don't see it anywhere on the eyepiece;
the logo seems live a V enclosed in a full circle that
touches the V at three points).

The eyepiece says: "Long eye relief. Fully multi-coated"

That should make it a low-dispersion lens, right? As in
low chromatic aberration, right?

Their performance when taking tough pictures is... well,
not as good as I would have expected. For instance, these
two are images of Venus, taken with a reflector telescope,
f=1000mm, 8-inches, and eyepiece projection with the 12mm
eyepiece. The chromatic aberration is horrible:

http://www.mochima.com/personal/2004-04-24/Venus1.jpg
http://www.mochima.com/personal/2004-04-24/Venus2.jpg

Also, this Jupiter shot (taken with the exact same setup)
shows a horrible amount of chromatic aberration:

http://www.mochima.com/personal/2004-04-24/Jupiter1.jpg

(the blue aberration was on the same side -- the Venus
pictures were inverted with a photo editing program; the
Jupiter shot wasn't)

I think the only element that could introduce that
distortion would be the eyepiece, right? I mean, any
distortion coming from the glass that covers any of the
mirrors should be negligible, right?

So, my question: Am I doing something wrong, or am I
just facing a basic limitation of the glass I have?
(was it possibly due to atmospheric/seeing conditions?)

If my glass is simply not good enough for my expectations,
what would you recommend? How much (ballpark figure)
should I expect to have to pay for an eyepiece that would
give me good quality pictures? (or maybe I should be
thinking in terms of alternative techniques instead?)

Thanks for any comments/advice!

Carlos
--

On Thu, 29 Apr 2004 10:09:39 -0400, Carlos Moreno
stepped up to the plate and batted:


So I have these eyepieces that are presumably good quality
(they cost me approx. 100 US$). I think the brand is TeleVue,
but I'm not sure (I don't see it anywhere on the eyepiece;
the logo seems live a V enclosed in a full circle that
touches the V at three points).

These would be Vixen ep's

G../0

So I have these eyepieces that are presumably good quality
(they cost me approx. 100 US$). I think the brand is TeleVue,
but I'm not sure (I don't see it anywhere on the eyepiece;
the logo seems live a V enclosed in a full circle that
touches the V at three points).

The eyepiece says: "Long eye relief. Fully multi-coated"

That should make it a low-dispersion lens, right? As in
low chromatic aberration, right?

Their performance when taking tough pictures is... well,
not as good as I would have expected. For instance, these
two are images of Venus, taken with a reflector telescope,
f=1000mm, 8-inches, and eyepiece projection with the 12mm
eyepiece. The chromatic aberration is horrible:

http://www.mochima.com/personal/2004-04-24/Venus1.jpg
http://www.mochima.com/personal/2004-04-24/Venus2.jpg

Also, this Jupiter shot (taken with the exact same setup)
shows a horrible amount of chromatic aberration:

http://www.mochima.com/personal/2004-04-24/Jupiter1.jpg

Those images lateral color, which is not the same as chromatic aberration.
Lateral color (i.e. the image separating into a short spectrum) arises from
several areas. The most obvious is atmospheric dispersion (which is also
reponsible for the Green Flash). Secondly, most all eyepieces have some amount
of lateral color if the object is not centered. Thirdly, almost all camera
lenses, especially those on consumer digital cameras, have large amounts of
lateral color when the object is not precisely centered. If you held your
camera lens up to the eyepiece and tried to take images of planets below 60
degrees above the horizon, through the eyepiece, then it's almost a given that
you ended up with all three effects.

I would say that most of your color problems are due to your imaging technique.

Roland Christen

Carlos Moreno wrote:
So I have these eyepieces that are presumably good quality
(they cost me approx. 100 US$). I think the brand is TeleVue,
but I'm not sure (I don't see it anywhere on the eyepiece;
the logo seems live a V enclosed in a full circle that
touches the V at three points).

I don't have much to add to what Roland said, but these are Vixen
eyepieces, probably the Lanthanum series (the non-wide-angle type).

The eyepiece says: "Long eye relief. Fully multi-coated"

That should make it a low-dispersion lens, right? As in
low chromatic aberration, right?

No. Long eye relief just means that you can hold your eye about 20 mm
back from the eye lens, as opposed to some eyepieces where you seemingly
need to hold it right up against the glass. Fully multi-coated simply
means that the glass surfaces are coated in such a way as to maximize
light transmission and minimize light loss due to reflection.

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

Brian Tung wrote:

The eyepiece says: "Long eye relief. Fully multi-coated"

That should make it a low-dispersion lens, right? As in
low chromatic aberration, right?

No. Long eye relief just means that you can hold your eye about 20 mm
back from the eye lens

Oops. Yeah, I knew that part. The way I wrote it was a bit
ambiguous -- when I said "That should make it...", the "that"
referred to the multi-coated part.

Fully multi-coated simply
means that the glass surfaces are coated in such a way as to maximize
light transmission and minimize light loss due to reflection.

Oh, I see. Now that you mention it, I recall reading the
specs for Meade's UHTC or the Celestron counterpart, the
Brightstar coatings.

I thought that would be somewhat related to make the surface
less dispersive -- like the achromatic or apochromatic lenses.
Am I correct in thinking that those technologies are also
based on special coatings? Or is it a different technology
altogether?

As a follow-up (though I'm pretty much repeating the same
original question :-)), what should I be looking for to
improve the quality of pictures? (reduce chromatic aberration)
Should I just go to my local astronomy store and ask them
what they recommend? Any brand I should be specifically
looking for? (or any brand I should try to avoid?)

I guess I would be willing to spend around 200 US$ if I can
get a good quality 10mm eyepiece that would allow me to take
good planetary images with very little chromatic aberration
(am I being over-optimistic?)

Thanks!

Carlos
--

Again with my writing things in a very ambiguous way:

Carlos Moreno wrote:

Fully multi-coated simply
means that the glass surfaces are coated in such a way as to maximize
light transmission and minimize light loss due to reflection.


Oh, I see. Now that you mention it, I recall reading the
specs for Meade's UHTC or the Celestron counterpart, the
Brightstar coatings.

I thought that would be somewhat related to make the surface
less dispersive -- like the achromatic or apochromatic lenses.

Here, I'm talking again about the multi-coated spec of my
eyepieces, and not about the UHTC.

Am I correct in thinking that those technologies are also
based on special coatings? Or is it a different technology
altogether?

Here, I'm referring to the achromatic and apochromatic
technologies.

Carlos
--

Carlos Moreno wrote:
I thought that would be somewhat related to make the surface
less dispersive -- like the achromatic or apochromatic lenses.
Am I correct in thinking that those technologies are also
based on special coatings? Or is it a different technology
altogether?

It's a different technology altogether. For an explanation of how
achromatic lenses, see my article "The Color Purple" at

http://astro.isi.edu/games/chromatic.html

Apochromatic lenses are similar, but they use different glasses in
different combinations to achieve even better color correction (as well
as a host of other benefits). They don't come cheap, though. The old
adage--"fast, good, cheap; pick any two"--applies as well here as
anywhere.

The coatings that are applied to eyepieces to reduce reflections cannot
do anything about chromatic aberration. They only improve light
transmission.

I'm afraid I can't answer your photography questions. I don't really
know enough about that field to say anything with confidence.

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

Carlos Moreno wrote:
Brian Tung wrote:

As a follow-up (though I'm pretty much repeating the same
original question :-)), what should I be looking for to
improve the quality of pictures? (reduce chromatic aberration)
Should I just go to my local astronomy store and ask them
what they recommend? Any brand I should be specifically
looking for? (or any brand I should try to avoid?)

If the problem is atmospheric dispersion (which appears likely), you
could try a wedge prism. see
http://homepage3.nifty.com/~cmohk/coming2003/08.html for some
information, or refer to a Sky & Telescope issue article on the subject
sometime last year.

Or simpler, image near the zenith ...

"Carlos Moreno" wrote in message ...

eyepiece. The chromatic aberration is horrible:

http://www.mochima.com/personal/2004-04-24/Venus1.jpg
http://www.mochima.com/personal/2004-04-24/Venus2.jpg

Also, this Jupiter shot (taken with the exact same setup)
shows a horrible amount of chromatic aberration:

http://www.mochima.com/personal/2004-04-24/Jupiter1.jpg

(the blue aberration was on the same side -- the Venus
pictures were inverted with a photo editing program; the
Jupiter shot wasn't)
I think the only element that could introduce that
distortion would be the eyepiece, right?
snip

No. Don't blame your eyepiece or any other optical
element; unless you count the atmosphere.

What you're seeing is almost certainly atmospheric
dispersion. The atmosphere is separating the planet's
light like a prism disperses light into its spectrum.

Try looking at an object near the zenith where the colors
should not be a problem.

--
Hilton Evans
-----------------------------------------------
ChemPen Chemical Structure Software
http://www.chempensoftware.com

Hilton Evans wrote:

No. Don't blame your eyepiece or any other optical
element; unless you count the atmosphere.

What you're seeing is almost certainly atmospheric
dispersion. The atmosphere is separating the planet's
light like a prism disperses light into its spectrum.

Try looking at an object near the zenith where the colors
should not be a problem.

I'm going to check the data for Jupiter. It is true that
Venus was perhaps at no more than 30 or 35 degrees above
the horizon when I took that image.

But Jupiter, I would estimate that it was no more than
10 or 15 degress from the zenith. True, there is a lot
less chromatic aberration than with Venus, but still, I
find that it is a lot.

For the next session, I'll pay closer attention to where
exactly the targets are.

Thanks!

Carlos
--