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Effect of aperture on chromatic aberation?



 
 
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  #11  
Old May 11th 12, 06:42 PM posted to sci.astro.amateur
Paul Ciszek
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Posts: 110
Default Effect of aperture on chromatic aberation?


In article ,
Martin Brown wrote:

I have to say that for picture of the moon and sun I would be inclined
to use the sharp focussed green channel as luminance and then tint it
using the chrominance information from the red and blue channels.


Can this be done in GIMP? If not, what program would you recommend?

It sounds like you are saying that even though the blue and red images
may extend beyond the edge of the moon as defined by the green image,
that information will be tossed--the image will be "trimmed" to fit the
green image.

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pciszek at panix dot com | --Thomas Jefferson
  #12  
Old May 11th 12, 09:01 PM posted to sci.astro.amateur
Chris.B[_2_]
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Default Effect of aperture on chromatic aberation?

I think your moon images have more problems than mere chromatic
aberration. If you aren't using a driven mounting the image is
constantly moving across the sensor. (as you have already mentioned)
The telephoto device is simply making your lens far "slower" than it
already is. This makes your problems infinitely worse by optical
leverage/ magnification plus much longer exposures.

If the false colour was the only problem you could simply switch to a
B&W image to sharpen it nicely. Chromatic aberration, within reason,
has little or no effect on the visual sharpness of the image. My extra-
focal, hand-held images are often colour fringed. One or steps of
colour reduction in free image handling software, like PhotoFiltre,
will easily remove any criticism of false colour on the moon.

Image movement on the sensor is one of the main reasons I hate
"ABSOLUTE PROOF OF UFO'S EXISTENCE!!!" shots on YouTube. Had they
simply held the camera very still their results, though very small,
might still have some slight ephemeral interest. Instead of which they
zoom to the "max" and handhold their camera to destroy all chance of a
worthwhile video. Or even a frozen still image. You are, following
exactly the same route, I'm afraid.

If you want to improve your chances of capturing something more useful
you could build a dirt cheap, Poncet mounting/platform for your camera
and hand crank a screwed rod drive. Place the platform on a solid wall
and you should enjoy an improvement in your images. An alternative is
to buy a cheap secondhand, equatorially _driven_ equatorial mounting
to hold your camera. Ask you local astro society/club if there are any
such mountings going cheap or available to be borrowed. Or make one
with plywood and hinges and screwed rod drive.

  #13  
Old May 11th 12, 11:35 PM posted to sci.astro.amateur
William Hamblen[_3_]
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Posts: 16
Default Effect of aperture on chromatic aberation?

On 5/10/2012 10:22 PM, jwarner1 wrote:


Paul Ciszek wrote:

Way back in undergraduate optics, I was told that reducing the aperture
of a lens mitigates various problems such as spherical aberration, and
of course it makes focusing less critical (aka depth of field). What I
am not sure of is whether a reduced aperture reduces chromatic
aberration. It would seem to me that if you make focal distance less
important, then the effect of red and blue light focusing at different
distances should also become less pronounced.

If I can get a hold of some solar film in time, I would like to attempt
to photograph the annular eclipse of the sun in May and the transit of
Venus in June. The way I see it, I should have plenty of light to
spare, and my camera is probably a long way away from being diffraction
limited, so why not use a small aperture?

The chromatic aberration issue with my camera is really only apparent
when attempting astrophotography. Here is one example:

http://www.flickr.com/photos/3585314...t-721576231290
10923/lightbox/

(view the 1024 pixel version for best results.)

--
Please reply to: | No nation is drunken where wine is cheap.
pciszek at panix dot com | --Thomas Jefferson


Part of the theory for reducing the outside edge of a lens,
or mirror, has to do with the statistical fact that most over or
under corrected zones are found at the edge of an optic - has
nothing to do with the optical theory of spherical or chromatic
aberration per se. Its just a statistcal production fact that most
departures from an ideal parameter in a circular optic are usually found near
the edge of an optic.


If your lens or mirror is spherical, the spherical aberration varies
inversely with the square of the focal ratio which means for a constant
focal length inversely with the square of the diameter of the aperture.
For a given lens the blur circle due to chromatic aberration is a
small fraction of the diameter of the aperture. Statistics notwithstanding.

Bud

  #14  
Old May 12th 12, 12:32 AM posted to sci.astro.amateur
Thomas Womack
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Posts: 206
Default Effect of aperture on chromatic aberation?

In article ,
Paul Ciszek wrote:
Maybe someone here can answer another important question: Given the
resolution you see in the moon picture, do I have a chance of getting
a recognizable silouette of Venus during the transit, if I can find a
way to mount the solar film?


Yes; Venus is about 1/40 the angular diameter of the Sun when
transiting (it's 115 times smaller in diameter than the Sun, but three
times closer to Earth than the Sun is), about 60 arc-seconds across;
the image scale on your Moon is about 3.2 arc-seconds per pixel so
Venus will be 20px across, rather bigger than the obvious crater in
the middle of the mare at the right of your image.

http://www.chiark.greenend.org.uk/~t...venus-2004.jpg
is eyepiece-projection of the last transit of Venus ( the right-hand
side is the limb of the Sun, the left-hand side is the limit of the
eyepiece, it's quite near the start of the transit ); it's a pretty
obvious event.


  #15  
Old May 15th 12, 09:01 AM posted to sci.astro.amateur
Martin Brown
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Posts: 1,707
Default Effect of aperture on chromatic aberation?

On 11/05/2012 21:01, Chris.B wrote:
I think your moon images have more problems than mere chromatic
aberration. If you aren't using a driven mounting the image is
constantly moving across the sensor. (as you have already mentioned)
The telephoto device is simply making your lens far "slower" than it
already is. This makes your problems infinitely worse by optical
leverage/ magnification plus much longer exposures.


His telephoto device is a front element multiplier - essentially a
dedicated x3 telescope fitted to the front lens so it doesn't alter the
lens speed at all. It does have a bit of chromatic aberration though.

If the false colour was the only problem you could simply switch to a
B&W image to sharpen it nicely. Chromatic aberration, within reason,
has little or no effect on the visual sharpness of the image. My extra-
focal, hand-held images are often colour fringed. One or steps of
colour reduction in free image handling software, like PhotoFiltre,
will easily remove any criticism of false colour on the moon.


Actually separating to RGB shows the green to be very good, blue not bad
and red a slightly different size and fuzzy.

Image movement on the sensor is one of the main reasons I hate
"ABSOLUTE PROOF OF UFO'S EXISTENCE!!!" shots on YouTube. Had they
simply held the camera very still their results, though very small,
might still have some slight ephemeral interest. Instead of which they
zoom to the "max" and handhold their camera to destroy all chance of a
worthwhile video. Or even a frozen still image. You are, following
exactly the same route, I'm afraid.

If you want to improve your chances of capturing something more useful
you could build a dirt cheap, Poncet mounting/platform for your camera
and hand crank a screwed rod drive. Place the platform on a solid wall
and you should enjoy an improvement in your images. An alternative is
to buy a cheap secondhand, equatorially _driven_ equatorial mounting
to hold your camera. Ask you local astro society/club if there are any
such mountings going cheap or available to be borrowed. Or make one
with plywood and hinges and screwed rod drive.


His shots are not at all bad for a standard camera with an add on.

--
Regards,
Martin Brown
  #16  
Old May 16th 12, 09:26 PM posted to sci.astro.amateur
Paul Ciszek
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Posts: 110
Default Effect of aperture on chromatic aberation?


In article ,
Martin Brown wrote:

I have to say that for picture of the moon and sun I would be inclined
to use the sharp focussed green channel as luminance and then tint it
using the chrominance information from the red and blue channels.


Well, my first picture of the sun looked absolutely colorless except
for a bit of chromatic aberration at the edges. Honestly, I expected
to look sightly yellowish with a bit of color difference around the
sunspots. Anyway, since there appeared to be no visible color to lose,
I said to heck with it and just isolated the green channel:

http://www.flickr.com/photos/3585314...n/photostream/


--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

  #17  
Old May 17th 12, 06:27 PM posted to sci.astro.amateur
Chris.B[_2_]
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Posts: 2,410
Default Effect of aperture on chromatic aberation?

On May 16, 10:26*pm, (Paul Ciszek) wrote:

Well, my first picture of the sun looked absolutely colorless except
for a bit of chromatic aberration at the edges. *Honestly, I expected
to look sightly yellowish with a bit of color difference around the
sunspots. *Anyway, since there appeared to be no visible color to lose,
I said to heck with it and just isolated the green channel:

http://www.flickr.com/photos/3585314...zes/l/in/photo...


Not bad at all!

Venus will be about the size of the lowest sunspot in your image.

  #18  
Old October 5th 12, 05:22 AM
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