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#11
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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. -- Please reply to: | No nation is drunken where wine is cheap. pciszek at panix dot com | --Thomas Jefferson |
#12
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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
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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
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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
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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
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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
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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
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Last edited by jonson010 : September 29th 14 at 11:17 AM. |
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