#CHROMATIC ABERRATION HOW TO#

It is only seen at the edges of an image. Lateral Chromatic Aberration is the cause of color fringing. It can be noticeable at the peripheries of extremely bright portions of an image. Axial chromatic aberration results in blurred colors in front of and behind the focus position due to the differences in each color’s focal point.

#CHROMATIC ABERRATION HOW TO#

I'm not sure how to make three and four variable charts in two dimensions but that's what we really need for this topic.Axial chromatic aberration is a variation in the length of each wavelength of light and lateral chromatic aberration is a variation in the magnification of the different colors of light becoming more visible at the image periphery. Visually, there are three factors to consider aperture, focal ratio and the magnitude of the object being viewed.įor photography there are four factors aperture, focal ratio, the magnitude of the object being photographed and the length of the exposure. My six inch f/5.9 put ridiculous purple around stars as faint as magnitude 8 and noticeable bloat on stars down to mag 12 in a 4 minute exposure. The above is most certainly not true for any kind of long exposure photography where star bloat and purple fringes ruin the image of almost every star in the field of view. At magnitude 5, 7 or 9 there is no CA difference between a $900 achromat and a $9 000 APO. My experience recently suggests that, for a six inch f/5.9 achromat visually, there is no CA visible on stars fainter than magnitude 3.5. There is also no differentiation in the chart between visual and the far more noticeable photographic CA levels. And why am I looking at those.I'm not.Ĭharts like this always leave out the magnitude of the object being viewed.

Personally I don't see it but on extremely bright objects. Images are crisp and CA is greatly reduced. There have been times when seeing was nasty and the views were nasty.almost a wash.The moisture in the atmosphere causes the larger objective to act like a CA magnifyer.while on exceptionally good seeing nights the airy disk is not bloated.hens not appearing to contribute to the CA level.

I think this is basically why Rob can get very good views with his 10 inch F/15's and stuff when the simple analysis suggests that CA would be a big problem. But anything larger, you are getting into range where the the resolution will be limited by the seeing rather than by the optics. The airy disk in a 10 inch is about 1/2 arc-second, certainly half arc-second seeing is common enough that it one would resolve the airy disk and therefore any chromatic aberration. In smaller scopes, I think this is a reasonable way to evaluate chromatic aberration but as telescopes become larger the chances of resolving the airy disk become less and less and so it is no longer such a useful tool. However since in the 6 inch, the size of the airy disk is half that of the 3 inch, the chromatic aberration is doubled and to bring back to the same relationship, the focal length and therefore the focal ratio must be doubled. The size of the chromatic blur is the same in a 3 inch F/10 as it is in a 6 inch F/10. One fact to remember though is this (that is if I remember it correctly)Ĭhromatic aberration is evaluated in comparison to the size of the airy disk. I have never looked through any of these scopes but I do remember reading a comment by Roland that with the Yerkes scope, you moved the focuser something like a quarter of an inch to focus the different colors. I have never looked through one, but I can't imagine how they ever discovered anything with such CA levels. 83Ĭan anyone comment about the chromatic aberration that they have observed thru the historic large telescopes? So all of these scopes, at least by calculation should have horrible chromatic aberration, off the chart to the left and lot's worse than my 6' f/5 Jaegers with a CA of. (at Swarthmore College) 24' (610 mm) with a 11 meter focal length (f/18) has a CA of. 24' (610 mm) with a 6.7 meter focal length (f/11) has a CA of. 26' (660 mm) with a 9.9 meter focal length (f/15) has a CA of. The Lick 36' ( 910 mm) with a 15 meter focal length (f/16.5) has a CA of. Yerkes 40' (1020 mm) with a 19.4 meter focal length (f/19) has a CA of. The formula for finding the CA Ratio is f/ratio divided by diameter of the objective (in inches). I used the Chromatic Aberration (CA) Ratio to take a look at how some of the largest observatory refractors in the United States 'stack up'.

Chromatic aberration