Telescope Reviews: Chromatic Aberration on planetary disks

Chromatic Aberration on planetary disks

While comparing two large 100mm binoculars, I made these observations relative to Chromatic Aberration (CA). Several very bright stars were observed, along with Venus, Saturn and Jupiter. CA, to varying degrees, was observed with both binoculars on all, or nearly all, bright objects, the brightest objects showing the most CA. In one model the CA blur appears blue, in the other model it appears slightly red. With some other models used in the past I noted a pronounced yellow or green can be seen.

Jupiter provides us with the information for the topic of this discussion. When viewing Jupiter, we get to see one of the brightest objects in the sky, the planetary disk, and at the same time four moons, which, for binocular viewing, can be considered point sources.

Most have noticed how difficult it can be to clearly focus binoculars on a planetary disk. Attempting to get a clean focus on Saturn or Jupiter, CA can interfere with a clean sharp focus. To understand why this happens you must consider some of the mathematics of CA.

Not all wavelengths of light come to focus at the same focal distance. A doublet lens (most binoculars) is used to bring the focal length of two wavelengths closer together, usually blue and red. Normally their focal point will be slightly longer than that of yellow/green, the wavelength to which we are most sensitive. Generally the violet is ignored because of our low sensitivity to that wavelength.

In an f8 to f10 system, a doublet correcting the focal lengths of blue and red may result with the blue/red focal length varying from the yellow focal length by 1/2000 to 1/3000 of the stated focal length. For longer slower lenses, the variance can be less. Fast refractors increase the affect of CA. In shorter faster systems, the variance is greater, maybe now only corrected between 1/2000 to 1/1000 the stated focal length. This describes binoculars, which are usually near f4.

To get a feel for the magnitude of the focus differential between the yellow focal length and the blue/red focal length, in a 22x100 binocular, approx 460mm focal length, if corrected to 1/1000, the difference in focus is 0.46mm. That is nearly a 30° turn of the focus dial. If corrected to 1/2000, the turn would be 15° of the focus dial.

In a good corrected doublet, the size of the blue/red CA blur can be 3x the size of the Airy disk. That is, for a 100mm lens with an Airy disk radius of 138/100 = 1.38 arcseconds, the blue/red blur may have a radius of 1.38 x 3 = 4.1 arcseconds. For faster systems (binoculars), this blur size will increase.

Now take our example of Jupiter. Generally, I focus my binoculars on a nearby star. Then I slew to a planet to observe. Doing this, you may notice Jupiter’s moons appear to be in focus, but the disk seems out of focus. Attempting to correct the focus on the disk throws the moons out of focus. Why is this? This happens because CA will not allow us to bring all the colors of the spectrum to focus at once.

When focusing on a star there is insufficient bright light to produce the affect of CA. The focus is more towards the yellow wavelength focal point. The moons will appear to coincide with this focus since they meet the same relative conditions of insufficient bright light to produce CA and small enough to be considered a near pinpoint. Even the largest moon that is near 1.5 arcseconds in diameter, if bright enough, due to CA would only have a ghost image appear about 5 arcseconds in diameter. Even in 22x or 25x binoculars, magnified to an apparent size of 110 to 125 arcseconds, it would still be too small for the eyes acuity to see as larger than a point. At 22x, we would still see the moon as a point-like object. This helps explain why we can sometimes focus out most of the CA on very bright stars. They never appear larger than point-like and you can pass thru a range of focus with no detrimental affects. It also helps explain why the CA affects are not as noticeable in lower powered binoculars. The image may not be magnified large enough to broadly separate the colors.

However the planetary disk is very bright and large. Assume the disk presents itself to us at a diameter or 40 arcseconds, approximately Jupiter near its largest. Magnified at 22x, we see the disk at an apparent size of 15 arcminutes or larger. Every point of light on the disk forms an Airy disk, even continuously around the outer edges. If the blur image is 3x the size of the Airy disk (3 x 1.38”radius = 4.1), then the blur image of a bright 40 arcsecond object, the planet disk, will appear to be 4.1 blur + 40 + 4.1 blur = 48 arcseconds in diameter. This is about 20% larger than the precisely focused disk, and the CA blur ghost is easily seen. (Actually, when taking into consideration a full explanation of diffraction, the light forming Airy disks at the edges of the planet disk may not be quite as bright, and the blur would be somewhat smaller than I have stated.)

Focusing to a point between the yellow wavelength focal point and the blue/red focal point will gradually reduce the blue/red blur. As focus approaches the blue/red focal point, the yellow/green blur will increase in size. The smallest blur image will occur when focused midway between the two, but neither wavelength will be clearly focused. At no point can some slight blur be totally eliminated from the disk. For that we need a triplet apochromat lens or the inclusion of some exotic glass such as fluorite in the system. Keep in mind not all lenses are corrected to the same degree. It is possible even a triplet apochromat may not provide any more correction than a very well corrected doublet.

Any attempt to reduce CA or improve focus on the disk by moving focus closer to the blue/red focal length will move further away from the focus point of the near point-like moons. They are focused closer to the yellow wavelength focal point.

This is why we see CA on bright objects and why it can be so difficult in binoculars to suppress CA and focus on especially large bright objects. Is this cause for alarm? Certainly not! It is inherent in all doublet lens systems. It shows up more in shorter focal length fast systems such as binoculars. Understanding the affects of CA should help you better understand the limitations and uses of your equipment. Consider yourself fortunate if you can focus the CA out when viewing planetary disks, especially in high powered binoculars.

For your additional reading pleasure
Chromatic Aberration

edz

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Edited by EdZ (05/07/04 01:16 PM)

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