Telescope Reviews: Chromatic Aberration, Triplets, Semi-Apo, Apo

CHROMATIC ABERRATION (CA)

The most important thing the user should understand is that it is impossible to completely eliminate CA from a lens system. It can be reduced to an almost imperceptible level, but cannot be eliminated altogether.

This is an explanation of what chromatic aberration is and how it affects the overall image
Chromatic Aberration

This explains some of the math of CA
Chromatic Aberration on planetary disks

Chromatic Abberation is one of the least worrisome aberrations. In an astro binocular, it is only present in about 1% of the objects viewed. So it should be one of the least critical aberrations on which to base a decision. Judge your choice of binocular on all the features and known aberrations that make a difference in the view 99% of the time. Honestly, the planets are not binocular targets, so the moon is the only astro target that would be a consideration for judging whether one needs to consider CA in the choices of astro binocular.

For terrestrial viewing CA becomes an important factor. CA becomes a problem in extremely bright conditions, not in low light conditions. However, there are not too many people using 15x or 16x binoculars for terra viewing.

A Table Plotting Chromatic Aberration vs Objective Size and Focal Length
see this interesting post in the refractors forum that discusses CA.. From the post...
" It illustrates the relationship of aperture and f/ratio to displayed chromatic aberration in achromatic refractors. Roughly, telescopes with similar CA ratios can be expected to display similar levels of visual CA; as CA in achromatic systems is a function of objective diameter and focal ratio.

Using this chart, one can get an idea of how another refractor will perform based on the performance of whatever model one already has. "

See this thread for
A discussion of CA in premium and well corrected optics

A discussion related to chromatic aberration in several model binoculars took place embedded in this thread.
Obie vs Fujinon CA

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There are two different forms of CA

Logitudinal CA is caused by the fact that, even with precisely on-axis light, in the focused image from an achromat all the wavelengths do not have the same focal length. Typically light in the image of an achromat varies from blue to red focal length by 1/1800th to 1/2000th of the focal length of the objective lens. So, for a f/4 100mm binocular, the focal length of red to blue might vary by as little as 0.2mm. That may seem inconsequential to you, but that might represent a 15°-20° turn of a center focus dial on a binocular.

Light that is not focused precisely does not make the minimum sized point of light in the image. If the minimum diameter circles of light for each wavelength is at a slightly different focal length, that means when focused precisely at any one color (usually yellow) that the Airy disks of all other wavelengths (colors) have slightly larger sizes and they overlap the yellow image, forming CA in the on-axis image. This is not generally seen by the observer unless you slightly defocus the image. But rest assured it is there and it has an affect on contrast of fine detail. This may possibly be seen in the inability to bring the focus to a precisely small point of light. An achromat that is not corrected to even 1/1800th f may have a difficult time forming a precisely small image of even a moderately bright point source.

Lateral CA is caused by the fact that the light rays forming the image across the lens all have slightly different lengths to a focal plane but the focus mechanism wants to focus across a flat plane. For all the points of light in an image to be in focus at exactly the same time, our focus mechanism would be required to focus across a slightly curved image plane. That doesn't happen. Since the system is incapable of providing that precision, we see the light spread across that plane at varying amounts of out-of-focus.

If you were to draw several half inch wide concentric circles over the field of view, you would see that in the outer fov ring you see all one color, green, in the next ring in all another color, purple. The color you see is dependant on how far off-axis the image is in the plane. The further off axis, the longer the physical length of the light ray beam to the plane. The varying lengths of light rays causes each color to be out of focus more-so than some other color at different distances off axis. This form of CA is entirely due to where the light falls across the plane.

See this thread Question about CA....

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This comment recently generated a discussion about Achromat doublets, Triplet objectives, Semi-Apo and Apochromatic lennses, means of achieving better color correction or lower CA.

I answered:
I doubt you will find any binocular anywhere on the market that is a true Apo, whether it is labeled as such or not, unless possibly you are up in the range of several thousands of dollars.

The fact that it is a triplet should indeed lead to some degree of better color correction, but it is not a gaurantee. While it usually takes a triplet to make an Apochromat, triplet is no indication that any instrument, binocular or telescope, is an Apochromat. It simply means that a third element of glass is incorporated into the design to give a slightly better color correction. In a binocular, it is probably still an Achromat.

A typical Achromat has two colors corrected, usually red and blue, so their focal lengths will be within approximately 1/2000th the focal length of yellow. An instrument would need to be corrected to within 1/8000th to 1/10,000th the focal length of yellow to be considered an Apochromat.

Generally the term Semi-Apo is applied to instruments that have been corrected to within 1/4000th F. It is very likely some of the instruments on the market, both telescopes and binoculars, are not corrected to 1/4000th F, and yet are still called Semi-Apo. It could be these instruments are better than 1/2000 corrcted, perhaps 1/2500 or 1/3000. Frankly, any improvement over the level of correction of a standard achromat should be appreciated as it is not particularly easy to achieve and rather expensive to incorporate into the design. I would question if any of the triplet binoculars are reaching a level of correction to 1/4000 F.

edz

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Edited by EdZ (07/07/06 10:53 AM)

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