Color Correction

When manufactures mention color correction like 8X color corrected on refractor lense what exactly are they comparing the color correction to?

Lets take synta as a comparison for example.

1. Best of the bunch achromat
2. an average achromat ?
3. Below average achromat?

Because of quality control your going to get quite a variety of amount of color in a normal achromat.. what are they comparing to?

And then go a step further... Synta's 100f9 ed says "Zero" color while Burgess 101f5.5 says 8 times color color correction.. Both use fpl53? glass.

Thanks

Mark Visser

I think it is hard to know what they mean by "8x
better color correction" without more information.
For example, they could mean the focus shift between
certain spectral lines (certain colors) is 8x less,
but without some formal definition, it is hard to know.

Color correction depends more on the design of the lens
and the glasses used, than on the "quality control." Most
Synta 150 F8 achromats I have seen have very similar color
correction, inspite of small unit-to-unit optical
differences.

Again with the Synta 100 F9 ("zero" color) and
Burgess 101 F5.5 (8x color) it is hard to know what they
mean without some definition. While both lenses may use
FPL53 glass, you would expect the faster F5.5 lens to
show more false color. But claiming "zero" color is
meaningless -- I have never seen a lens with zero color.
Even the best lenses have some false color if you look
hard enough.

>When manufactures mention color correction like 8X color corrected on refractor lens what exactly are they comparing the color correction to? >>

An achromat defocuses the red and blue wavelengths by 1 part in 1800. That means that an 1800mm focal length lens would focus green about 1mm closer to the lens than red and blue. If a lens of 1800mm FL has 8 times better color correction, then the green focus would lie only .125mm from the blue and red.

All achromat lenses have approximately the same percentage of color error. The perceived color error depends on the focal ratio of the lens. A fast lens with normal achromat color will look like it has more color error than a slow achromat. The reason for that is simple geometry.

In order to make a fast lens color free, one needs to use special glass called ED of fluorite and combine it with the right matching glass in a doublet or triplet configuration. Doing so can reduce the color error, or difference in focus point to be anywhere from 4 times to 20 times better than an achromat.

Whereas an F9 lens might look quite color-free with 8 times better than achromat error, an F5 lens with the same design will show color error on bright objects. The only way to match the performance of that F9 lens would be to have an F5 lens with 14 times better color error.

Correcting color error is only one of many defects that will be present in a lens, and it is not the main component to having a sharp view. One can have achromat color error, but have a superbly polished and figured lens where all the other Sidel aberrations are well corrected, and as a result that lens will easily beat a poorly made apo lens on the planets and even deep sky objects.

My personal feeling is that a large number of people are being unduly influenced by one or two aspects of a refractor, and thinking that if this was perfected, then the scope will perform to the utmost limits. If the cost is low, then so much the better. When low cost is the goal, and people want something like low color error, manufacturers have no choice but to concentrate on that one aspect while reducing the performance of the other fundamentals.

Many years ago Zeiss made a line of semi-apo lenses. These had only 2x better correction than a normal achromat, but they were made to very high performance standards, and will easily outperform any of the better color corrected inexpensive offerings coming from China today. The views through a Zeiss AS objective are outstanding. I sold a number of 6" and 8" lenses when they were still available, and still remember the magnificant planetary views. Even the little 4" AS lenses were exceptional. And yet, their color correction was only 2x better.

Roland Christen

Mark,

Color correction is the science of making all colors focus at the same place. In any glass, one color will focus at a slightly different place than another. Therefore, one side will be fringed with violet, and the other will look yellow (if I recall correctly). (The main object in the image also is muddied by this phenomenom, but it does not show as much because there are too many colors in the middle.)So, the telescope makers put together different combinations of lenses and types of glass so that all three colors are "corrected" to focus at the same place.

A lens showing zero color will show no such false color. But I have am not familiar with the expression "8 times color correction."

(As usual on the forums--where one does not know exactly where the other guy is coming from--please forgive me if I have explained the obvious to you. I am sure the explanation will help some lurker if not you.

Alex