Less CA with quality in Achro?

Would a high quality achromat objective produce less chromatic aberration than a poor quality objective? I have a Hardin 90mm f/11 with a fairly poor lens, my 60mm old Jason is much sharper, and am wondering if a "Vixen spec" lens will give less CA than I see in the Hardin, or only sharper images? My understanding is that CA is related to other aberrations, so I expect so, but would like some idea of just how much difference it might make.

Thanks,
Brad

Brad,
The first thing is to understand how CA in an achromat depends on lens aperture and focal ratio, if it's well made. A common rule of thumb is that CA will not be too obtrusive if the focal ratio is 3x the aperture in inches. If you mix in poor quality as a variable, all bets are off. CA is indeed related to other aberrations, but if well made and of proper design, CA can be minimized to the above extent, and spherical aberration and coma eliminated as well.

By this rule the Hardin should be a quite good lens if well made, which apparently it is not. You don't state the focal ratio of the Jason, but I imagine it is f/15, so it should be virtually color free and perform well up to about 100x, if well made, which apparently it is. By all accounts, Vixen spec lenses are very well made, but will show some CA in difficult circumstances, like any achromat.

Of course, there are telescope enthusiasts who are extremely sensitive to CA that might strike others as inconsequential, and insist on a higher level of color correction than possible in a normal achromat, and more power to them. Sirius or Venus are popular CA-revealers. If CA is visible under any test, however harsh, they argue, views under normal use will be somewhat compromised as well. I'm not very picky I guess, and am happy with my 5-inch, f/12 achromat, ( allegedly of decent quality, made by D&G) which only rarely (Sirius and Venus) shows "obvious" CA, to my eye.
Ron

Brad Bassett said:

Would a high quality achromat objective produce less chromatic aberration than a poor quality objective? I have a Hardin 90mm f/11 with a fairly poor lens, my 60mm old Jason is much sharper, and am wondering if a "Vixen spec" lens will give less CA than I see in the Hardin, or only sharper images? My understanding is that CA is related to other aberrations, so I expect so, but would like some idea of just how much difference it might make.

Thanks,
Brad

Brad:

To repeat and add to what Ron said... First different observers seem to have different sensitivities to false color. But as Ron said, the color correction in an Achromat is a depends on the focal ratio and the aperture, a larger aperture shows more false color, a shorter focal ratio shows more false color.

There are basically two rules of thumb for the color correction in an Achromat, the 3X rule and the 5X rule. These refer to the focal ratio of the scope divided by the scope's aperture as measured in inches. This ratio is sometimes referred to as the Chromatic Ratio. The way I state these rules is:

3X: An achromat whose focal ratio is at least 3 times the aperture as measured in inches will have acceptable color correction. For an 80mm scope this is about F/9.5. The standard 80mm F/11 has a "chromatic ratio" of about 3.5. I have owned several 80mm F/11s and all have had very similar color correction. There is some CA to be seen around bright targets like Venus, Jupiter and Rigel but it not overly distracting. Your 90mm F/11 Hardin with a C.R. of about 3.1 will show more chromatic aberration than a 80mm F/11, it should be quite apparent though not particularly distracting.

5X: An achromat whose focal ration is at least 5 times the aperture as measured in inches will have negligible chromatic aberration. For an 90mm Scope like your Hardin this is a Chromatic Ratio of F/17.7. Other scopes that fit this are common 60mm at the 700mm focal length. A scope with a CR of 5 will show essentially no false color, for practical purposes it will not affect the viewing but with care can be detected. Your Jason is likely either F/11.7 or F/15, both are quite free of false color.


My experience is that these rules of thumb are pretty accurate and that it is reasonably easy for a manufacturer to make an objective that achieves optimal color correction. One one would normally expect your 90mm F/11 to show considerably more false color than a 60mm F/11 or F/13, it just the way color correction works. Whether you Hardin has poor optics, that is another issue, is there something other than the level of false color that leads to believe the lens is not good on the Hardin scope?

This is worth reading I think:

http://geogdata.csun.edu/~voltaire/roland/color.html

There are other essays (at the bottom) and nice photos here:

http://geogdata.csun.edu/~voltaire/roland/

Jon

I have a Hardin 90mm f/11 with a fairly poor lens, my 60mm old Jason is much sharper, and am wondering if a "Vixen spec" lens will give less CA than I see in the Hardin,...

The "Vixen spec" is unlikely to give less CA,
unless its a longer F/ like F/15. The CA depends on the
glass types which are pretty universal for an achro, and
approximate curves which are unlikely to be very wrong.
All the achros I've seen have had pretty much the same
CA (for a given diameter and F/) regardless of how bad
the other aberrations were. Its not completely impossible,
but pretty unlikely.

Usually if they are using advanced glass types, terms
like semi-APO, APO, ED, SD, FPLxx, Fluorite, etc. will
be plastered everywhere. They won't be keeping it a
secret.

It's an interesting question. I hear people talking all the time about "well corrected achromats" as opposed to ordinary achromats, and it never made sense to me. I know that the designer can determine exactly which portion of the spectrum is "well corrected" (weighting it equally across the entire visual spectrum, or giving preference to red correction over violet correction, for example), but I was always under the impression that all achromats of a given aperture and focal length had essentially the same color error--assuming traditional crown and flint glass choices, of course.

That does, however, beg the question of whether poor execution of a design would result in different dispersion characteristics, or whether it would show itself just in other aberrations. My guess is that spherical correction and chromatic aberration are not linked--that the color correction would usually be unaffected by the execution of the lens. This, of course, ignores issues such as wedge error.