Does anybody make a Fluorite ca filter?

The chromacor was a sub aperture corrector to address chromatic aberration common in
refractors. It was very effective for visual use at eliminating the unfocused purple. It has been out of production for years.

Recently I have been wondering if the addition of a fluorite filter would deliver similar results, and if anyone has come out with a fluorite filter or an alternative to the standard minus violet filter. Thank you, Bill

Hi Bill,

First, Happy New Year (well, another 100 minutes or so for me) !!!

Second, I'm going to try and get Tom Dey or one of the refractor gurus to reply to your question, but first, I'll take a stab at a quick answer for you.

The "Fluorite ca filter" probably doesn't exist. The reason is that the ca you are asking about (in inexpensive doublet refractors) is introduced by the choice to use only two lenses: Usually Crown Glass and Flint Glass which have differing "dispersion" properties that aren't exactly complementary. The goal is to "inexpensively" make Red & Blue light focus at the same focal plane and have Green light focus at a very similar plane. The problem is that other colors (farther away from Green than Red or Blue = mainly Violet) don't focus close enough to be well corrected. There are a number of filters that you can explore to deal with the violet, but I digress . . .

The fluorite lense you refer to is an alternative glass that has "ED = Extra-low Dispersion", so that it gives less of this false color, but at a "price", since the Fluorite Glass is more rare and more expensive that what it replaces in the "two lense system" that is chosen because of the low cost. ED glass doublets work very well, but sometimes it is actually less expensive to a a third lense aka Triplet scopes.

The better way to go is to use one (or two or more) additional lense(s) to correct more things (like ca and flatness of field and spherical aberation, etc.) and bring a better color balance to the focal plane. The four lense Petval system uses four lenses so that more of the ca is taken care of compared to the two lense system, but is more expensive (You get what you pay for).

Your quote:

"The chromacor was a sub aperture corrector to address chromatic aberration common in refractors.
It was very effective for visual use at eliminating the unfocused purple.
It has been out of production for years."

as you say.

The cleverness of the chromacor (from the Ukraine, I believe) was that it corrected for (some or most) of the ca, if you carefully matched which chromcor you choose to go with "your" doublet, you could additionally address some or all of the SA = spherical aberation as well. The additional cleverness was that one could use the chromacor on a big (6 or even 8 inch), inexpensive doublet without any modification (by just inserting it in the 2 inch focusser at the "small end" of the refractor). Really a nice solution !!! Too bad they are getting hard to find !!!

Ed

P.S. Hey Tom, care to elaborate ??? Anybody else want to jump in ???

Bill Barrett said:

. . . eliminating the unfocused purple . . . fluorite filter or an alternative to the standard minus violet filter. Thank you, Bill

Hi again Bill,

Here's a good reference or jumping off point for "an alternative to the standard minus violet filter" that can be found in the "Members" section tab. I don't exactly agree on all the points and conclusions, but most of what is said is a very good write-up:

https://www.astromart.com/articles/article.asp?article_id=922

Cheers,

Ed