Chromatic Aberration Correction Eyepieces

[Refractordude]

Besides the Baader BCO, what other eyepieces are there for this achromatic flaw. Thanks

[Refractordude]

I know no eyepiece can do a 100 percent job, but what eyepieces do some cleanup of the violet color?

[notFritzArgelander]

To my eyes, none. I don't think the BCOs do in general either. There may be individual cases where a BCO might help with a particular scope, but the question is unrealistically simple. One needs polychromatic ray trace diagrams for a candidate scope and eyepiece pairing to sort this out.

[Lady Fraktor]

Well made modern Huygens have been rumored to do a fair job in longer focal length refractors so if you are using anything less than f/12 I cannot imagine there would be any meaningful difference.
Personally to me, at this focal length and longer CA is never a distraction anyways so not really concerned about it.

There is a Plossl eyepiece (variant) that JG tested last year that seemed to offer a bit of violet reduction with his 150mm f/5 refractor.
The review was at AF.net

[Bigzmey]

Other than a couple of anecdotal evidence, I don't believe EP can correct telescope's CA. On the other hand wide angle EPs can introduce lateral color even in CA free scope designs.

If you want to mask CA you can use yellow or semi-apo filter.

[Refractordude]

Well made modern Huygens have been rumored to do a fair job in longer focal length refractors so if you are using anything less than f/12 I cannot imagine there would be any meaningful difference.
Personally to me, at this focal length and longer CA is never a distraction anyways so not really concerned about it.

There is a Plossl eyepiece (variant) that JG tested last year that seemed to offer a bit of violet reduction with his 150mm f/5 refractor.
The review was at AF.net

I was reading that same review, which caused me to ask the question.

[Refractordude]

Other than a couple of anecdotal evidence, I don't believe EP can correct telescope's CA. On the other hand wide angle EPs can introduce lateral color even in CA free scope designs.

If you want to mask CA you can use yellow or semi-apo filter.

I have a #8 yellow that works like a charm. However, I don't like the yellow color for the moon and Jupiter. The 90mm aperture mask I made works also.

[Bigzmey]

Other than a couple of anecdotal evidence, I don't believe EP can correct telescope's CA. On the other hand wide angle EPs can introduce lateral color even in CA free scope designs.

If you want to mask CA you can use yellow or semi-apo filter.

I have a #8 yellow that works like a charm. However, I don't like the yellow color for the moon and Jupiter. The 90mm aperture mask I made works also.

Yes, stopping down the aperture increase your scopes focal ratio and as a result decreases CA. This however also decreases scope resolution. For F8 and slower refractors I would rather take CA, than stop down the aperture. For fast achro fracs and newts stopping down on planets and Moon is necessary evil.

I also saw a couple of comments that a prism diagonal can in some cases decrease CA. I have yet to verify that.

[Lady Fraktor]

When doing a star test if the first ring intrafocus has a red tinge a prism may help, if it is white or neutral a mirror will likely be better.

[j.gardavsky]

CA correction in the eyepieces

Hello all,

practically all advanced dsignes of the eyepieces correct to some extent either the longitudinal (center field) CA, or the lateral (peripheral) CA - the so called ring of fire, when designed for the achromatic and ED (not full APO) optical systems, like micrscopes, binoculars and telescopes.

I have done the first tests on the longitudinal CA in 2017, http://www.astronomyforum.net/telescope ... ieces.html , and further tests will be done with some added equipment next year.

The longitudinal (center field) CA is, when it is corrected, then as a rule on the blue wavelengths, see the above link.
The lateral CA correction is readily visible as a slim orange ring on the field stop pointing in the field. This type of correction is often seen in the Swarovski and ZEISS designs. Historically it has been the first time applied to some Abbe orthos, later to the Carl Zeiss Jena PK, and similarly ZEISS West.

An opposite to the longitudinal CA correction is the undercorrection, well visible as a blue ring on the field stop, pointing in the field. This is typical for most of the wide field and ultrawide field astronomy eyepieces.

Best,
JG