Definition: An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
I believe in this case he is talking about compound lenses, similar to camera lenses. As far as I know there is no single lens that will correct for all color aberrations. A spatial filter needs to focus all light, usually one color, to converge at a pinhole. Anything that doesn't is blocked, so when you are using optics of any kind, it introduces aberrations into your system. In holography it is best to have your reference beam clean of any noise introduced by these optics. A spatial filter does that.
OK...Maybe CDBEAM is not an "Optics Nub"....But....I still have much to learn !!
Thinking about it....typically...refraction is accompanied by the prismatic effect...But...the discussion regarding "Achromatic lenses"...new to me....Well...whatever....I am fascinated by Optics theory....but....I am learning all the time ! It's all good...
I am more of a " Hands-on" person....we each have our talents...and the valuable part is to share !!! There are not too many days that pass...that I do not learn something about optics on LPF. :wave::wave:
You know I'm not sure what effect chrome aberration might have on collimation. I know it matters when trying to focus an image or a large field of colors through a tiny pinhole. That is a question I don't have an answer to off the top of my head.