lenses

[mallordl]

how good are achromatic lenses

[mdstuart]

If the onjective lens is truly acromatic then it reduces false colours sometimess seen in cheap refractors...

[Capricorn]

No real answer to this.

In general an achromatic is defined as having 2 glass components and being designed to bring 2 wavelengths to focus at the same focal plane. The 2 glass components may be either glued together or as a separated doublet.

However many lens in scopes are doublets and seem to be classed as apochromatic. The William Optics Megrez 90 is a doublet lens (achromat?) and advertised as an apochromatic.

At the end the performance of the lens depends on how well designed, this in turn depends on the glass types used and to some extent the wavelengths used for the design.

[The Warthog]

A good achromat with a ratio of f/8 or higher should not give too much false colour. My f/9.5 gives very little, and that is easily cancelled by a minus violet filter. Before I had the -V filter, I used to put a yellow filter in for the same effect. My answer to your question would be, "Very good indeed."