Here is an interesting attempt to get an apochromat using the least possible expensive glass types. It is kind of an academic exercise since the actual cost of an objective depends on several different factors, not just the cost of the glasses. Almost all telescope objective designs with enhanced color correction use one or more lenses with very low dispersion with anomalous dispersion behavior. And those ED glasses are very expensive. Ohara FPL53 glass costs 18X the cost of BK7 glass while FPL51 glass costs 11X as much as BK7. Those glasses have a very low index and are not just expensive, they are also soft and not chemically robust.
The design shown here uses as its most expensive glass Schott LAK8, which is 5X the cost of BK7 glass. It a high index glass and is a hard glass with good chemical properties. It has anomalous dispersion and is the basis of an apochromatic design here, where it is paired with two Ohara glasses (BSM15 and BSM16) that are 2.5X the cost of BK7 glass and one very high index Ohara glass that is 4X the cost of BK7 glass and has anomalous dispersion. Because the dispersion differences are smaller with these glasses than with the very low dispersion ED glasses, the lens powers are higher and the tolerances would be tighter.
There are probably glass choices on both sides of the LAK8 positive lens that would allow the first two lenses to be cemented without any performance loss, since there is such a near match now of the 2nd and 3rd radii. I have not tried to do that. Then there would be a cemented triplet followed by the last single lens.
The OSLO file is included here. The color correction is very good and it is diffraction-limited over a field about 1/2 degree in diameter from .4358u to .7000u for a 4 inch aperture f/10 design. This is, again, kind of an academic exercise since glass cost is only one factor in the total system cost.
It looks like Schott and Ohara glasses that are described as being equivalent, such as the BK7 glass of Schott and the Ohara equivalent are actually almost identical and can be switched in a design with no effect. But anomalous dispersion glasses that are billed as being equivalent are actually slightly different and they differ slightly in their anomalous aspect. A very small difference but significant. Schott LAK8 glass, the key to this design here, has an Ohara equivalent but it is just different enough so that the design needs to be changed to get back apochromatic correction. Good to be aware of this.
Edited by Get Sirius, 18 June 2020 - 07:41 PM.