FC-100: what glass is the front element?

I have from reliable source it is Bk7, yet from other sources that the glass is more exotic combination than FS-102 resulting in better color correction. One difference of course is the fluorite element moved forward and coated. Anyone?

The color curves that Tak published suggest that the element is KzF2. Normally fluorite is coupled with KzF2 in order to achieve a lens that is well corrected with all spherical surfaces. This gets you pretty good color correction with some error in the violet. One can also use KF3 with fluorite to achieve all-spherical design, but with less good color correction. BK7 would not be a good match unless the focal ratio is long (F12 or longer) or unless the BK7 is split into two pieces to form a triplet lens.

Roland Christen

I removed the lens elements from the cell, and carefully cleaned them with acetone. First of all, a fluorite doublet is assembled just like any other, glass on metal, with an axial compression ring and a threaded ring. Foil spacers.

For a 23 year old lens, the rear fluorite element has fared very well, no hazing, staining or fracturing. It is of course uncoated.

The front negative meniscus is coated both sides and also is very clean. The interferometer shows that a very flat spot has been polished on the convex exterior side during the apparent rework, then recoated. This defect mirrors the wavefront error, with twice the apparent amplitude as expected.

A friend in the biz is attempting to restore the surface to a nice sphere; he will be polishing the coating off in the process, of course. I have a quote for 75 bucks for a single surface AR coating, if this works out