: Has anyone ever replaced the three spacers in an air-spaced achromat? If you have some experience with this, I have some questions: : 1. How critical is the thickness of the spacers? : 2. What are the best materials to use as spacers? : 3. What is the best way to make sure the spacers do not shift when the retaining ring is screwed back on.

: A second issue relating to the air-space distance, has anyone experimented with the seperation and/or rotating the doublet for better color correction and alinement. Excuse my spelling.

Try to find some adhesive backed, heavy aluminum foil. I found some in a Salvo Outo Parts store. It's made by Bondo and is intended for body repair. It measures about 0.0003" thick and can be layered to make up different thicknesses. I used three layer and cut three shims for my 4" f-10 achro. It worked well and I didn't increase the secondary spectrum. It's also inexpensive.