: 1. How critical is the thickness of the spacers? They should all be the same thickness and .001" accuracy is generally good enough relative to the spec. although they should all be cut from the same piece of material to maintain the same thickness as that determines how much tilt you put into the lens set. : 2. What are the best materials to use as spacers? Plastics are good these days although paper and so forth have been used. Metal is the least as it has no real shock relief ability. : 3. What is the best way to make sure the spacers do not shift when the retaining ring is screwed back on. Use some thin glue of some kind (Canada Balsam? paint?) to hold them to one of the lenses so they don't move around.

: 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. Go get a copy of gsum or other refractor design program and play with the air spacing. You will note that the color correction changes (generally for the worse from what the spec is although you can change where the 0 crosses are) and other abberations also change while doing this. If you're unhappy with your lens, try changing the gap from 0 to about 4 times the present gap and see where you're happiest. My bet is that you are just about the best point right now although a very slight movement may improve the image slightly for you.