Telescope Reviews: Understanding air-spaced achro objective spacing?

Hi all,

Most of us are aware that the spherical correction characteristics of air-spaced achromats (and probably ED's as well) can be altered by changing the distance between them. I've played around with this a lot on my C6-R and, more recently on my Astro Telescopes 102 achro.

My process is currently to use a yellow and/or green filter, star-test, alter the spacing, test again, repeat 'till the green/yellow star-test looks the best. Seems to work for the color it's tested under, but I can't be sure what impact it has on other colors, and how it affects contrast. I have noticed that as I improve the star test in green light the purple gets worse.

One possibility I'm toying with would be to make a Bahtinov mask, place a color filter on the eyepiece, focus on a star using the mask (in order to hopefully attain exact focus), change the color filter, re-focus and measure the difference in focal distance for red, blue, yellow, and green. Then, adjust the spacing significantly, re-test, and compare results. Hopefully this would help me understand what's going on, and help me make the best spacing choice.

I'm not really sure what color crossing I need at closest focus-- I assume that'd be yellow and green...

Anyone have some suggestions? Should this be posted in the ATM/DIY forum instead?

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Kerry