Spacing in Fraunhofer acrhomats

Hello all,

How does spacing affect the spherical correction of a Fraunhofer achromat? I have one which had a 1mm plastic spacer ring which got broken and could not be repaired, so I used several layers of aluminum foil to achieve spacing. The spacing went from about 1mm to a fraction of a mm (just enough to forestall the appearance of Newton rings).

The resulting objective performs fine for its purpose (terrestrial and guide scope use) but the star test reveals some very slight spherical aberration. I never bothered to test it when it was spaced at 1mm so I'm wondering if restoring the original spacing will correct the spherical error.

Thanks in advance.

-drl

Dan,

You mean that the original spacing of the doublet was ~1mm via the plastic ring spacer and you respaced it to just a fraction of a mm with three foil spacers? Question? Why would you not space to the same degree (thickness of spacers) as the original spacers? Also, are you saying that you spaced so that you wouldn't see Newton's rings? Could you see them originally with the old ring spacing?

From my limited understanding, basically, greater spacing can create overcorrection (extra focus rings appear strong while intra focus appear less clear or mushy). Relatively less space will create undercorrection, appearing opposite of the above.

If the focal ratio of the scope is quite long, you might just try acquiring a set of three foil spacers from a telescope manufacturer. This would be a base to start from. If they need to have more space, you could just add tiny burnished square(s) of gum wrapper foil, well burnished, no folded corners, to the spacer(s). Reason for this too is that the real foil spacers have tacky substance on one side (goes to the flint surface) and keeps the spacers steady and in place,,,MUCH easier to work with, especially when fianlly stacking lenses and remounting on station.

Good luck,
Bruce