[ATM] Re:(ATM) F/10 or F/8?

[jsgrmfg@gracemfg.com]

>Can anyone tell me if there would be any advantage in making a mirror
>in an F/10 vs. a F/8?  Seems like everything is a an F8.  This will be
>for a 6" mirror for primary use in a planetary scope.

  F/8 is common and often recommended because it is the longest focal ratio
that can be tested easily by a beginner.  An F/10 is too short to leave
spherical but so close to a sphere that the Foucault shadows are very
subtle and hard to read.  I read somewhere that if you are testing an F/10
and can see a definite Foucault "donut", you are already way overcorrected.
The lateral wire test might be a good solution for the subtle shadow
problem but building the tester might be more trouble and complication than
most beginners want.

Jim Sturtevant

_______________________________________________
ATM mailing list http://www.atmlist.net/