Re: [ATM] Question about "Null-testing"

["Bob May" <bobmay@nethere.com>]

Like any other test, the Ross and other null tests can lie to you if you
allow them to do so.  The test is still a variation of the Foucualt test so
I'd not assume better reading quality than what the basic Foucault test will
produce.
I've found that with gentle long focus mirrorrs that you still need to apply
a mask to the mirror to insure what the surface shape really is.  A 1/4 wave
error on a mirror can be very difficult to detect by mere inspection of the
surface but will pop out sharply when a mask is applied to the mirror.
Using interfrometery (there's a Yahoo group for that which is fairly active)
that will show errors a lot better than what the geometric tests like
Foucault and so forth.
Over the last few years there has indeed been a revolution of ideas on
testing as well as making mirrors and lenses.  Some of this has been a
leakdown of the info from the professional side of things due to books like
Malacara's Optical Shop Testing as well as others and then there is the
amateur side trying to do things cheaper and faster than the old style of
doing things.  Tile tools, thin mirrors and other such stuff has been well
discussed in the last 10 - 20 years and more is to probably come in the
future.  I've also been quite active in trying to use a TV camera instead of
the ol' Mk 1 eyeball behind the KE of the Foucault tester to do the reading
of the shadows on the mirror.  Who knows what the next thing to happen in
the field?
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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