From: Mark Holm 
To: IAPDK{at}ADMIN.DRAKE.EDU
CC: atm{at}shore.net
Reply-To: Mark Holm 


IAPDK{at}ADMIN.DRAKE.EDU wrote:

>>I'm not sure I understand your diagrams.  I imagine that what you are
>>describing has the folding mirror after the knife edge/mirror edge.
>>...
>>
>
>Sorry, there is no knife.  The mirror-air edge replaces the knife-air
>edge.  The knife is redundant.
>
>Paul Kline
>
Sorry, that is what I meant to imply.  If the mirror edge is smooth and
straight enough, I don't see any problem except the one of possibly not
being able to get your eye close enough to the mirror edge.

  How good does the flat need to be?  Probably any mirror good enough
for even a fair quality diagonal mirror is good enough, even if you are
pushing for a high quality primary.  Why?  Because the cone of light near
the cutting edge will be very small.  Only a very small patch of the flat
mirror will be illuminated.  Lets say a 35 mm diameter mirror has a  1/8
wave P-V surface eooro consisting of smooth curvature.  If the light cone
is only 1mm in diameter, the P-V error in that illuminated patch will
probably be about  1/280 wave, and that will be quite small enough.  The
only probalem with this rosy picture is that flats are said to often have 
a major portion of their errors close to the edge.  That is right where
your technique will be putting the light.
 In that case, more of the flat's error will be impressed on your
Foucault result.

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