Re: [ATM] Hill or hole?

[GARY FUCHS <gary@rcn.com>]

Dave,

Have you read this?

http://www.atmsite.org/contrib/Harbour/Foucault.html

If not, I believe it will help answer your question(s). It took me a few readings of the beginning section but once I'd "gotten" it I could think my way through what I see. At least I think I can...

I'll boldly take a crack at an answer and if I get it wrong I'm sure someone will correct me!

When you see the shadow advancing on the side that your knife edge cuts in from that means you are inside of focus for that region, annulus, or zone. The light being reflected from that side of the mirror hasn't crossed the focus point yet. So, say your knife edge starts on the left side and moves right and you see the shadow doing the same. You are inside focus for the area or zone where the shadow does that. Move the tester stage or whole tester farther back and try again. When the shadow advances from the opposite direction you are outside of focus for that zone. Move the tester back in and when the whole zone dims (nulls) evenly you are at focus - which it will only do if it's spherical. Where the shadow is in the other zones will tell you whether they are flatter or deeper than whichever one you're nulling.

Regards,

Gary Fuchs

---- Original message ----
>Date: Tue, 29 Jan 2008 23:58:29 -0500
>From: "David Bevel" <dhcb@bellsouth.net>  
>Subject: [ATM] Hill or hole?  
>To: <atm@atmlist.net>
>
>Hi All,
>
>
>
>I have been wondering about an issue I have concerning Foucault images for a
>
>while now and thought I would see what you folks have to say about my 
>conundrum.
>
>
>
>I have a mirror that has a problem. I have either a hill or a hole but not 
>sure which.
>
>Looking at images I have seen on various sights that provide examples of a 
>hole and a hill
>
>and having reviewed the remarks that accompanied the images I wonder how one 
>can tell
>
>what you have unless you know for a fact what either condition looks like in 
>reference to
>
>which direction your knife edge comes from  as it approaches the mirror. I 
>would think your
>
>image of the mirror produced with a knife edge that moves left to right 
>would be a mirror of
>
>one  produced with a rig that brought the knife edge in from right to left. 
>In essence a mirror
>
>defect could appear as a hole to one test rig and a hill to the other.
>
>So lets say I have image taken with a left to right moving mirror and my 
>image shows a deep
>
>shadow on the left side of the hole/hill and a brighter area on the right 
>while the rest of the
>
>mirror outside the hill/hole is bright on the left and darker on the right. 
>What have I got?
>
>I also, hope I don't have any prints of the image upside down.;) Now for you 
>highly intelligent
>
>deep thinkers this may be blatantly obvious but to a fellow who gets 
>headaches doing a simple
>
>tax return uh, No!
>
>
>
>Thanks in advance,
>
>
>
>Dave Bevel 
>
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