SV: ATM 8" f3.8 -- Worthy project?

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Dwight replied to Matt:


> ......The wire tends to make a ring
>at the RoC for that zone. On fast mirrors, this ring is
>quite distinct. 

This apparent free lunch may be misleading - the narrower the wire shadow appears, the 
more accurately you have to determine its position on the mirror. This isn't a 
disadvantage of the wire test as such (the same is true for the KE shadow), but rather a 
consequence of the general difficulty of measuring highly non-spheric surfaces with tests 
at the COC. I haven't tried it, but one way of optimizing accuracy would be using an 
accurate ruler, and read the position with a small telescope, as I believe Berry 
recommends for traditional Foucault - you may even have to compensate for the par
allax.

I also believe you have to be careful in ensuring that the wire appears at the same 
distance from center both sides - else you are not on axis.


>The disadvantages are that you need a
>bright point source as small or smaller than the wire
>and like I said, it hasn't been shown by any of the diffraction
>experts on this list as to the validity of using the center
>of the wires shadow as the measurement point ( although
>I'd think it was vary close to it ).

I haven't dug very deep into this, but it doesn't seem to me that the wire thickness or 
pinhole size are terribly critical - with a thinner wire, the shadow will be fainter, but 
not narrower. Also, if you use a slit, you need to align it with the wire. The shadows 
will be spread out in the direction along the wire/slit, but not in the direction you 
measure.

Nils Olof