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Re: [ATM] ATM Digest, Vol 53, Issue 21
I am not so sure about this
If you consider a mirror of radius=h and ROC=R, the the focus of center and
edge rays differ by an amount:
p=h^2/R.
The transverse aberration of an edge ray whenthe KE is at the center ray
focus position, is:
t=p*h/R
For a 10" F/5 mirror this amounts to more than 300um, to one side only. This
is enough to let the entire 200um slit image pass the knife edge. In this
case narrowing the slit will affect what you see: I suppose that the bright
parts of the Foucaultgram become dimmer with a narrower slit. This might
decrease the swamping of subtle details in the grey parts. However, I don't
think it will affect the measurment of overall correction.
Diffraction of the slit gives no practical problem with slit sizes down to
30um, according to Jean T...
I have used both, but never did an objective comparison. Anybody on the list
actually did ?
-Arjan
> Quite true, the slitless and slitted tester operate identically when the
> KE
> closes down enough to show shadow detail. I've taken hi-res foucaultgrams
> that
> illustrate that very fact - and using a very narrow slit doesn't improve
> detail, merely adds diffraction artifacts. If anybody's interested in
> seeing
> them I'll send you a copy. BTW I now use a 200 micron slit on my tester -
> same
> as you.
>
> I just didn't get what the original question was about...
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