Re: [ATM] Slit and source arrangment part 3.

["Arjan te Marvelde \(hetnet\)" <arjan.te.marvelde@hetnet.nl>]

The issue is that evaluation with a KE is neccessarily breaking the 
rotational symmetry of the (intended) conic: it really only has meaning in 
the direction perpendicular to the knife edge (i.e. usually horizontal). 
This is why offset in this direction will influence the measurements and 
offset along the KE won't.
This holds for the horizontal diameter (equator) in theory, but in most 
practical cases also for areas nearby. I can imagine though, that an error 
may be introduced when using a Couder mask on very fast mirrors, with 
considerable vertical offset between source and KE.

If you want to make a measurent without breaking the rotational symmetry of 
the conic, the lightsource must be a point and the KE be replaced with a dot 
of a size similar to the point source. This could be constructed with a 
glass reticle or photonegative or so, but it will be very hard to achieve 
zero offset between source and dot. Maybe an optic fiber could serve as a 
source here.
Theoretically you would see a ring on the zone under evaluation.

- Arjan

> Actually, you are wrong here.
> A slitless tester will not show errors in the direction of the
> line between the source and reading edge.  Typically this is in
> the vertical direction.  It will show accurately errors in the
> horizontal direction tho.  Putting the source and the KE
> horizontally won't show any problems in the vertical direction
> but will show all of the error in the horizontal direction and
> those, assuming a vertical alignment of the KE, will definitely
> be in error.
> Only the alignment where the distance between the source and the
> KE from the mirror will show a proper null for a figure of
> revolution properly.

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