(Jon Isaacs) wrote in message ...

It appears to me that this scope does a reasonable job, seeing 6 diffraction
rings at high power is a sign things are pretty good.

I am wondering if you just asking too much of this scope...

I think not. Any scope of mine that showed 5 or 6 bright diffraction
rings
in focus would be headed back to the pitch lap. All my scopes have
strehls
at or above .8, and none shows that many bright rings, filtered or
not.

Refractors with the typical Faunhofer design, as this one is, are
aplanatic, meaning not only being comma free, but are nulled to 3rd
order spherical correction if all surfaces are spherical(easy for
machine polishers) AND if the glasses are close to the proper indices
and dispersion values. This is a problem in cheap, massed produced
refractors, and when these values are
off (especially for objectives this fast), errors in third order
correction and/or color correction are common. The former can be
overcome with figuring one of the surfaces--usually R1. A green light
strehl of at least .8 should not be too much to ask for considering
the ease and low cost of the fabrication of these massed produced
refractors. Resolving high contrast features near the
limits for the aperture should not be a problem for a short achromat,
but don't expect great contrast on planets--even with an error free
example of
one of these scopes.


Dan Chaffee