Re: [ATM] two mirror folded Newtonian

["vladimir sacek" <vla@toast.net>]

Nils Olof Carlin wrote:

> A Newtonian secondary has no optical axis to bother about, the
> hyperboloid Cassegrainian secondary has.

Doesn't matter. The consequences of moving it from its perfect position will 
still result in the field shift bringing coma to the field center (unless it 
slides in the plane of its surface, which is possible but unlikely 
scenario).
>
> A placement error of the secondary of a Newtonian is readily
> compensated by adjusting its tilt - coma is only determined by the
> miscollimation of the primary's axis at the focal plane. Not so with
> the hyperboloid secondary of a Cassegrain - if a sideways displacement
> is compensated by adjusting the tilt, the optical axes are misaligned.

And so are with the Newtonian. Logical assumption is that we start with 
nearly
perfectly collimated systems. If so, any change in the diagonal placement 
can be
compensated by adjusting the tilt only if the focuser is also re-adjusted. 
Not a
simple matter. Btw. why should compensating decenter by adjusting the tilt 
be
preferred to fixing the decenter itself?

>
> Also, the distance primary to secondary is quite uncritical in a
> Newtonian - far from so in a Cassegrainian.

Not true. If we start from near perfect alignment, and the diagonal alone
changes its distance from the primary (defined by the location of the point 
of intersection
of diagonal's surface and primary's axis), it will directly cause the 
primary
axis to move by as much off the field center, bringing in the coma.

The Cassegrain is also less than half as sensitive to miscollimation of the 
primary
alone. A 0.1 degree primary tilt would result in little over 1/4 wave RMS of
coma in the field center. In the Newtonian, it would bring 5.6mm off-axis
point to the field center (0.6 waves RMS of coma). Comparatively, the 
numbers
are not kind to the Newtonian.

Vlad


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