Re: [ATM] Collimating, the eccentric way

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

About elliptic secondaries:

I haven't thought much of it, but...  A  "standard" diagonal with short:long
axis 1:sqrt(2) will look like a circle when seen at an angle of 45 deg, from
infinity. Seen from focus, it will be an ellipse, but how much difference
from a true circle? I did a calculation for f/4.5, and found it will be an
ellipse flattened in the direction of the tube, the longer axis about 1%
larger than the short axis - the difference is less with larger f/ ratios,
and bevels on real-world secondaries may matter more.

Or one practical problem: you can Both tilting and rotating the secondary
will move the reflected axis "sideways", and if you move it one way by
tilting, and back to centered by rotating, you may end up with a skewed
secondary, looking like a "tilted" ellipse. This won't cause astigmatism or
such nastiness, but the secondary may be decentered sideways by perhaps a
significant amount. If present, this would likely be detected by looking at
the secondary from a distance (near the COC, with the reflection of your eye
pupil flooding the primary) - the secondary or its reflectionwould look
tilted.

Not the major collimation problem, but one that perhaps deserves a look.

Nils Olof

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