[ATM] Initial collimation

[Bob May]

No, the secondary is a flat mirror.  As such, there hsould be no
focus point on it.  Now, the edge shape is an elipse which is
generated by putting the glass at an angle and cut into a
spherical shape.  This gives an eliptical shape to the edges of
the mirror which is what is being discussed.
In all of these discussions, rememver that optically a flat
mirror does nothing to the optical properties of the telescope.
It is merely used to redirect the light beam from one point ot
another and, as a result, is not used when calculating the
optical characteristics of a telescope mirror/lens system.
All of the discussion here has been on how to put the flat
secodary mirror into the "perfect" spot to maximize the light
passing by the mirror and nothing more.  For short scopes, it
does help to get the mirror positioned a bit away from the
focuser as the 100percent illuminated field is a bit offset from
the center of the paraboloid's axis, go draw out what the ligh
path is for the edge of the primary fo what happens.  You will
see that the cone of convergence of the light will indicate that
getting a full illumination of the focal plane concident with the
axis of the primary means that the further end of the flat
secondary needs to be a little further away from the axis than
the nearer side of the flat secondary but this only comes into
play when the actally viewed image is large enough to actually
see a part of the focal plane that doesn't have full
illumination.  At higher powers, the EP will be seeing only the
fully illuminated field  For those who only use higher power
EPsl, it comess that a smaller secondary can be used which will
improve the contrast by a small fraction of a percent.  I'll also
note that the loss of brightness from using a small secondary is
a small loss as a 50percent loss of brightness is only a
magnitude of brightness and in a really widefield scope, it is
fairly unobservable as the coma of the scope will give at least
that much loss of the brightness to a star's image anyway.
Bob May

rmay at nethere.com
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