Re: ATM How to test a Gregorian secondary mirror

[Anthony Stillman <atmer@flash.net>]

>Gregorian telescope, ....how to test the
>secondary mirror.


Wah!

The classical gregorian's secondary is a prolate spheroid (ellipsoid).  As
such it may be null tested by using unequal conjugates.  That is, placing
the light source and knife edge or ronchi screen on axis and different
distances from the mirror.  As the closer of these will shadow the more
distant, a convenient implementation is to use a fiber optic light source
close to the mirror.  One such light source, described the archive, buries
the non-business end of the fiber in an LED.  I've used this arrangement
several times and have always been disappointed with the low illumination.
These days I use a 12 volt "grain of wheat" light bulb, run at 9 volts, hot
glued edge on to a popcicle stick or thin one inch wide stick of balsa.
Also, I'm careful to align the bulb so that the filament is also edge on.
Though, for some reason it never seems to stay that way.

As this a null test, ronchi lines will appear straight and under the knife
the mirror will gray uniformly when the figure is correct.

The location of the conjugates is important.  The conic constant CC, of the
secondary of a classical gregorian is: { - ((m+1)/(m-1))^2 }. The
eccentricity e, is: { (m+1)/(m-1) }  when m, POSITIVE for a gregorian, is
the factor of magnification.  The distance between the two foci is: { 2*a*e
} where a is half the major axis.  Finally, the distance from the center of
the mirror to the nearest focus is { a*(1-e) }


Anthony