Re: [ATM] Testing Cassegrain Secondaries

[Mike Lockwood <melockwo@uiuc.edu>]

Cary,

Guy Brandenburg wrote:
> Seems to me that the best method would be to first
> grind, polish, and figure a CONCAVE mirror that has
> precisely the same (but opposite) figure as the convex
> one that you want. Then you build a monochromatic
> light source and you use interference testing to see
> how close the two surfaces are. If the interference
> fringes are straight, you have hit the nail on the
> head perfectly. You can easily quantify the number of
> fringes that a straight string crosses.

This is the technique of a test plate.  Texereau used spherical test 
plates, I think.

If you are only TESTING secondaries, this method is very labor 
intensive, because you have to fabricate the test plate.  For weakly 
corrected secondaries with smaller conic constants, figuring the test 
plate to a sphere of the right ROC, photographing the fringes, and 
then doing some analysis might be sufficient.  However, I think in 
most cases testing is more easily done in autocollimation, or with a 
convex lens through which the mirror can be Foucault tested.

If you are making NEW secondaries from scratch, test plates work very 
well.  I make two new secondaries for two Cassegrains in this manner:

http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/16in_CUAS_cass/secondary/secondary.html
http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/12in_cass/constr3.html

Make sure you get the whole link into your browser - sorry they are so 
long.  These Cassegrains are excellent performers.

As Mark Holm later pointed out, you do have to match the radii, but 
once you get your test plate figured, you just work away the high 
zones as shown by the fringes, just like figuring a flat.  I did not 
leave my test plates spherical - I corrected them, and tested with the 
Foucault test.

For figuring the convex mirror, petal laps and a turntable are very 
useful.  As a bonus, when you are done you can box up the test plate 
and use it again if you even want to make another secondary with the 
same ROC and conic constant.

Bottom line, test plates work.  I have a friend that used a test plate 
to figure a 6" Cass secondary with a conic constant of -8.  He said 
this type of aspherical shape was quite doable.  (I don't know the ROC 
of that secondary.)

	Mike Lockwood

_______________________________________________
ATM mailing list http://www.atmlist.net/