RE: [ATM] Dall Kirkham secondary update

["Scott Milligan" <starzkey@charter.net>]

Rod:

If you make the test plate match with one piece off-center a bit with
respect to the other and note the position of the zonal irregularities, you
may be able to distinguish whether or not those zones are in the cx or the
cc piece.  For example; make the match such that the center of the cx piece
contacts just inside the edge of the cc, and note where the zone
corresponding to the central hill seen in the centered match is.  If it
stays put in the center, then you know that this zone is in the cc, but if
it moves out to the edge, it is in the cx.

Make sure to clean off any dust particles etc before making the match, and
don't slide the two disks across each other when in contact for any reason.
(Scratches).  Also, I would advise waiting for any accumulated heat in one
or both disks to dissipate fully before interpreting a match that is close
to showing finished; pyrex test plate matches can show false astigmatism,
power, and zonal irregularities when not at thermal equilibrium.

Scott Milligan


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Rod Brackenridge
Sent: Monday, November 22, 2004 7:20 AM
To: atm@atmlist.net
Subject: [ATM] Dall Kirkham secondary update

Hi,

Last week several people assisted me with my confusion over testing and
working a convex secondary.  As Bob May advised, I used longer strokes in
order to reduce the convex secondary's edge.  With my original hard lap
there was no change in the surface.  I then made a softer lap and work is
progressing nicely.  The number of bulls eye fringes gradually reduced and
now I have 8 fringes over the secondary.  They are not straight.  There is a
hump in the centre and a slight trough further out, both a little under a
half a fringe in size.  I think this is caused by zones on the reference.
Someone else tested my reference sphere and noted a slight hill and ring I
had not seen when using a ronchi grating.  They were just visible with a
foucault test.  I think these zones are caused by my polishing machine.  

I intend to work the reference by hand and remove the zones.  Hopefully I
will then have nearly straight lines.

Thanks for all the help.

Rod Brackenridge
Mt Martha
AUSTRALIA


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