Re: ATM about dog biscuit and pitch lap channels

[Krajci Tom Det5 57 Wg <Tom.Krajci@barksdale.af.mil>]

From: Hugues_Laroche@aia.com

>As per the thread running about effects of channels
>width and depth, is it not recommendable, to avoid
>"dog biscuit" effects, to make the laps small squared,
>deep channels i.e. more "flexible"? Does the same
>not improve behaviour for figuring fast or strongly
>deformed mirrors?

For the last eighteen mirrors I've used 1/32" wide/deep channels and hard
pitch that doesn't flow much.  The mirrors I make are slow (f/6 to f/10.5)
and on the test stand it's very easy to see ripple/dog biscuit at longer
f/ratios.  If my room temperature is not too cold I have almost no sign of
dog biscuit.  The air in the room has to be very, very still before I can
see dog biscuit.

Also, I make my channels in a 120 degree/triangle pattern instead of 90
degree/square.  I scrape the channels by "freehand" and don't care if my
spacing/alignment is somewhat random/disordered. . .in fact I want that to
some extent.  Maybe these things also help suppress dog biscuit?

>Sorry, that are just questions, I hope not to be
>"beside my shoes" as would say French people...

Interesting expression!  ;-)  I don't know what an English equivalent would
be.

Tom Krajci