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Re: [ATM] Figuring question

Hi,

Jim Burrows wrote:
> Sixtests agrees with Figure XP (as always), zone 7's deviation relative to 
> RMS-minimizing parabola (R = 3149.891 mm) is -20 nm and the edge is ~+50 
> nm.  Also, the RMS surface error is only 17.5 nm (±2.5 nm, assuming 
> readings are ±0.002") = 0.852 Strehl.  It's good enough unless you are a 
> perfectionist, in which case you want to proceed VERY carefully - no 
> overshooting.  I agree with Mike Lockwood, it's probably easiest to move 
> toward a slightly shorter parabola, R = 3149.791, for which you have a +60 
> nm high center, implying one of Texereau's "deepening the center" strokes, 
> but only a couple of minutes before testing again to see if you've gotten 
> the Strehl up to the pefectionist's level.

I certainly am a bit of a perfectionist!  But so few people really 
give the outer zones the time they deserve, which is a shame.

I don't understand why AMMs (amateur mirror makers), who have plenty 
of time to make their mirror, will readily attack "zones" farther in 
on the mirror (often near the center) with fervor and dogged 
perfectionism while they are strangely content to leave the outer 
zones all out of whack (usually markedly undercorrected).  I see this 
a lot in commercial mirrors, too.

The outer zones are arguably the MOST important because they have the 
most AREA.  The corrections between the outer two or three zones are 
absolutely critical to performance at high powers.  Unfortunately the 
P-V ratings AMMs typicaly rely on don't take the area of the zone into 
account (though strehl does!).

Yes, the mirror had a claimed Strehl of 0.85 or so, but the transverse 
error was 4+.  That's not done in my book.  The mirror maker should 
not be afraid of losing the figure they have - if they got there, they 
can get back!

The difference will be evident when staring at a planet at 600X some 
steady evening - if you take the time (and practice) to get the outer 
zones right, you will have a magical experience at that power (which 
is 50X per inch for the 12.5" mirror in question) that will probably 
never be forgotten.

	Slowly correcting the outer zones of a 32" F/4,

	Mike Lockwood

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