Re: [ATM] Re:(ATM) F/10 or F/8?

[Michael Peck <mpeck1@ix.netcom.com>]

At 10:59 6/14/05, jsgrmfg@gracemfg.com wrote:


>aperture.  Accuracy criteria that may be more than adequate for deep sky
>observers fall short of the mark for the dedicated moon and planet watcher.
>Only the highest quality optics can take full advantage of those wonderful
>split seconds when our busy atmosphere takes a break and gives us a glimpse
>of an unblurred universe.

OK, here are some numbers for a 150mm diameter sphere. First recall that 
Marechal's lambda/14 criterion is just satisfied at f/8.22 -- this is 
basically equivalent to Rayleigh's 1/4 wave tolerance. At f/9.56 the 150mm 
diameter sphere just satisfies a corrupted version of the first 
Danjon-Couder criterion, namely the geometric rays just fill up an Airy 
disk at the circle of least confusion. The columns of the following table 
are (1) f/, (2) relative transverse aberration (RTA - for those who believe 
in the magic bullet theory of light propagation), (3) P-V (waves at 550nm), 
(4) RMS (waves at 550nm), (5) Surface RMS (nm), (6) Strehl ratio. Sorry if 
columns don't line up.

8       1.71    0.26    0.078   21.3    0.79
8.22    1.57    0.24    0.071   19.7    0.82
9.56    1.00    0.15    0.045   12.5    0.92
10      0.87    0.13    0.040   10.9    0.94
12      0.51    0.08    0.023     6.3   0.98

The quantities in columns 2 through 5 scale like D/F^3 if you'd like to 
check some other cases.

Just for comparison if you go to James Lerch's writeup at 
<http://lerch.no-ip.com/atm/Robo_V_Inter/> he has interferometry on 14 
mirrors made or owned by members of his ATM club. The measured Strehl 
ratios on those mirrors range from 0.46 to 0.95. Excluding the one clunker 
that was later refigured the range goes from 0.72 to 0.95. There's 
apparently one guy in James' club who consistently turns out small mirrors 
of very high quality. Just about everyone else makes light buckets.

Mike Peck


------
Michael Peck
mpeck1@ix.netcom.com

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