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

[jsgrmfg@gracemfg.com]

Michael Peck wrote:

>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

No doubt about it, a spherical 6" F/10 is a fine mirror. Better than you
are likely to get in a commercial telescope and better than most people
make for themselves. For 99% of visual observing and photography, such a
mirror, if it is also smooth, will give entirely satisfactory service. But
the remaining 1% are planetary observers.  The objects they seek, the
images that make their hearts pump faster are very small and, more
importantly, very low in contrast. The tiny 6% of light energy that strays
beyond the Airy disk (.94 Strehl) is negligible if you are looking at the
Ring Nebula but can make the difference between visible and invisible if
you are looking for subtle changes in the Great Red Spot. For the planetary
observer there is no such thing as a primary mirror that is better than it
needs to be. For him, if for no one else, a 6" F/10 should be parabolized.

Jim Sturtevant

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