Re: ATM Mime-Version: 1.0 (was ATM More Spheroidalage)

[Anthony Stillman <atmer@flash.net>]

In responce to Michael Peck's post.

>Anthony Stillman wrote:
>
>>Reverse engineering Couder's rule of thumb we see that it amounts to one
>>wave wavefront error.
>
>No, the formula in Texereau is correct.

I agree Couder's formula is correct.  It is the results of this formula I
present.


>What you're forgetting is you get to choose your reference wavefront by
>refocusing.

Wavefront is wavefront.  Piston either surface any amount you please and
the range in the difference does not changes.


>One wave of spherical aberration at the *paraxial* focus turns
>into 1/4 wave SA at the diffraction focus.

Longitudinal aberration, wavefront error, is not transverse aberration.
Rayleigh criterion references wavefront error, 1/4 wave wavefront error,
which for a spheroid at the diffration focus amounts to 1/16 of a wave of
transverse spherical abberration.  And thus, a tolerably degraded image.
The title to Texereau's table is just plane wrong and what the table itself
implies, what Couder's formula implies is that 1/4 wave of transverse
aberration, one wave of wavefront error is tolerable.   Well, perhaps this
is true, I do own a Meade.

Anthony



Updated Table

 transverse abb.    1/40       1/32       1/16        1/8       1/4
   diameter

     80 mm          14.0       13.0       10.3        8.2       6.5
    100 mm          15.0       14.0       11.1        8.8       7.0
    150 mm          17.2       16.0       12.7       10.1       8.0
    200 mm          19.0       17.6       14.0       11.1       8.8
    250 mm          20.4       19.0       15.0       11.9       9.5
    300 mm          21.7       20.1       16.0       12.7      10.1
    400 mm          23.9       22.2       17.6       14.0      11.1
    750 mm          29.4       27.3       21.7       17.2      13.7