Re: ATM P-V vs. RMS

[Aplanatic@aol.com]

Hi Ken:

> Dave Rowe.  Did you ever claim a wave rating for your Slevogt? <SP?>  Do
>  you think anyone who saw his picture of the Rosette will forget it
>  anytime soon?  

Well, thanks Ken.  The Slevogt was tested and figured using a 10" f/6 
Newtonian as a collimator.  I hand figured the mirror in this test telescope, 
but did not get it much better than about an eighth wave P-V wavefront, it 
being my first paraboloid.  Using the test telescope, the Slevogt showed 
about 2.5 mils of residual longitudinal aberration when I quit fooling with 
it.  Given the error in the test telescope and the residual aberration in the 
Slevogt, it's hard to claim that the Slev is much better than 1/4 wave P-V 
wavefront, and maybe a little worse.  The star test shows significant 
departures from perfection, and when using a star as the source for the 
Foucault test, I can see a number of defects in the optics including 
considerable astigmatism, which changes with temperature.  (This is 
associated with the way I mounted the primary mirror, not great in 
hindsight.)  Nonetheless, these imperfections do not substantially affect the 
performance of the camera when using film, and medium format film has such a 
large image frame that 20 micron stars look like points anyway, even when the 
image is enlarged to 16X20" prints.  I would not call my Slev a good visual 
instrument, but it does work well as a camera.

Notice that I freely use P-V as a measure of the quality of the test 
telescope.  At the time that I figured it I did know about the virtues of RMS 
and Strehl, and subsequently have not bothered to reenter the data to find 
out how well I really did.

Dave Rowe.