RE: ATM 8"f/7 astigmatism

[Mark Suchting <masuch@cia.com.au>]

At 07:37 PM 10/4/2003 -0500, you wrote:

>  Note:
>  I took a piece of alum. foil, put it on the kitchen counter (hard surface)
>and pressed the point into the foil, the pin did not go through the foil.

Dave ,

Hone the needle point , by rubbing it on  an oily  knife sharpening stone, 
or even fine abrasive against a piece of glass.  Fold a wad of foil to many 
layers, then stab the foil with the needle and unwrap it to find your 
smallest hole.

  If your needle hole is bigger than the Airy Disc for a spherical mirror 
at the center of curvature, in your case , at F14 , the hole needs to be 
smaller than 20 micron. If not , you will see an image of the pinhole with 
your eyepiece, not a real Airy Disc. But in defocussing, the image you 
should still get a feel for slight astigmatism by watching how the light 
comes to focus and the circularity of the defocussed discs.

All these bench tests are best done with your mirror spherical.

~Mark  Suchting