Re: ATM Figuring/Testing my 8" f8

[Aplanatic@aol.com]

Paul Kline wrote:

> I am in the process of figuring my first mirror, an 8" with 66" fl (~ f8).
>  
>  It should be possible to null test this mirror against an elipse with near
>  focus at 89.8" and far focus at 20'9".  To be safe I place the light
>  source (led + pinhole) at 89" and find the far focus a little beyond 21'.

I hope I don't understand your test methodology.  If you are trying to null 
test a *parabola* but placing a light source at ~89 inches and examining the 
mirror with a KE at ~210 inches, then you have a problem.  If the mirror 
nulls in this arrangement then you don't have anythin close to a parabola, 
but an ellipse (which you mention, hence my confusion).  

The ellipse you get by this null test will give you big, ugly blobs for stars 
when the mirror is focused at infinity, roughly 3 arcseconds in diameter.  
Not good.

Dave Rowe.