Re: [APML] Another intro

[Chris1011@aol.com]

In a message dated 8/22/2001 12:38:38 PM Pacific Daylight Time, 
katzung1@home.com writes:


> The radius of curvature for the mag 5 secondary system is -160 mm
> and that for  the mag 2 secondary system is -3287 mm. Much larger radius of
> curvature = much flatter field


Field curvature is a function of the secondary radius. In the case of the 
9.25, the secondary is shallower, but not by enough to significantly decrease 
the field curvature, i.e. it is not flat. By going from F2 to F3, the field 
curvature goes from about 7" to about 14". The downside is that the secondary 
intercept is larger by the same ratio. That means that the secondary 
obstruction has to be larger to provide the same unvignetted field by a ratio 
of 3/2. One way to get around this is to make the secondary a bit larger, say 
2.5/2, and to let the off-axis image vignet more. Unfortunately, there is no 
free lunch.

In the above example quoted from Rutten, the secondary radius of a 2x system 
is very weak and close to that of the primary mirror. If they are the same, 
the field is theoretically flat. However, in order to cover a minimum of a 1" 
field for example, the secondary size is close to 50%, and the baffling 
required for a fully baffled system would result in a 60% secondary shading.

Roland Christen