From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi Goran
 You need to consider what portion of the secondary contributes
to each spot of the field of view. In the case of a typical secondary used
for visual use, each spot at the focal plane used a circle that is only
about 0.5 inches less than the minor axis. This means that the secondary
needs to be good over most all of its surface. In the case of the filter,
each spot at the focal plane uses only a vary small amount of the filter.
One has to visualize the effect for all the ray paths that contribute to
that spot. Anything outside that has no effect. Dwight


>From: "Goran Hosinsky" 
>
>Is the distance from focus not an important factor? I have learned that
>when using filters you should
>place them as near focus as possible, just far enough off that you
>cannot see the dust on them.
>Goran
>La Palma
>
>mdholm{at}telerama.com wrote:
>
>>Certainly it is foolish to become mesmerized by primary mirror quality and
>>miror cell induced deformations without also considering the quality of the
>>secondary mirror.
>>
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