Re: Secondary Size

[akelly@ghgcorp.com]

In reply to:

>       If I go with a 1.5" secondary, I will only have a .25" (6.4mm) zone
>of 100% illumination at the eyepiece.  The obstruction of the primary is 19%.
>(I am assumming that the secondary holder's lips will subtract approx. 1/8"
>from the secondary minor axis.)
>
>       If I go with a 1.83" secondary, the zone of 100% illumiation will
>be .6" (15mm), while the obstruction of the primary will be 23%
>
>       Contrast versus illumination?  I would appreciate any suggestions.


        I am having trouble verifying your calculations (for the size of
your 100% 
illumination zones).... they can only be consistent if I assume you are
using two different "throw distances" from the the secondary to the focal
plane for the two different secondaries.  Is this in fact the case?
        Regardless, here is my two cents on your alternatives:  go with the
1.83 m.a. secondary.  With the 1.5 you will have only a 0.3 degree fully
illuminated FOV, while the 1.83 will yield close to 0.75 degree.  This
discrepancy will be noticeable when using wide fields on extended objects.
The ever-so-slight loss in contrast between 19% and 23% obstruction will not
be detectable by normal humans under almost any seeing conditions.  Accurate
collimation and surface smoothness mean oh-so-much more.

                                        AL Kelly