[ATM] where to place the secondary?

["Fernando Ruiz" <gm.ferruiz@gmail.com>]

Hello everybody

I am building the secondary cage for a 12.5"  f/5 truss dobsonian and have
doubts about where to place the secondary mirror. In KB's book it is
recommended that the the distance from the cage's outer top to the mirror
center should be at least 7". The total length of the cage being 10", this
leaves
3" from the mirror center to the bottom edge. It is also recomended that the
holes for the attachment screws be at 1" from the top end of the tstruts.
These numbers fit very well with Astrosystem spiders and holders.

If one makes the spider himself,  what are the minimal distances from the
mirror center to the cage's outer top and bottom edges.

Thanks in advance for you answers.
Fernando.
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