ATM Spider vanes (4 or 3?)

[Adam Perkins <scopeguru@yahoo.com>]

Hi,

Its about time to begin construction of my secondary
cage.  I originally planned on a four vane spider, but
have begun to reconsider.

My understanding is that vanes set at 90 deg angles
create 4 diffration spikes (a 4-vane setup).  Spider
vanes set at 120 deg apart (a 3-vane spider) will
create 6 diffration spikes.

So,  if the total intensity of the light spread due to
diffraction is equal for both arrangements, the
three-vane geometry would seem to be preferable as it
distubutes the light more uniformly and over a greater
area of the field.  The 3-vane arrangement is more
similar to a curved spider's diffraction than the
4-vane arangement.  The extra vane in the 4-vane
design would seem to add even more scattered light to
the view, in addition to the inferior diffration
pattern of the 4-vane system.

It just seems obvious to use only as many supports for
your secondary as necessary to hold it steady.  Three
tensioned vanes...

Has anyone on the list tried a three vane spider on a
scope of my size (12.5")?  I've seen three-vane
spiders  only on smaller scopes so far.  Are there
mechanical problems with the three-vane support?  Am I
overlooking obvious problems?

Just wondering...


Adam Perkins
Covington, Louisiana


__________________________________________________
Do You Yahoo!?
Thousands of Stores.  Millions of Products.  All in one place.
Yahoo! Shopping: http://shopping.yahoo.com