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[Fwd: Re: ATM Collimating Newtonians]

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> if you do not implement the offset, you can still fully illuminate a
> 35mm frame so long as you use a
> big enough diagonal. Does this make sense?
> 
> --
> Clear skies,
> Steve Bell

Ahh yes it does but.....
I simply took a piece of cardboard and modeled a 10" f2 system (I drew
it) and discovered what I beleive to be a much simpler collimation
method that is purely objective and not subjective (which in my opinion
the methods being described are EXTREMELY subjective - center this and
do not center that and if your f ratio is this it might look offset etc
etc). 

First it is obvious that you do not need to use an offset but if you do
not you will need to
1)  oversize the secondary OR 
2)  you should expect to see off center PRIMARY reflections on the far
side of the secondary. 

I have in this process answered my oft posted question using the example
and not just hypotheticals. The laser CAN show "collimation" in an
uncollimated system. Bare in mind the way the cheshire AND the
autocollimator work they can also do the exact same thing!!!! The
problem is that the system is NOT necessarily collimated to the incoming
light axis but is rather tipped one way or the other depending on the
secondary placement. If you do not use an offset AND you look for
concentric primary in secondary reflections (as described by everyone
and their brother) then you will HAVE to tilt the primary to get a
closed system in any of the three devices, laser, cheshire,
autocollimator! This will mean the center of the optical path will NOT
be centered in the focuser and there will be coma! This was a very
interesting discovery. I have noted in my 30" that I "felt" objects were
sharper off center (towards the primary - which is exactly where the
optical center line would be!!!). THis explains it and shows me it is
NOT collimated. The method I found looking over this model assumes first
you have checked your focuser for parallelism to optical axis and inline
with the centerline of the diagonal bolt - this can be done readily and
has been reported, I can supply details if needed. 

This being done then do the following steps:
1)      Mark the diagonal with a small dot (whiteout) at the point of the
offset. THis is the 
exact center - the hypotenuese of the offset x,y axes. My calculated
offset is 0.25" so there will be a mark 1/4" away from secondary center
and towards the focuser. 
2)      Next using a laser (borrow one - it should be collimated and thus you
do NOT rely on your eye) move the secondary up or down on the spider
until the dot is on the mark. This means the vertical placement is
complete. 
3)      Assuming the secondary mirror is close to the right size for the
scope : 
adjust the tilt of the secondary  ( in the plane of the focuser - the
laser dot should stay centered throughtout - put it in occasionally and
confirm this) until one of the edges of the primary reflection is
coincident with the secondary edge. At this point (first time through
this step) the other edge of the primary will be well within the
secondary. If the edge of the primary that is lined up with the
secondary is the far one (away from focuser) then move the diagonal away
from the focuser using the spider adjustments. If the edge lined up is
closest to the focuser move the secondary towards the focuser by a small
amount. DO NOT adjust the depth - leave the bolt locked. 
4)      Repeat step 3 until the edges are seen equally on the secondary
(concentric primary reflection on secondary). This will be the optical
center of the primary (optical axis) AND a tilt of 45 degrees and should
provide perfect collimation!!

I have never seen an easier procedure that should get you there - they
are all very subjective! THis is frustrating. Unfortunately equaling
frustrating is the constant rain so another new moon has come and gone
(tomorrows forecast is more of the same). I will try this when I get a
chance but I am convinced it will work.

G.

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