Re: [ATM] side obstruction

[William Chang <williamichang@yahoo.com>]

Jonathan & Steve,

Having done the simple experiment of holding a sheet of paper in 
front of the aperture, and also having asked for help raytracing 
the effect, I believe a straight-line side obstruction (truncation
of the aperture) will result in no additional spikes and negligible
effect on contrast.  The diffraction pattern will be slightly 
elliptical but still smooth.

Two ideas come out of this observation.

(1) Deliberately truncating the aperture shortens the light
path, so a smaller secondary (CO) could be used.  What is the
quantitative trade-off of losing 10/12.5/15% on an edge, vs
a 2/2.5/3% reduction in CO (assuming f/5)?

(2) Truncate the mirror on two opposite sides by 10-15% each.
Now the scope can be made 20-30% narrower which has significant
mechanical advantages (in large apertures).  The loss of light
is insignicant (about 10% area), and the contrast effect ought
not be visible, again based on simple experiments.

I proposed such a "SlimScope" on SAA and AstroMart, and even
considered commissioning a 20" thin mirror truncated to 15" width,
for airline carry-on (!).  I'm serious.  Anyone here up to the
task? :-)

Also, if anyone has a badly chipped mirror, I know what to do
with it!

-- William (Saratoga, CA)

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