RE: [ATM] side obstruction

["Koehler, Steve" <Steve_Koehler@securecomputing.com>]

William,

> 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.

Actually, I believe the spike is just as strong as with the rectangular side
obstruction.  Since this is a wave optics effect, I would not expect ray tracing
to show the effect.  I'm quite sure that this effect (the spike) should be
visible in your experiments when viewing a bright star in focus.  Be sure to
introduce the straight-line obstruction at a diagonal to the spider vanes to
make it most apparent.

> 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)?

However, your diagonal can only be smaller in the same direction as your side
obstructions, so I don't think you really gain anything.  In other words, this
doesn't, in itself, result in a smaller percentage of obstruction.

> (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.

If your mirror is 15x20, then it will act like a 15" mirror for resolution of
detail in one direction, and like a 20" mirror for resolution of detail in the
other direction.  To optimize this arrangement, you would want a customized flat
that is narrower in the 15" direction.

-- Steve Koehler
   steve_koehler@securecomputing.com


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