Re: [ATM] side obstruction

[William Chang <williamichang@yahoo.com>]

Thanks to everyone who replied.  To elaborate:

(1) What I referred to as "raytracing" was Zemax PSF (point spread 
function) 3D plots, which is capable of showing spikes, but didn't 
for the edge-truncated mirror.

(2) By truncating one edge, the focuser can be brought closer to
the secondary, allowing a smaller CO -- hence my question what is
the quantitative trade-off between edge truncation and CO.

And, while observing planets through both reflector and refractor,
no effect could be seen of the edge-obstruction (using a sheet of 
paper), until the obstruction is very large.  I'm not a quantitative
double-star observer so can't comment on that....

Hope this helps,  -- William


> > From: William Chang <williamichang@yahoo.com>
> To: atm@atmlist.net
> Date: Tue, 19 Apr 2005 21:31:47 -0700 (PDT)
> Subject: Re: [ATM] side obstruction
> 
> 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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