Re: [ATM] Ronchi Confusion

[Mark Cowan <toolontop@yahoo.com>]

Hi Art:

Typically one would place a Ronchi grating at the
focus of the 'scope, with eyepiece removed.  This
could be built into a eyepiece casing or a film can.

With a bright star centered, you should see a pattern
of lines.  As you move the grating in and out thru
focus the lines will grow fewer and larger, get all
crazy, and grow smaller and more.  You're looking for:

a) Straightness of lines
b) behavior as you go thru focus

If the lines look straight all the time you're in good
shape.  "How straight" they are is a large topic to
investigate, and whether they have "hooks" or "wavers"
or other oddities can in theory tell you the condition
of the mirror's figure.  

If they appear to "twist" as you go thru focus the
mirror is exhibiting astigmatism, which can come from
the mirror, the mirror mount, the secondary, or its
mount.

It's hard to do a a quantitative evaluation this way -
a lot of people treat it more as a pass/fail kind of
test - if it looks straight and well behaved there's
not much to be concerned about.

Cheers,
Mark Cowan
Salem, OR


--- artbianconi@blast.net wrote:
> Where does one place the Ronchi grid when using a
> star test?
> 
> and could you please describe the test procedure?
> 
> Thanks
> 
> Art
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/



	
		
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