From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


To put it another way, a lens of sufficently long focal length will have
little color error.  This is basically what the astronomers of the early
days did to keep the color down on their scopes.  My 200" refractor is
a short focal length example of that technique as it has a 4.25"
single lens out at the far end and produces a fair bit of color aberration
- at higher powers you adjust focus by balancing the red and violet fringes
about a star to the same size and you are "focused" - which makes
the scope a bit interesting to use.  The scopes used back then had focal
ratios even longer than that short focal ratio scope to keep the color
down. I'll note that the microwave stuff that you've been doing is for
fairly narrow band stuff, often a fair bit less than a quarter octave in
range. This tends for the errors due to the changing frequency minimal as
the change of spacing is not very large and the focus error is usually
quite small as a result.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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