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


The Huyghenian eyepiece has the aperture stop in the middle of the eyepiece
where it is inaccessible to working with.  The lens, being as simple as it
is also runs into curvature of the field and astigmatism as well as color
errors at the edge.  Typical FOV of the eyepiece is in the 35-45 deg. range
with the wider views being more aberrant.  The Ramsden eyepiece has a
problem with a focal plane being on one of the surfaces so you get to see
the dust on that surface very nicely.  It is usually built a bit off from
that perfect condition to eliminate the inspection of cleanliness of the
user.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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