Hershel's Telescope Making

Ran across this article and thought those here would find it interesting. Has a photo of a 10" reflector he made. The 7 footers seemed to be the most popular ones he made and sold.

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=AST&bibcode=2004JHA....35...45S&letter=.&classic=YES&defaultprint=YES&whole_paper=YES&page=45&epage=45&send=Send+PDF&filetype=.pdf

One thing that has always puzzled me is that Herschel did most of his observing with a 18.7 inch of 20 ft focal length. That is only f/12.8. I don't know at what angle the mirror was tilted (I don't think he actually observed on axis with his head in the way). If the tilt was about 1.4 degrees giving 2.3 inches clearance with the incoming light the resultant image is quite horrible if it was a figure of revolution.(pic) So how was he able to see so much planetary detail and split close doubles that other observers couldn't with such an image? Did he polish an off axis parabola?

Ed Jones

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