LtE in CMO #270

From Thomas A DOBBINS

Dear Masatsugu: 　

Bill and I recently completed a popular nostalgia article for the June 2003 issue of Sky & Telescope about the last perihelic opposition of Mars prior to the Space Age - the opposition of 1956. I have attached a copy of the manuscript for your hopeful enjoyment. Perhaps a bit of the more obscure history will be new to you.　

Warmest regards, as ever

(1 March 2003 email)

Dear Masatsugu:

I read with great interest Teruaki Kumarmori's success with wedge prisms for eliminating the effects of atmospheric prismatic dispersion in Issue #269 of CMO.

It may interest some readers to learn that wedge prisms are available in the United States from Edmund Industrial Optics

(dohttp://www.edmunptics.com ).

These prisms are made from BK7 crown glass. They are all 25.0mm diameter and are available with wedge angles of:

1 degree 56 minutes (nominal deviation 1 degree) Cat. No. F45-555 $45.30

3 degrees 52 minutes (nominal deviation 2 degrees) Cat. No. F45-651 $45.30

7 degrees 41 minutes (nominal deviation 4 degrees) Cat. No. F45-650 $45.30

The late British optical genius Horace Dall of Luton, England used to compensate for atmospheric dispersion when observing with his 15.5 Cassegrain by using a two-element achromatic Barlow lens with the elements separated not by the usual cement like Canada Balsam but with an inert fluid "indexing oil" retained by capillary action. Because the internal radii of curvature of the Barlow lens were equal but opposite, one element could be displaced with respect to the other by means of a thumb screw acting against an opposing spring bearing against the opposite side of the movable element. Others have employed Ramsden eyepieces with a fixed field lens and an eye lens that could be rotated in an arc.

Kind regards,

(24 March 2003 email)

Tom DOBBINS (Coshocton, OH, USA)

kmdobbins@coshocton.com

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