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Originally Posted by acropolite
Ian, you have too much time on your hands....  We define a motor as a device that propels a car or vehicle, the poms call a car a motor, a rocket motor is nothing like a car motor, motor skills have nothing to do with internal combustion and our muscles don't have pistons. Gay used to be happy.... Where do we stop. The english language evolves, why not just evolve with it....  |
Phil Phil Phil, While english does indeed evolve, scientific and physics terms have very fixed meanings and are generally absolute in their meaning.
The English slang expression "Motor" for a car is derived from shortening the term "Motorised Carriage", which was a very accurate description of the early motor carriage. A motor however is defined as a device that converts any form of energy in to mechanical energy, hence it is applied to both mechanical devices that produce motion, and also to human muscles.
As for Gay, the word was hijacked quite late on, from memory in the late 50s or 60s and was used due to the colourful nature of the lifestyle lead by many of that sexual persuasion.
jjnettie
There's some innaccuracies in that quote you posted. Firstly an Erfle is not a generic lens design per se, it's a very specific design in it's proper form, being a 5 element design with 2-1-2 element layout. The Erfle is named after Heinrich Valintin Erfle, who worked for Steinheil & Soehne before taking up a post at Carl Zeiss. The true Erfle has an apparent FOV of around 55° with some modern designs running out to 70° FOV.
For what it's worth, the so called "Five element Plossl" is not a plossl at all, as it was designed by Herr Erfle and used the "Erfle" as the basis of it's design, rather than the Plossl which is a much earlier design by George Simon Plossl (d. 1868). So called Super Plossls (Meade etc) are just using the Plossl name for marketing purposes, same with the name Erfle with respect to most so called Erfle designs. Hence the confusion amongst many amateur astronomers.
syzygy You are quite corrrect, the actual point of focus is in front of the diagonal/secondary mirror, as it is 'behind' the tertiary mirror in a Coudé focus system. (When viewed from the front of the telescope tube)
Using "prime focus" to define Newtonian, Coudé, Cassegranina focus etc did NOT start to happen until the mid to late 70s and was a specifically American twisting of true terminology. As far as I can recall it was Meade and Celestron that pioneered this travesty in their advertising and the gutless editors at S&T and Astronomy Now magazines aquiessed rather than do the correct thing and refuse to run advertisements with such poor copy.
Ian
pot stirring on a cloudy night