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#21
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Nagler's DeLite eyepieces
On Thursday, July 2, 2015 at 7:13:29 AM UTC-6, wrote:
Terms such as: Exotic Expensive Low dispersion High dispersion High performance Highly corrected Short focal ratio Long focal ratio all tend to be relative and open to interpretation. That's true. "Helpful Person" was being hypocritical and unfair. What he said was: You should think before you write. Apochromats have been designed and built in large quantities long before the existence of low ED glass. if you want people to listen avoid making sweeping incorrect statements. The sentence in the middle is the substantive one. And I have to admit it's open to serious criticism. Yes, apochromats were 'designed and built in large quantities' without "low ED glass" - they had *three elements*, and instead of using an element made of special glass, they used an element made of *calcium fluorite*. Some of the very earliest apochromats used a space filled with oil as an element, but that kind of telescope was not manufactured in large quantitied snd sold to amateurs. As far as I know, it was only the "low ED glasses" that made it possible to sell telescopes with *two-element* objectives as apochromats; that is their significance. Anyhow, I found these interesting pages - http://www.telescope-optics.net/achromats.htm http://www.telescope-optics.net/apo_refractor.htm http://www.telescope-optics.net/semi...o_examples.htm and http://geogdata.csun.edu/~voltaire/tmb/definition.html John Savard |
#22
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Nagler's DeLite eyepieces
On Thursday, July 2, 2015 at 9:36:48 AM UTC-4, Quadibloc wrote:
On Thursday, July 2, 2015 at 7:13:29 AM UTC-6, wsne... wrote: "Helpful Person" was being hypocritical and unfair. What he said was: You should think before you write. Apochromats have been designed and built in large quantities long before the existence of low ED glass. if you want people to listen avoid making sweeping incorrect statements. The sentence in the middle is the substantive one. However, that was not the sentence to which I was alluding, the last one was. And I have to admit it's open to serious criticism. Which is another reason why, after seeing him nitpick RichA's language, it was necessary to point out H.P.'s hypocrisy. |
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