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

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.