Nagler's DeLite eyepieces

On Tuesday, 30 June 2015 08:42:03 UTC-4, Helpful person wrote:
On Tuesday, June 30, 2015 at 4:15:31 AM UTC-4, Quadibloc wrote:

In principle, exotic glasses aren't needed in order to make a highly-corrected
lens.

It is true, though, that the oldest and simplest glasses, if used exclusively,
do prevent some aberrations from being corrected: hence, the Petzval portrait
lens, which used astigmatism to compensate for an uncorrectable curvature of
field by flattening at least one image plane.

However, the "new" glasses which solved that problem can hardly be termed
'exotic' by today's standards.

What exotic glasses let you do is make a well-corrected lens lighter and
smaller. And they let you do more ambitious things. So while the original
Nagler did not use any exotic glasses, it did have the 'kidney-bean' problem,
and the ones with wider fields do all use exotic glasses.

John Savard

Correct. However, there are some techniques that have not been used which obtain good performance without expensive glass or extra elements. Sorry, can't discuss but I'm sure I'm not the only one to have experimented with eyepiece design.

http://www.richardfisher.com

Yeah, I remember the design for a non-ED apochromatic refractor too. Like the million mile tire, or the latest miracle battery, it never amounted to much.

On Wednesday, July 1, 2015 at 1:25:14 AM UTC-4, RichA wrote:

Yeah, I remember the design for a non-ED apochromatic refractor too. Like the million mile tire, or the latest miracle battery, it never amounted to much.

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.

http://www.richardfisher.com

On Wednesday, July 1, 2015 at 10:27:01 AM UTC-4, Helpful person wrote:
On Wednesday, July 1, 2015 at 1:25:14 AM UTC-4, RichA wrote:

Yeah, I remember the design for a non-ED apochromatic refractor too. Like the million mile tire, or the latest miracle battery, it never amounted to much.

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.

http://www.richardfisher.com

You should be taking your own advice, of course.

What I note about APOs that -don't- use exotic glass is that they usually have longish focal ratios, making them far less attractive than they might otherwise be.

On Wednesday, 1 July 2015 17:12:04 UTC+2, wrote:

What I note about APOs that -don't- use exotic glass is that they usually have longish focal ratios, making them far less attractive than they might otherwise be.

Define "far less attractive" in the context of Apochromatic telescopes.
Are we talking appearance, hype or real world optical behaviour?
The longer focal lengths will allow fewer and thinner elements with potentially faster cool-down times.
One of the weaknesses of [some] APOs is their temperature lag in typically falling evening temperatures in comparison with achromats.
Improved depth of focus will reduce the need for slow motion focusers and constant need to refocus.
Longer focal lengths will allow longer, less exotic eyepieces for the same power.
A longer instrument will place the objective higher above typical ground thermal issues.
A longer focal length will allow better overall correction with reduced aberrations.
Field curvature and higher order chromatic aberrations may be reduced.
Unlike yourself, I claim no great authority nor Solomon-like wisdom on these, or any other, matters.

I am quite prepared to be corrected on any of these claims by the real opticians amongst us.
I look forwards to it with an open mind. Fearlessly prepared to be corrected.
Their responses will be educational, based on reality, have a scientific basis and enlightening.
Unlike anything you have ever posted on s.a.a. as the forum's leading pedant, ignoramus and grouch.

On Wednesday, July 1, 2015 at 1:10:44 PM UTC-4, Chris.B wrote:
On Wednesday, 1 July 2015 17:12:04 UTC+2, wsne... wrote:

What I note about APOs that -don't- use exotic glass is that they usually have longish focal ratios, making them far less attractive than they might otherwise be.

critter's bloviations deleted

A 12-inch f/6 Newt is probably worth the trouble, a 6-inch f/12 refractor is probably NOT worth the trouble.

On Wednesday, 1 July 2015 10:27:01 UTC-4, Helpful person wrote:
On Wednesday, July 1, 2015 at 1:25:14 AM UTC-4, RichA wrote:

Yeah, I remember the design for a non-ED apochromatic refractor too. Like the million mile tire, or the latest miracle battery, it never amounted to much.

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.

http://www.richardfisher.com

You can build a spherical mirrored Newtonian with no aberration, but how long would it have to be? Impractical scopes do not count.

On Wednesday, July 1, 2015 at 9:12:04 AM UTC-6, wrote:

What I note about APOs that -don't- use exotic glass is that they usually have
longish focal ratios, making them far less attractive than they might otherwise
be.

Yes. The earliest apochromatic lenses either used conventional glasses that
were slightly off the 'glass line', or used one element of transparent oil or
water.

Today, we don't have to go to such lengths. If we don't want to use Calcium
Fluorite, or glasses mixed with it, we *could* build an apochromat using an
element made from _acrylic plastic_.

That would still mean thicker lenses for the same focusing power. For
*astronomy*, it's a drawback that plastic changes its optical properties with
temperature about 100x as much as glass does. Since amateur astronomers go out
at night when it's colder.

John Savard

On Wednesday, July 1, 2015 at 3:48:57 PM UTC-6, RichA wrote:

You can build a spherical mirrored Newtonian with no aberration, but how long
would it have to be? Impractical scopes do not count.

Still, qualified statements rather than sweeping ones have the virtue of being accurate and avoiding confusion. And one can be a little less impractical - spherical elements can correct for both spherical aberration and coma using somewhat less extreme constructions. (They'll have quite a bit of central obstruction, though.)

John Savard

On Wednesday, July 1, 2015 at 11:10:44 AM UTC-6, Chris.B wrote:
On Wednesday, 1 July 2015 17:12:04 UTC+2, wrote:

What I note about APOs that -don't- use exotic glass is that they usually
have longish focal ratios, making them far less attractive than they might
otherwise be.

Define "far less attractive" in the context of Apochromatic telescopes.
Are we talking appearance, hype or real world optical behaviour?

Well, if you've got a long focal ratio, you don't need an apochromat for a good
image as badly, for starters. However, I'm not really aware of actual examples
of apochromats that don't use exotic glass being offered for sale. Instead, the
apochromats I'm aware of either use three elements, one of exotic glass, or two
elements, one of exotic glass.

The two-element ones are the less fancy ones, of course.

John Savard

On Thursday, July 2, 2015 at 8:41:48 AM UTC-4, Quadibloc wrote:
On Wednesday, July 1, 2015 at 11:10:44 AM UTC-6, Chris.B wrote:
On Wednesday, 1 July 2015 17:12:04 UTC+2, wrote:

What I note about APOs that -don't- use exotic glass is that they usually
have longish focal ratios, making them far less attractive than they might
otherwise be.

Define "far less attractive" in the context of Apochromatic telescopes.
Are we talking appearance, hype or real world optical behaviour?

Well, if you've got a long focal ratio, you don't need an apochromat for a good
image as badly, for starters. However, I'm not really aware of actual examples
of apochromats that don't use exotic glass being offered for sale. Instead, the
apochromats I'm aware of either use three elements, one of exotic glass, or two
elements, one of exotic glass.

The two-element ones are the less fancy ones, of course.

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.

"Helpful Person" was being hypocritical and unfair.