Proposal for an APO "standard:" TMBs 100mm f8

How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?

Before any other apo makers freak out, I figured this scope
would be a good baseline since it's correction matches the
airy disks fairly well. There may be better corrected scopes
out there. I don't know because the mfgs don't generally tell
anyone!

If other scopes could be measured, a database of sorts could
begin to be built.

You probably need to be a member of the group to access this:

http://tinyurl.com/5y9of

RichA wrote in message . ..
How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?

Before any other apo makers freak out, I figured this scope
would be a good baseline since it's correction matches the
airy disks fairly well. There may be better corrected scopes
out there. I don't know because the mfgs don't generally tell
anyone!

If other scopes could be measured, a database of sorts could
begin to be built.

You probably need to be a member of the group to access this:

http://tinyurl.com/5y9of


APO-Max telescope was significatly better corrected. As well as
100mm F/10 Zeiss APQ - they better serve as "reference" apo.

VD

On 25 Nov 2004 22:31:51 -0800, (ValeryD)
wrote:

RichA wrote in message . ..
How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?

Before any other apo makers freak out, I figured this scope
would be a good baseline since it's correction matches the
airy disks fairly well. There may be better corrected scopes
out there. I don't know because the mfgs don't generally tell
anyone!

If other scopes could be measured, a database of sorts could
begin to be built.

You probably need to be a member of the group to access this:

http://tinyurl.com/5y9of


APO-Max telescope was significatly better corrected. As well as
100mm F/10 Zeiss APQ - they better serve as "reference" apo.

VD

No, I suggested it as a reference because it's spot sizes
at different wavelengths fairly match the theoretical
size. I'm sure there are better ones, but they aren't needed
to establish a standard. But thats no reason not to
take note of better scopes. From this point, other scopes could be
compared, and judged better or worse.
For instance, if you compare it with the spot diagram of a Tak FS102,
you can see it is markedly closer to meeting the airy disk size
than the Tak, especially in the blue-violet region. But undoubtedly,
the 100mm f10 Zeiss APQ is better, probably because of the longer
focal ratio. The TMB would serve as a baseline
measurement. If there is another scope like it, it could serve
just as well. The whole thing is just a way of assessing basic visual
colour error, what we see in the eyepiece and the spot diagram serves
that purpose well.
So, if someone sees a refractor they might want to buy, they could
look at the spot diagram and determine roughly what kind of colour
error they should expect.
Lets not forget, one of the first customers for a large TMB doublet
ended up disappointed in the colour error he saw. Had he enough
information about it to begin with, he may have chosen another scope.

RichA wrote:

How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?


Where did you publish the prescription, that we may examine the
spot sizes on our own?

RichA wrote:

How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?


This isn't TMB's but just a short overnighter 100mm f/8 apo design just
to see if I could get there, and study temperature sensitivity:

Lens Units: Millimeters
Entrance pupil diameter 100mm
EFL 800.056mm
Catalogs: Schott, Ohara

Y Field angles 0º, 0.5º

W# Value(µm) Weight
1 0.410000 1.000000
2 0.550000 1.000000
3 0.720000 1.000000

Surf Radius Thickness Glass Diameter
OBJ Infinity Infinity
STO 1023.5 15.28 LAKN13 104
2 -128.81 0.535 104
3 -127.89 8 LAL59 104
4 471.41 0.8 104
5 296.73 14 S-FPL52 104
6 -486.23 789.4662 104
IMA Infinity 13.99374

Nice sharp broadband central imagery at 20ºC, 88%EE within 20µm circle
at field edge. Pricey glass. 0.41µm focus sensitive to temperature, 3%
MTF drop at 30ºC, 10% MTF drop at 0ºC, best focus shortens with
decreasing temperature.

This brings up an interesting point: how sensitive is the color
correction and focus for commercial apos over temperature? Reasonable
temperature range might be -15ºC (5ºF) to 38ºC (100.4ºF). The CTE and
dn/dT of the glasses and especially the higher dn/dT of any spacer
fluids would have to be part of the overall multiconfiguration design to
produce an apochromat that performs well when cold, comfortable or
warm. Comments?
Mike

On Fri, 26 Nov 2004 14:16:07 +0000, "Richard F.L.R. Snashall"
wrote:



RichA wrote:

How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?


Where did you publish the prescription, that we may examine the
spot sizes on our own?

The TMB group on Yahoo. I didn't provide a link because you
have to join the group to see the files.

On Fri, 26 Nov 2004 12:35:23 -0600, Mike Jones
wrote:

RichA wrote:

How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?


This isn't TMB's but just a short overnighter 100mm f/8 apo design just
to see if I could get there, and study temperature sensitivity:

Lens Units: Millimeters
Entrance pupil diameter 100mm
EFL 800.056mm
Catalogs: Schott, Ohara

Y Field angles 0º, 0.5º

W# Value(µm) Weight
1 0.410000 1.000000
2 0.550000 1.000000
3 0.720000 1.000000

Surf Radius Thickness Glass Diameter
OBJ Infinity Infinity
STO 1023.5 15.28 LAKN13 104
2 -128.81 0.535 104
3 -127.89 8 LAL59 104
4 471.41 0.8 104
5 296.73 14 S-FPL52 104
6 -486.23 789.4662 104
IMA Infinity 13.99374

Nice sharp broadband central imagery at 20ºC, 88%EE within 20µm circle
at field edge. Pricey glass. 0.41µm focus sensitive to temperature, 3%
MTF drop at 30ºC, 10% MTF drop at 0ºC, best focus shortens with
decreasing temperature.

This brings up an interesting point: how sensitive is the color
correction and focus for commercial apos over temperature? Reasonable
temperature range might be -15ºC (5ºF) to 38ºC (100.4ºF). The CTE and
dn/dT of the glasses and especially the higher dn/dT of any spacer
fluids would have to be part of the overall multiconfiguration design to
produce an apochromat that performs well when cold, comfortable or
warm. Comments?
Mike

For all your optical liquid needs, please visit:

http://www.cargille.com/

RichA wrote:

How about the spot diagrams for the TMB 100mm f8 posted on the
TMB apo group? It looks like this scope could be a kind of
benchmark since it keeps the colours just inside the airy disk?

I looked at the TMB spots - nice. Would be interesting how TMB spots
hold together over temperature range.

I worked a little more on reducing variation over temperature, resulting
in this 100mm f/8 design. Almost no variation in performance over
0º-30ºC except for BFL shift.

Lens Units: Millimeters
Entrance pupil diameter 100mm
Catalogs: Schott, Ohara

Y Field angles 0º, 0.5º

W# Value(µm) Weight
1 0.400000 1.000000
2 0.550000 1.000000
3 0.720000 1.000000

Surf Radius Thickness Glass Diameter
OBJ Infinity Infinity
STO 1467.61 15.00 S-BSM21 104
2 -153.67 1.52 104
3 -152.09 9.00 S-LAL59 104
4 837.00 0.05 104
5 370.89 15.00 FPL52 104
6 -360.51 * Below 104
IMA Infinity 13.989

Temp EFL BFL
0ºC 799.661 792.668
20ºC 800.000 793.024
30ºC 800.173 793.207

1/20 wave P-V central imagery over 0.4-0.72µm, 87%EE within 20µm circle
at field edge - gives spots very similar to TMB's 100mm f/8, and over
0º-30ºC temperature range.

Mike

Impressive theoretical performance, which comes at a cost of instability,
however. The main problem I see with this design is a very high sensitivity to
cooldown. During cooldown on cold nights the middle and rear elements lag the
front one by about 20 fringes or so. If you change R3 by 20 fringes, you have a
severely undercorrected lens.

If you rely on an airgap to produce a large amount of spherical correction, as
is the case in your design, the entire lens will be thermally unstable because
airgaps are by their nature unstable. Unlike a lens element, where the rear
surface will bend by the same amount as the front, the rear part of an airgap
will not bend the same amount as the front part under changing temperature
conditions. Only when the two glasses are at exactly the same temperature front
and back, will the airgap have the exact shape that you wish it to have.

Roland Christen


I worked a little more on reducing variation over temperature, resulting
in this 100mm f/8 design. Almost no variation in performance over
0º-30ºC except for BFL shift.

Lens Units: Millimeters
Entrance pupil diameter 100mm
Catalogs: Schott, Ohara

Y Field angles 0º, 0.5º

W# Value(µm) Weight
1 0.400000 1.000000
2 0.550000 1.000000
3 0.720000 1.000000

Surf Radius Thickness Glass Diameter
OBJ Infinity Infinity
STO 1467.61 15.00 S-BSM21 104
2 -153.67 1.52 104
3 -152.09 9.00 S-LAL59 104
4 837.00 0.05 104
5 370.89 15.00 FPL52 104
6 -360.51 * Below 104
IMA Infinity 13.989

Temp EFL BFL
0ºC 799.661 792.668
20ºC 800.000 793.024
30ºC 800.173 793.207

1/20 wave P-V central imagery over 0.4-0.72µm, 87%EE within 20µm circle
at field edge - gives spots very similar to TMB's 100mm f/8, and over
0º-30ºC temperature range.

Mike

John Savard wrote:
A basic triplet lens, crown, flint, and fluoride, will, for different
focal ratios, provide a certain level of color correction. The longer
the focal ratio, the better the correction.

That depends, as you well know, on the way in which that color correction
is expressed. If you express it as fractional variation in focal length
over a range of wavelengths, then it does not depend on the focal ratio.
An achromat of whatever size, whatever focal ratio, will exhibit variation
in focal length of about 1 part in 2,000 over the usual spectrum (C to f?).

The reason why it looks less conspicuous at slow focal ratios is that the
Airy disc has a larger linear size. When it becomes large enough, the
different spot sizes for different colors all get obscured by the enlarged
Airy disc.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
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