Long versus short focal length refractors

I recall reading one time that since refractors
effectively spread out the light spectrum, the
shorter the focal length, the greater the spread.

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective. Thus, a refractor with a focal length
of three inches (76.2mm) needed to be 3x3 or
F9. Or 27 inches (686mm) in order to hold the
spread of the light spectrum below one wave.

How accurate is this? Anyone know more?

Jerome Bigge
Member, Muskegon Astronomical Society
Author of the "Warlady" & "Wartime" series.
Download at "http://members.tripod.com/~jbigge"

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic (bright
light) response of the eye. It applies to normal achromats which have a color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared waves,
then there is no way that an achromat can meet the 1 wave criterion even in
absurdly long focal ratios. This is why apochromats were developed with ED or
Fluorite elements and matching crowns or short flints to effectively reduce
this focal length variation.

Roland Christen

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic (bright
light) response of the eye. It applies to normal achromats which have a color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared waves,
then there is no way that an achromat can meet the 1 wave criterion even in
absurdly long focal ratios. This is why apochromats were developed with ED or
Fluorite elements and matching crowns or short flints to effectively reduce
this focal length variation.

Roland Christen

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic (bright
light) response of the eye. It applies to normal achromats which have a color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared waves,
then there is no way that an achromat can meet the 1 wave criterion even in
absurdly long focal ratios. This is why apochromats were developed with ED or
Fluorite elements and matching crowns or short flints to effectively reduce
this focal length variation.

Roland Christen

Chris1011 wrote:

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic (bright
light) response of the eye. It applies to normal achromats which have a color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared waves,
then there is no way that an achromat can meet the 1 wave criterion even in
absurdly long focal ratios. This is why apochromats were developed with ED or
Fluorite elements and matching crowns or short flints to effectively reduce
this focal length variation.

Roland Christen

Is there any one person who is credited with making the first
apochromatic
lense system?


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Chris1011 wrote:

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic (bright
light) response of the eye. It applies to normal achromats which have a color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared waves,
then there is no way that an achromat can meet the 1 wave criterion even in
absurdly long focal ratios. This is why apochromats were developed with ED or
Fluorite elements and matching crowns or short flints to effectively reduce
this focal length variation.

Roland Christen

Is there any one person who is credited with making the first
apochromatic
lense system?


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Chris1011 wrote:

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic (bright
light) response of the eye. It applies to normal achromats which have a color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared waves,
then there is no way that an achromat can meet the 1 wave criterion even in
absurdly long focal ratios. This is why apochromats were developed with ED or
Fluorite elements and matching crowns or short flints to effectively reduce
this focal length variation.

Roland Christen

Is there any one person who is credited with making the first
apochromatic
lense system?


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"starman" wrote in message
...
Chris1011 wrote:

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of
C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic
(bright
light) response of the eye. It applies to normal achromats which have a
color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared
waves,
then there is no way that an achromat can meet the 1 wave criterion even
in
absurdly long focal ratios. This is why apochromats were developed with
ED or
Fluorite elements and matching crowns or short flints to effectively
reduce
this focal length variation.

Roland Christen

Is there any one person who is credited with making the first
apochromatic
lense system?
Carl Zeiss.
The 'key' is that the term 'apochromat', actually has a meaning, that is
often 'bent' a little on the modern scopes. Basically, you can use
combinations of multiple glasses, especially with special glasses like
flourite as one element, that correct for chromatic aberration at more than
two points in the spectrum. He realised that with the right mix, you could
correct for CA at three points, and also for spherical aberration at two.
Many scopes sold as 'apochromats', don't actually meet this definition, but
instead units that give better chromatic correction than an 'achromat',
often use the label (technically these are really 'semi-apochromats').
The original 'apochromat', was for microscope eyepieces, at the tail end of
the 19th century. Flourspar, wasn't available at the time in large enough
pieces for anything much else.

Best Wishes

"starman" wrote in message
...
Chris1011 wrote:

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of
C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic
(bright
light) response of the eye. It applies to normal achromats which have a
color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared
waves,
then there is no way that an achromat can meet the 1 wave criterion even
in
absurdly long focal ratios. This is why apochromats were developed with
ED or
Fluorite elements and matching crowns or short flints to effectively
reduce
this focal length variation.

Roland Christen

Is there any one person who is credited with making the first
apochromatic
lense system?
Carl Zeiss.
The 'key' is that the term 'apochromat', actually has a meaning, that is
often 'bent' a little on the modern scopes. Basically, you can use
combinations of multiple glasses, especially with special glasses like
flourite as one element, that correct for chromatic aberration at more than
two points in the spectrum. He realised that with the right mix, you could
correct for CA at three points, and also for spherical aberration at two.
Many scopes sold as 'apochromats', don't actually meet this definition, but
instead units that give better chromatic correction than an 'achromat',
often use the label (technically these are really 'semi-apochromats').
The original 'apochromat', was for microscope eyepieces, at the tail end of
the 19th century. Flourspar, wasn't available at the time in large enough
pieces for anything much else.

Best Wishes

"starman" wrote in message
...
Chris1011 wrote:

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective.

It's a rule of thumb which applies over a limited wavelength spectrum of
C to F
(656.3 red to 486.1NM blue) which corresponds roughly to the photopic
(bright
light) response of the eye. It applies to normal achromats which have a
color
error of 1 part in 1800 (focal length variation) over that spectrum.

If you include the shorter wavelengths of violet and the longer infrared
waves,
then there is no way that an achromat can meet the 1 wave criterion even
in
absurdly long focal ratios. This is why apochromats were developed with
ED or
Fluorite elements and matching crowns or short flints to effectively
reduce
this focal length variation.

Roland Christen

Is there any one person who is credited with making the first
apochromatic
lense system?
Carl Zeiss.
The 'key' is that the term 'apochromat', actually has a meaning, that is
often 'bent' a little on the modern scopes. Basically, you can use
combinations of multiple glasses, especially with special glasses like
flourite as one element, that correct for chromatic aberration at more than
two points in the spectrum. He realised that with the right mix, you could
correct for CA at three points, and also for spherical aberration at two.
Many scopes sold as 'apochromats', don't actually meet this definition, but
instead units that give better chromatic correction than an 'achromat',
often use the label (technically these are really 'semi-apochromats').
The original 'apochromat', was for microscope eyepieces, at the tail end of
the 19th century. Flourspar, wasn't available at the time in large enough
pieces for anything much else.

Best Wishes