The Cooke Triplet: Answering Another of My Own Silly Apochromat Questions

It is my suspicion, therefore, that a refracting telescope based on H.
D. Taylor's design _would_ provide views of the heavens that were
absolutely stunning

There is nothing earth shattering about Taylor's design. It uses one abnormal
dispersion flint in triplet configuration to achieve a modest reduction of
secondary spectrum, not 10x less.

Using NORMAL glasses in any kind of configuration, doublet, triplet, quadruplet
etc. will yield the same color correction as any normal achromat, no matter how
you bend the glass. To get any reduction of secondary spectrum requires at
least one abnormal glass that lies off the Abbe line, and this includes short
flints, and ED/fluorite glasses.

If you have doubts about what I say, then join the ATM Optics software group
and pose it as a challenge to the members:
http://groups.yahoo.com/group/ATM_Optics_Software/

Or you can do some design yourself and will quickly learn what is real and what
is imaginary in optic land.

Roland Christen

a refracting telescope based on H. D. Taylor's design

There is nothing earth shattering about Taylor's design. It uses one
abnormal dispersion flint in triplet configuration to achieve a modest
reduction of secondary spectrum, not 10x less.

It is unclear if this response was to Taylor's 'Cooke Triplet', as
titled, or to Taylor's 'Photovisual' triplet, as described.

But Taylor's Photovisual was a very significant advance beyond the
refractors of its day. Abbe / Zeiss and C. Hastings were involved in
similar work, but Taylor was right there with them, and Cooke was ahead
of them in production, which was unfortunate since the glass
deteriorated badly.
Some details on the glass, and more text than I really should send out
in a post to a newsgroup, are below, part of a text on HDT found on my
site:
----
In 1892, when Taylor was 30, came the introduction of Taylor's
ambitious design, the Cooke Photo Visual telescope objective, patent
17,994/92; probably the first triplet apochromat, using new Schott
glass. These objectives could be used as photographic or visual
telescopes, since the color correction extended across a wide range of
colors. They were made f18, although faster scopes were possible, f18
provided superior achromatism, a larger plate scale, and allowed
fabrication of shallower spherical surfaces. The design suffers from
steep curves on the center element, a 5 inch negative center element was
polished to .07 inch thick at mid-diameter. Even at f18, the center
element of this objective requires two concave surfaces of steep
curvature, making this element very thick & limiting the aperture of the
lens.
As of 1894, the glass elements were as follows: The outer element
was biconvex and of Schott baryta light flint (O 543), index of
refraction 1.564 for the D ray, reciprocal of dispersive power 50.6.
The center element was biconcave, of a new Schott borosilicate flint (a
type of O 164), with an IR of 1.547 and reciprocal of dispersive power
50.2. The inner element was a meniscus of a light silicate crown glass
of lower dispersion than standard crown (Schott O 374), an IR of 1.511,
reciprocal of dispersive power 60.4. Both pairs of inner surfaces had
matching profiles. The rear surface had a radius of curvature roughly
equal to twice the focal length Dispersion was corrected by controlling
the radii of the elements. The air space between the second and third
elements was critical and used to correct sphereochromatism. The image
plane was flat and free of coma over a few degrees.
The lens is not particularly sensitive to squaring on, and can be
squared using the rear concave surface in autocollimation. Fabrication
of the glass elements is not difficult, with three concave surfaces, and
the two inner convex surfaces have the same curvature as the adjacent
concave surface. After the three concave surfaces were tested,
defective figuring of the inner convex surfaces was detected by
introducing liquid between the surfaces, if the defects disappear, then
a fault is suspected in the concave surface under test.
The P-V lens was fitted in a cell especially designed for this
objective. The steep curves needed for the center element meant that it
had to be very precisely centered in its cell, for any lateral motion
introduced significant coma. A metal cell that would restrain the lens
at a very cold temperature would expand with heat, and at a higher
temperature would allow the lens to move unless it was restrained. All
refractors have this problem, but this objective has a tolerance for
centering errors that is far tighter than others. Temperature
compensation is therefore necessary, and the cell design must factor the
coefficients of expansion for the glasses and for the cell components.
Smaller photovisual objectives used a modification of the standard cell,
and to center the elements there were three protrusions from the inner
wall, one of which was spring loaded.
It was probably Frederick Cooke who designed the cell used for larger
objectives. The inner wall used three equidistant small blocks to
restrain the lens elements; two bronze blocks fixed to the bronze cell,
and a third block made of two sliding bronze wedges. Each wedge is
carried on a thick strip of zinc, that is as wide as the cell is tall
and lays against the inner wall of the cell, from one fixed block to one
of the wedges. Each zinc strip is attached at its end to a fixed block
and free to move with its bronze wedge. An increase in temperature
causes expansion of the outer bronze cell, which would loosen the lens
elements. However, zinc has a much higher coefficient of expansion than
does bronze, and the zinc strips expand even more, to force the wedges
together, which widens the split block as one wedge rides against the
other. Compensation for temperature is controlled by the composition
and dimensions of the zinc strips and by the angle at which the wedges
are cut. These are very precise assemblies, the glass and metal fitted
to a few thousandths of an inch. Cooke warned users that the screws in
the outer case of the cell held the zinc strips in place and should not
be turned. During assembly or disassembly, any tilting of lens elements
will cause them to be wedged, and the center element has a slightly
rounded profile to prevent this.
In the 1900 catalog of T. Cooke & Sons, P-Vs were available with
apertures from 3 inches, at 20 pounds; to 15 inch at 800 pounds; and
"special quotations will be given for larger sizes". The photo visual
objective is described as free from secondary spectrum and from
spherochromism, as follows:
"total abolition of the secondary spectrum"
"...free from spherical aberration for all colours simultaneously"
"....largest possible field of good definition"
".....The curves are such that a ray parallel to the optic axis, traced
through the margin of the objective, enters and leaves the flint lens at
approximately equal angles....This condition secures freedom from
optical effects of flexure" (this refers to any sagging of the thin
flint element in the cell).
The 1900 catalog includes testimonials from customers of the P-V,
including The Observatory at the Cape of Good Hope, whose 8 inch P-V was
used by David Gill. Edward Crossley bought a 9 inch in 1895, to replace
his 9.3 inch objective in the Cooke telescope he purchased in 1867.
Norman Lockyer wrote in 1898, after four years of use; he found the P-V
lenses to be excellent collimators for spectroscopes, because all colors
exited parallel. He also used them for telescopic objectives for
spectroscopy, which allowed focusing to both ends of the spectrum
without a swing back. P-Vs were used by Lockyer in spectroheliographs
to image two monochromatic images, of different wavelengths, on one
plate.
A catalog circa 1930 from Cooke, Troughton & Simms, carries the P-V
model; 4 inch aperture for 40 pounds, to 12 inch at 720 pounds. Since
these are f18s, thus the 12 inch is 18 feet in length; and the catalog
notes that lenses have been made up to f60 in focal ratio. The
objectives were fitted into the standard Cooke tubes, but the tube was
then lined with non-reflective material; and a larger focuser was
available, since the focuser could "be made to pass a field of view one
inch less than the aperture of the object glass". This catalog also
lists telescopic doublet achromats to 26 inches aperture.
Henry King notes that a photovisual objective is a compromise, that
does not permit the most precise positional astronomy. It is designed
to allow the user to visually focus the instrument and insert a
photographic plate without refocusing, but they did not replace
instruments dedicated to astrometry. Taylor also wrote about
compromises, in the 1896 edition of ATTO, that since the P-V was most
used with Huyghenian eyepieces, the color correction of the P-V was
overcorrected to be fully achromatic when used with a Huyghenian
eyepiece at a magnification of 50 times the aperture, or 200 on a 4 inch
glass. At lower powers than this, the telescope is undercorrected, at
higher powers it is overcorrected. These effects are very small but
illustrate the compromises that are part of optical design.
This lens was by far the best available during its time, and was a
genuine advance in the technology of telescope objectives. P-V
telescopes were offered up to 15 inches aperture, but the largest known
P-V models are two 12.5 inch aperture telescopes, one for Rio de Janeiro
in 1894. The other was made for Robert Ball at Cambridge Observatory,
mounted in the Sheepshanks polar coude telescope designed by Howard
Grubb, and used for astrometry. This telescope was decommissioned and
in 1947, the objective was moved to the Northumberland equatorial
telescope, which had an identical focal length. In 1988, it was
replaced with a doublet by Jim Hysom of AE Optics. Like all P-V lenses,
it had a problem: every 20 years, it needed repolishing.
There was a severe problem hidden in the objective. The borosilicate
glass used for the center element was a new innovation from Schott, and
Taylor had not learned that some of the exotic elements in the glass
were reactive and would chemically react to air, moisture, and
pollution. The glass inevitably loses its transparency with a very fine
crazing or frost. All three elements can be hazed. All P-V lenses
require reworking every 20 years, more or less, usually less. The 12.5
inch objective for Cambridge retained an acceptable polish for about 25
years, and the center element had been repolished three times by the mid
1970s. The second reworking, circa 1950, left the element quite thin
and necessitated careful handling and reassembly. The third polishing
was in 1972 and the reworked lens was very difficult to align and
maintain in centration. Adding to the difficulties was the loss of
orientation marks on the perimeter of the elements. Other photo-visual
objectives did not endure this long. Norman Lockyer equipped his Solar
Physics Observatory with 6 P-V lenses of 3 to 12 inches aperture, and
within 2 to 7 years, all were marked with a fine crystal growth on
internal surfaces. Lockyer notes that some lenses were kept in a well
heated room, and those require close examination to detect the hazing.
Others were used under varying temperatures, though well ventilated, and
the problem was severe. The inner surfaces of the front and especially
the back elements were the problem areas, and not the borosilicate
center element. Water vapor was believed to be the cause, glass absorbs
water, which liberates the alkaline components in the glass, forming
carbonates on the surface. Lockyer notes that it is quite difficult to
replace a lens in its cell with the proper centration, and therefore
only one lens was disassembled. Taylor's 1907 reply to Lockyer notes
that in damp climates such as Calcutta, the crystallization has been
found to corrode the polished glass surface. From the 1907 JBAA, in a
reply to Lockyer's problems, Taylor objects, that the lenses were older
than Lockyer's text would indicate; "the objectives were put
together....at least 12 months" before the telescope was mounted.
Taylor continues, that sulphuric acid treatment was successful, the lens
retained it's polish during the treatment, and only in areas like
Calcutta has the crystallization mandated a repolishing.
Crossley's 9 inch objective, from 1895, was moved to New Zealand in
1907, and is now at Carter Observatory. The lens was shipped back for
repolishing to Cooke & Sons in 1914, and again in 1950-51 to Grubb
Parsons. It now has a new matte finish, and in addition has suffered
from water seepage. The inner lens is slightly hazed from deposits of
alkali carbonate but is still polished.
In the sequence of journal articles and catalogs during the first
decade of this century, Lockyer first appears as an impressed consumer
in the 1900 catalog, with a testimonial from 1898, and by 1907, he was
writing for MNRAS that he'd had problems for seven years. Taylor and
Cooke defend the materials used, writing in many publications & speaking
to many meetings. Already in 1894, Taylor was writing that a year & a
half of experiments allows Cooke to guarantee the permanence of the P-V
objective. He continues with the first publication of his idea that a
tarnished lens surface transmits more light than a freshly polished one;
claiming that even if some slight tarnishing were to take place over
many years, as is typical for flint glasses, that would not necessarily
have any ill effect. (MNRAS 54:5, 1894) It is possible that his work
as a pioneer of lens coating had its genesis in problems with the P-V.
Howard Grubb was quoted that 20 years experience was needed for
claims of permanence. In April 1894, 'The Observatory' printed a reply
from Cooke & Sons, defending its use of the glass by mentioning
consultations, experiments, and visits to Jena. Both firms were rather
strident and antagonistic.
In the May 1894 issue of Astronomy & Astrophysics, during the third
year of production of the P-V, an anonymous reviewer notes that the
interior lens is 'slightly liable' to tarnishing, but that "Cooke & Sons
guarantee the permanence of objectives". Taylor replied to the article
with a longer rebuttal, describing the haze on the glass as 'iridescent
but perfectly transparent', a product of gas lamps, and that pure damp
air has no effect on the glass. In any case, the inner surfaces of the
objective is 'hermetically sealed'. A new melt of Schott glass will be
modified to further perfect the lens.
There are questions about the nature of the tarnish. In the 1896,
second edition of ATTO, chapter 15 is added, on care of objectives, and
'tarnish' is described as a 'transparent film of varnish' that viewed in
reflection appears a dull grey brown or blue, and occurs on flint
elements after a few years. Tarnish is described as not a "dull milky
film, visible by scattered light". In 1896, Cooke & Sons had not yet
seen any 'milky decomposition' in their glass; though they had "heard of
it taking place with certain phosphate glasses in tropical countries".
From the 1900 catalog: "nonwithstanding certain unsympathetic
criticism which has proved too hasty.....Cooke & Sons can testify to the
permanence of the glass."
Jumping forward 80 years, the 1983 edition of ATTO reprinted the 1897
instructions for the P-V, noting in the 1983 preface that 'corrosive
industrial atmospheres' can attack some of the glass types.
In retrospect, all known Cooke Photo-Visual triplets have become
hazed to the extent that the objective is unusable. The sequence of
tarnish to haze is variable with humidity, temperature, and time. In
response to these problems, Cooke issued cleaning instructions, offered
factory service, tried & adopted new glasses as they became available,
and very much tried to put the problems in the best possible light.
Cooke & Sons described the tarnish as a controlled, predictable,
beneficial process. For example, in the 1921 article, "A New
Anastigmatic...Telescope", p73, Taylor writes: "In the telescope
described in the present paper about 40 per cent of the light is
transmitted when the surfaces are freshly polished. About five of the
surfaces retain their polish, while the others tarnish and then transmit
more light."

Jena made apochromatic doublets, and the U.S. made Hastings doublets,
used glass that deteriorated similarly or worse. Schott had deleted O
374, the borosilicate used in the center element, by 1907, because it
was 'wanting in permanency'. Taylor replaced it with O 599, and
objectives made after 1907 had a third glass recommended by Schott.
Lockyer's problem objectives used the earlier formation, and those using
599 were the less hazed.


--
=============================================
Peter Abrahams telscope.at.europa.dot.com
The history of the telescope and the binocular:
http://home.europa.com/~telscope/binotele.htm

"Peter Abrahams" telscope.at.europa.dot.com wrote in message
4...
a refracting telescope based on H. D. Taylor's design

There is nothing earth shattering about Taylor's design. It uses one
abnormal dispersion flint in triplet configuration to achieve a modest
reduction of secondary spectrum, not 10x less.

It is unclear if this response was to Taylor's 'Cooke Triplet', as
titled, or to Taylor's 'Photovisual' triplet, as described.

But Taylor's Photovisual was a very significant advance beyond the
refractors of its day. Abbe / Zeiss and C. Hastings were involved in
similar work, but Taylor was right there with them, and Cooke was ahead
of them in production, which was unfortunate since the glass
deteriorated badly.
Some details on the glass, and more text than I really should send out
in a post to a newsgroup, are below, part of a text on HDT found on my
site:
----

SNIP!

Interesting read. Thanks!
--
Jan Owen

To reach me directly, remove the Z, if one appears in my e-mail address...
Latitude: 33.662
Longitude: -112.3272

Peter Abrahams telscope.at.europa.dot.com wrote in message . 44...
a refracting telescope based on H. D. Taylor's design

But Taylor's Photovisual was a very significant advance beyond the
refractors of its day. Abbe / Zeiss and C. Hastings were involved in
similar work, but Taylor was right there with them, and Cooke was ahead
of them in production, which was unfortunate since the glass
deteriorated badly.

Not true. We have one such 7" Cook triplet here in three miles from my home.
It works just fine.


VD