The Wayback Machine - https://web.archive.org/web/20071103085804/http://etoile.berkeley.edu:80/~jrg/MIDDLE/

Major telescopes from Lord Rosse to about 1980

From 1800-1840 Fraunhofer's work on optical glass and its systematic application to refractors led to instruments such as the Dorpat 24 cm refractor.

Modest refractors were able to compete with or even excel reflectors with apertures up to 1.2 m (Herschel)---a measure of the two great weaknesses of the reflectors of the time:

Poor efficiency through low reflectivity
Problems of mechanical manipulation in such sizes.

Development of the reflector after Herschel was due to

Lord Rosse (William Parsons) & William Lassell in Great Britain.

In the 1830's,Rosse systematically investigated casting large speculum (Cu:Sn) mirrors.

The major problems were:

Fracturing due to temperature gradients that occur on rapid cooling
Crystallisation through cooling too slowly.

Herschel used high Cu content alloys for his largest mirror with bad consequences for the reflectivity.
Some workers avoided crystallisation by rapid cooling.
Rosse showed that this softened the material and led to an inferior polish.
Rosse compromising on the cooling rate

Rosse produced blanks up to

90 cm in 1839 and
180 cm (60 inch) in 1842 using an optimum alloy for polishing (68.2\% Cu and 31.8\% Sn).

Rosse pioneered lightweighted, built-up construction.

The Cu-Zn alloy ribs with the same CTE as the speculum faceplate.
Rosse detected no difference in optical performance between the 90 cm massive cast blank and the built-up one.
However, he decided on the massive cast approach for his 6-foot.

Five blanks were cast for the 6-foot

The first and last were polished
The other three breaking because of uneven temperatures in cooling.

Rosse was the first telescope maker to develop a polishing machine systematically.

Another Rosse innovation was the first use in a major telescope (1.82 m) of the whiffle-tree support invented by Thomas Grubb

Three plates support the mirror weight on universal joints at the centre of gravity of the three mirror sectors.

In turn, each support three more plates on universal joints.
With four stages, 81 supports were finally used at the back of the mirror.

Modern mirror supports are based on Grubb's whiffle-tree or the Lassell's astatic principle.

Rosse's largest 1.82 m primary had a focal length of 16.5 m giving f/9.0,

Only slightly shorter than Herschel's.
The asphericity of a given conic varies linearly with the size of the mirror and as F³
For a parabola, the difference from the circle with the same vertex curvature is

dz = y/(512 F³)
2.4 microns for Rosse's 6-foot mirror

Rosse parabolised from the sphere of equal vertex curvature formula by flattening the outer edge of the mirror.

The modern method abandons the equal curvature reference sphere, so that the radial aspherising function can be freely chosen

dz = -(c_par - c_sph) y²/ 2 + c_sph³ y⁴/ 8
If the two terms are made equal at the edge of the aperture then material must only be removed in a zones.
The amount of material removed is reduced and the zone of the zero removal can be chosen at will.

Rosse apparently was the first to introduce zonal testing using masks.

He knew, like Herschel, that a paraboloid does not produce a perfect image if the test object is not at infinity.
Rosse calculated the focus shift for different zones and measured this with his zone masks.

Rosse recognised the disadvantage of field coma caused by Herschel's tilted mirror

Reverted to the Newtonian, made possible by the better reflectivity of his speculum.

Recognising the mechanical problems of Herschel's largest telescope, Rosse renounced entire sky coverage.

Instead, the iron tube swung between walls on trunnion bearings along the meridian.

By wedging the bearings on each side the telescope could observe an object on the equation for about 1.5 hours.

Rosse discovered the spiral structure of external galaxies and found

that parts of some spiral nebulae could be resolved into stars.

Not yet a modern telescope.

About the same time William Lassell also made fundamental advances.

Lassell's casting techniques and alloys were similar to Rosse's and successful up to 1.22 m (4 feet) at f/9.2.

Lassell's largest telescope was set up in Malta in 1861.
This telescope possessed three notable features

A primary mirror support system based on astatic levers (invented for a 9-inch telescope in 1842)
A fork-type equatorial mount
An open tube to permit natural ventilation of the air

The 1.22-m telescope had a multi-lever astatic support; the precurser of the majority of support systems of subsequent telescopes.

Lassell, erroneously, believed that the lever arms had to be kept horizontal.

Lassell designed the telescope tube to be rotatable about its axis.
Experience demonstrated that this rotation made no difference and that the levers worked well in all telescope attitudes.
Lassell's error arose from unfamiliarity with the new equatorial mount.
With an Alt-Az mount, lever arms arranged horizontally in the zenith position would have stayed so at all elevations.

Lassell's rotating tube did provide easy access from the observing tower to the Newtonian focus.

If Lassell had used the Cassegrain form, this telescope could have rated, from its opto-mechanical form, as the first modern reflector; its speculum mirror belongs to the pre-modern era.

Another important telescope is James Nasmyth's 20-inch (51 cm) built ~ 1845.

It used Cassegrain form

Nasmyth added a flat to send the beam through the hollow altitude axis to a fixed focus
This invention and makes the Nasmyth telescope the direct precursor of the ESO's NTT, the Keck and VLT.

Nasmyth's 20-inch and a 15-inch Newtonian-Cassegrain built by Thomas Grubb (1835) for Armagh Observatory represented the first successful manufacture of a convex Cassegrain secondary for a major telescope, almost 200 years after its invention.

The Great Melbourne Reflector Fiasco

The last telescope of the speculum mirror era was the 48-inch Melbourne reflector.

A committee, chaired by Robinson, included Rosse, John Herschel and Lassell

Lassell offered his own 48-inch telescope operating in Malta.

The offer was refused because a ``more manageable'' instrument was desired

This decision was based on the telephoto advantage of the Cassegrain form

A 1.22 m primary with f/7.5 and m₂ = -5.54.
Notable for a significant reduction in primary f/no. compared with Herschel's telescopes.

Combined with the Cassegrain form, the fast primary, presented a new dimension of optical manufacturing and testing difficulty.
The contract was given to Thomas and Howard Grubb who proposed a 27-pad whiffle-tree support
The tube was lightweighted for ventilation and reduction of windloading.
No dome, only a sliding-roof, a direction which is being proposed again for modern telescopes.

Had the committee decided on silver-on-glass mirrors, it could have been the first successful modern telescope.

They preferred speculum, believing that silver was risky in the Melbourne climate.

The first successful chemical silvering was displayed at the Great Exhibition 1851

Silver-on-glass was used by Foucault in a 10 cm telescope and in 1857

Subsequently used in reflectors up to 80 cm
The larger ones profiting from his knife-edge.
The largest, 80 cm, was completed in 1862.

The decision to use speculum in the Melbourne reflector, taken five years after the completion of Foucault's first silver-on-glass telescope appears as an inexcusable blunder,
Tarnishing of speculum was well-known and frequent repolishing was required, a complication avoided by silver-on-glass.
The extreme temperature changes and humidity of Melbourne was very unfavourable to speculum

The building gave inadequate dust and wind protection.

After only 15 years the the director attempted to repolish the tarnished primaries but the optics never worked again.

The Melbourne reflector is seen as one of the greatest tragedies in the history of telescopes. In 1904 G. W. Ritchey wrote:

``I consider the failure of the Melbourne reflector to have been one of the greatest calamities in the history of instrumental astronomy; for by destroying confidence in the usefulness of great reflecting telescopes, it has hindered the development of this type of instrument, so wonderfully efficient for photographic and spectroscopic work, for nearly a third of a century''.

The lessons of the Melbourne reflector are fundamental and a warning:

Design by a committee rather than dedicated individuals.

Previous successful telescopes (by Herschel, Rosse and Lassell) were designed and built by enthusiasts who optimized and used them.

Failure to involve the designer-manufacturer in the erection and optimization of the telescope on site
Failure to give authority to an astronomical director of enthusiasm, vision and astronomical and technical competence to ensure a science program suited to the telescope and the necessary technical expertise to maintain it.

Glass Optics Telescopes up to the Palomar 200-inch

Reflectors made steady progress without exceedind the aperture achieved by Rosse (1.82 m) before the end of the nineteenth century.

In the late 1880's the refractor reached its zenith with Alvan Clark's 36-inch (Lick) and 40-inch (Yerkes)
In 1859 Foucault invented the knife-edge test, the first scientific test of telescope mirrors of high sensitivity.
After 1865 silvered glass dominated mirror technology.

In spite of tarnish by sulphur and moisture and poor reflectivity in the far blue
One of the most important advances in the history of the reflecting telescope

Ease of replacement
Average high reflectivity compared with speculum
Favored the development of the Cassegrain because two reflections were acceptable.
Glass is much lighter than speculum
Photography also favored the reflector because of its completely achromatic

Foucault, Draper, Brashear, H. Grubb, With, Calver, Martin, Eichens, Gautier and Common were the principal successful manufacturers up to 1900.

Calver made a 36-inch (91 cm) silvered glass mirror for Common in 1879.

Common attempted to make a 5-foot (152 cm) Cassegrain using a blank with a hole cast in it
This work revealed problems arising from normal crown glass
High expansion coefficient of (~ 80 x 10^-7)
Low thermal conductivity

The most notable pre-1900 reflector used Calver's 36-inch (91 cm) silvered glass mirror in a Newtonian telescope by Common for Edward Crossley's private observatory, Halifax, England.

This telescope was presented to the Lick Observatory in 1895
Re-figured and remounted by H. Grubb.
The remarkable feature of Calver's mirror was its fast f/5.8 primary focal ratio
The asphericity of the Calver mirror was about four times higher than similar sized mirrors of Rosse.
The Crossley was really the first modern reflector

Introduced modern astrophysics through the work of James Edward Keeler.
This historic telescope is still in operation

George Willis Ritchey

The turn of the century saw the emergence of one the greatest telescope builders, George Willis Ritchey

G. E. Hale was instrumental in building the 40-inch Yerkes refractor but recognised the potential of the reflector.

G. W. Ritchey had made a 23-inch (60 cm), f/3.9 (!) reflector which he set up at Yerkes in 1901 after his appointment by Hale.

Ritchey was interested in photography and recognised that higher speeds (f/numbers) were essential.
In 40 years, primaries progressed from f/9.2 (Lassell, 1861) to f/7.5 Melbourne (Grubb, 1869), to f/5.8 Crossley (Calver, 1879), to f/3.9 (Ritchey, 1901).
Impossible without Foucault's invention of the knife-edge test.

Ritchey's 60 cm was the first telescope to possess all the following features characterising a modern telescope, albeit with a modest size:

Glass mirrors (silvered)
A high speed primary (f/3.9)
A Cassegrain focus for spectroscopy with a fixed spectrograph
An open frame, ventilated tube
An equatorial mounting

Ritchey's figuring techniques and testing was a milestone

His method of parabolising avoided working the edge of the primary since the aspherising function can be chosen at will by varying the curvature of the reference sphere relative to the vertex curvature of the desired paraboloid.

This does not represent minimum removal of material, and some material must be removed right up to the edge.

In testing the primary mirror at the center of curvature Ritchey found that it was unsatisfactory to employ an eyepiece in determining the focus of the successive zones

The difficulty was due to the change of curvature, especially in the outer zones.

If zones of the usual width (15-20mm), were used it was found that the difference of focus of the inner and outer parts of a zone was so great that the image in the eyepiece showed evidence of strong aberration; while if narrow zones (3-4mm) were used the image in the eyepiece was very indistinct, as a result of the strong diffraction from the edges of the screen.
Finally it was found that very narrow zones or arcs could be used by employing the knife-edge instead of an eyepiece for determining the position of the focus
This method was found to be so accurate that, with some practice, the focus of a zone could be determined without difficulty to within 0.04 mm.

The primary was not only tested at its center of curvature, but also in auto-collimation and double-pass with a plane mirror of similar size.

A plane mirror of the necessary high quality and in a diameter sufficient for a 1.5-m primary was an undertaking in its own right.

The way was open for Ritchey & Hale and to produce large telescopes of modern form.

Hale had procured a 60-inch (1.52 m) plate glass blank of about 20 cm thickness from the St. Gobain glassworks in Paris.
Because of the greater size of the 60-inch Ritchey relaxed the f/number to f/5.0 compared with f/3.9 for his 24-inch
The 60-inch telescope completed in 1908

It was the first telescope to offer Newton (f/5), Cassegrain (f/20 & f/16) and Coude foci (f/30).
Ritchey recognised the importance of avoiding temperature changes in the mirror and strict rules were established to achieve this by control of the conditions in the dome
Images were as good as those obtained in the optical shop.

Photographs at the Newton focus after 11 hours exposure showed images < 1"

A new standard of optical quality, above all for a telescope of this size and speed.

At last, a versatile, manoeuvrable telescope of very high quality was available with a size only surpassed (still) by Rosse's 6-foot telescope of 1845"

The 60-inch telescope at Mt. Wilson spelled the definitive death-knell of the large refractor.
Together with W. Herschel's 20-foot focus telescope, Ritchey's 60-inch was arguably the greatest relative advance in astronomical observing potential ever achieved. realised.

Anticipating the success of the 60 .inch telescope while it was still to be built n 1906 Hale ordered in similar blank of 100-inch (2.54 m) diameter which was delivered in 1908.

Because of the high glass mass (4.5 t), the melted glass was poured from 3 separate pots leading to bubble concentrations at the joints.

Ritchey at first refused to work the blank for fear the bubbles could provoke a breakage.
Three further castings failed or were too thin, so Ritchey agreed to work the original blank.

Optical work started in 1910 and took over five years.

Test procedures were similar to those used for the 60- inch, except that a quantitative Hartmann test was added.

The aspect ratio of the blank was 8.0 and the relative aperture of the finished mirror f/5.l.
The telescope has an English cradle-type mount.

This does not allow access to the pole but offers symmetry and rigidity.

The light grasp of the 100-inch Mt. Wilson telescope was almost 3 times that of the 60-inch.

Together, these two telescopes transformed astrophysics and cosmology.

Observations by Hubble and Humason (1924) of M31, and other galaxies, had sufficient angular resolution to Cepheid variable stars to be observed and their distances determined.
Proved definitively the island universe theory of W. Herschel that spiral nebulae were external galaxies similar to the Milky Way system.
This led in 1929 to Hubble's redshift law of the expansion of the universe and the Big Bang theory of cosmology.

Oct	NOV	Jan
	03
2006	2007	2008