>From "Reflecting Telescope Optics" by R. N. Wilson Volume I second edition
(Springer Verlag, 2000) page 15
"Apparently he was also the first to use, systematically, pitch polishers
cut into squares. His discovery of Uranus in 1781 was made with an excellent
7 foot telescope with 6.2 inches aperture made in 1778."
"As his projects became more ambitious in size, Herschel was forced to use a
higher copper content in casting his speculum metal blanks to avoid
fracture. Typically for large blanks he used 73% copper and 27% tin. The
lower tin content made the mirror more prone to tarnish."
and then on pages 18-19:
" A blank cast in London in 1785 was polished and gave reasonable results. A
second blank cracked in cooling (Herschel did not understand the necessity
of slow cooling in the casting furnace) and a third was given an even higher
proportion of copper, a guarantee of rapid tarnishing. The focal length was
40 feet (f/10), a formidable mechanical undertaking. TGhe giant telescope
went into operation in 1789 and gave "pretty sharp images". The extra
light-gathering power enabled Herschel to discover two further satellites
of Saturn, Enceladus and Mimas. But, compared with the 20 foot telescope,
the 40 foot was a relative failure. The high content of copper in the mirror
caused rapid tarnishing; but, abover all, Hreschel had reached a telescope
size where mechanical problems became the limiting factor, rather than the
problems of optical figuring, though these were also formidable. His simple
support system, a radial iron ring, gave rise to considerable flexure
problems when the telescope was used at appreciable zenith distance. This
was exacerbated by the relatively thin mirror, although this had the
advantage of reducing the thermal sensitivity. Herschel used the
:"front-view" form which bears his name, having already experimented with it
with the 20 foot."
"For freshly polished mirrors, Herschel determined a reflectivity of 67%,
justifying his use of the Herschel focus. The optical quality of his smaller
telescopes must have been extraordinarily good. In a well-conceived
experiment using terrestrial objects he establishedthat a telescope of 8.8
inches would show a "real disk", as distinct from the "spurious disk", of
0.25 arcsec. This suggests clearly that the telescope was diffraction
limited, a concllusion supported by his observations when the telescope was
stopped down. His observational achievements indicate that his best
telescopes, including the 20 foot, were "seeing limited" for the seeing in
the Slough in England at that time, a quality of resolution not
significantly improved till the 20th century."
....
"Herschel was so much in advance of his colleagues, bosth astronomically and
technically, that a further 50-60 years were to pass before his achievements
could be surpassed. "
----- Original Message -----
From: "P T Chambers" <ptchamb@No-Spam>
To: "The Astronomy Connection" <sf-bay-tac@No-Spam>
Sent: Saturday, April 26, 2003 1:20 PM
Subject: Re: [TAC] Astro Quiz part?
> Hi
> I hate to mention this but Bronze is an alloy of Tin and Copper. Hence
> Bob Cz's reply.
>
> ---------
> Phil Chambers [ptchamb@No-Spam] (S.F. Bay Area - Calif. USA)
>
> On Sat, 26 Apr 2003, Rashad Al-Mansour wrote:
>
> > I was surprised to read this:
> >
> > The passage below is from a bio I found on Herschel.
> >
> > "At the end of the Seven Years war, William Herschel came to live in
England
> > and found work firstly in London as a copyist and then in Durham as a
> > teacher. At the same time, he continued his linguistic, mathematical and
> > astronomical studies and also polished the first bronze mirrors that he
was
> > to use in his first telescopes."
> >
> > Just imagine, cleaning your mirror meant that you also had to re-figure
it!
> >
> >
> > ----- Original Message -----
> > From: "Bob Czerwinski" <bczerwin@No-Spam>
> > To: "'The Astronomy Connection'" <sf-bay-tac@No-Spam>
> > Sent: Saturday, April 26, 2003 11:49 AM
> > Subject: RE: [TAC] Astro Quiz part?
> >
> >
> > > > > Just a guess, based on the material used by Lord Rosse: Tin
> > > > > and Copper?
> > >
> > > > Nope.
> > >
> > > Platinum (platina), maybe? That's from about the mid-1700s. Pretty
> > > gray, though.
> > >
> > > ..Bob...
> > >
> >
> >
>