Thanks RC,
Looks like Rich was right as well!
Further research shows that Herschel's telescopes were well received all
over Europe and were even exported to North Africa, Arabia, India and China.
In fact at this point in his life Herschel was making 4 times the money
annually selling scopes than was his salary from the King as Royal
Astronomer.
But still, I would hate to be the one who had to clean any of his mirrors!
;-)
Rashad
----- Original Message -----
From: "Richard Crisp" <rdcrisp@No-Spam>
To: "The Astronomy Connection" <sf-bay-tac@No-Spam>
Sent: Friday, April 25, 2003 2:05 PM
Subject: Herschel's scopes: materials etc was Re: [TAC] Astro Quiz part?
> >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...
> > > >
> > >
> > >
> >
>