Hey! Thanks, Rashad.
Herschel's sister is someone from the past who would
be really interesting to talk to.
Rich
>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...
>> > > >
>> > >
>> > >
>> >
>>