Sky and Telescope - July 2017 - 31
NAMING OF PARTS The astronomer Eugène Antoniadi, famous for his planetary studies, drew this side view explaining the siderostat's components for a magazine titled Knowledge (1900). For smooth tracking, the forked mirror support (M) floated in mercury (inside N).
were most sensitive, the other with the usual red-blue correction for visual use. The crown-glass elements of both objectives were to be mounted on rollers so they could be separated
from the ﬂint-glass elements for cleaning.
Deloncle also pushed another innovation, encouraging
Gautier to use his mechanical skills to develop machines to
grind and polish optics to their ﬁnal shape, without the timeconsuming step of ﬁnal hand retouching necessary for ﬁne
astronomical optics (as is still usually required to make ﬁne
optics today). This technology, if successful, would promise "an immense step forward" toward refractor objectives
as large as 2 meters, wrote the French astronomer Camille
Flammarion in a popular article about the Paris Telescope.
Casting the massive, 2-meter siderostat mirror proved difﬁcult. Édouard Mantois, regarded as the world's best maker of
large glass blocks, turned down the job as too big. So it went
to Georges Despret, director of the Jeumont glassworks. Only
one of Despret's 12 casts proved acceptable (ironically, the
ﬁrst). Mantois went on to cast the less unwieldy 1.25-meter
ﬂint and crown blanks for both objectives.
Gautier's company had never made optics before and
needed nine months to grind the mirror sufﬁciently ﬂat.
Polishing the ﬂat was also time-consuming, because half
an hour of cooling was required after every two minutes of
s k y a n d t e l e s c o p e .c o m
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