Practical applications of high and low-reflecting films on glass

Practical applications of high and low-reflecting films on
glass
John Strong
To cite this version:
John Strong. Practical applications of high and low-reflecting films on glass. J. Phys. Radium,
1950, 11 (7), pp.441-443. <10.1051/jphysrad:01950001107044100>. <jpa-00234293>
HAL Id: jpa-00234293
https://hal.archives-ouvertes.fr/jpa-00234293
Submitted on 1 Jan 1950
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LE
JOURNAL
DE
PHYSIQUE ET LE RADIUM.
TOME
11,
JUILLET
19~O,
PAGE
PRACTICAL APPLICATIONS OF HIGH AND LOW-REFLECTING FILMS ON GLASS
Johns
Sommaire.
-
By JOHN STRONG.
Hopkins, University, Baltimore, Maryland.
Description (accompagnée
de
projections)
des
équipements d’évaporation thermique
utilisés pour aluminier les grands miroirs de télescope, en particulier le réflecteur Crossley de l’Observatoire Lick, les miroirs de 60 et de 100 pouces du Mont Wilson et, enfin, le miroir de 200 pouces de
l’Observatoire Palomar.
L’aluminiure des miroirs de célestat et des réseaux est discutée.
Discussion d’un procédé pour réduire la réflexion sur le verre, au moyen de couches déposées
par évaporation thermique. Ce procédé utilise une première couche à gradient dont l’indice au
contact du verre est le même que l’indice N du verre. Cette première couche a une épaisseur indéter-
minée
(environ 03BB/2) et
son
indice
superficiel
est voisin de N2 : elle
ne
d’épaisseur 03BB/4 est déposée
présente
pas de couleurs d’inter-
première, réduisant ainsi
efficacement la réflexion. On utilise, pour former ces couches, des substances qui adhérent fortement
au verre et présentent une grande résistance à l’abrasion. Ce procédé a été découvert aussi par Nadau
(brevet canadien n° 418.289).
férences. Une deuxième couche d’indice N et
sur
la
Introduction.
The purpose of this article is
supplement my former publications on applications of thin films to astronomical telescopes [1],
to enhance their reflectivity. This activity began
with my discovery of a practical method to evaporate
aluminum films. Owing to the support of the
Observatory Council at California Institute of
Technology, it was possible for me to coat many
mirrors extending from amateurs 6 inch mirrors
to the Crossley Reflector of Mt Hamilton, and
the 60 inch and 100 inch mirrors of Mt Wilson,
and finally to the 200 inch mirror of Mt Palomar.
In addition to a report of aluminizing the last I
shall report here, briefly, on a novel method of
decreasing the reflection of glass which supplements
my original publication on that art.
-
to
The 200 inch mirror.
Figure I shows the
chamber in which the 200 inch mirror was
aluminized. It is 1 g feet inside diameter and 7 feet
high inside. The walls and ends are made of o.5 inch
steel plate, the ends being reinforced with twelve
radial 13 inch I-beams. It weighs 3o tons.
The gasketed seal between the chamber and its
base plate appears in figure I at the third rung down
from the top on the ladder.
The two Westinghouse 1000 cubic feet per minute
pumping units, on the top of the chamber, produced
the necessary vacuum (about io-I mm of mercury).
-
vacuum
Fi g. 1 .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01950001107044100
~~I.
442
One of the five platform arcs which surrounded
the mirror, and afforded a walk from which workmen
could wash the mirror face, is shown standing
Fig.
between the chamber and the open hatch of the
observing floor in the telescope dome. Two of
these platforms appear again in figure 2.
2.
Fig. 3.
Figure
shows the mirror after being aluminized.
Hendrix, of Mt Wilson Observatory,
cooperated with me during the summer and fall
of 4~ and future aluminizations are now his respon2
Mr D. 0.
sibility.
The bottom and
top volumes in the chamber
are separated by the mirror face, and a rubber
gasket between the mirror and the chamber wall.
The necessary high vacuum was thus obtainable in
the upper volume of the chamber, while only a
moderate vacuum was possible in the lower part,
because of the many lubricated working parts
443
in the permanently attached mirror support.
The use of a silicon monoxide overcoat [2] was
considered but this was abandoned when Dr Babcock’s measurements showed only about I per z o0
deterioration of the reflectivity of the 100 inch
mirror in seven years. (This mirror was re-aluminized in 1947. Incidentally, the primary mirror of
the 48 inch Schmidt telescope was first aluminized
at the same time.)
The oil cleaning method of Colbert and Weinrich [3] was used. One of the holders of felt pads,
used to rub the mirror face, is shown in figure 2
sticking in the elevator-motor cage, and also, in
reflection, at the left side of the mirror face.
Aluminum was evaporated from 175 tungsten
coils, all in a plane 20 inches above the face of the
mirror. One half inches of 4o mil aluminum wire
The coils were in
was evaporated from each coil.
circular arrays of 20, 4o, 60, 70 and 100 inches
radius with 10, 20, z 4, 36 and 95 coils in each array.
I do not use a chromium substrate film either for
astronomical mirrors or for our diffraction gratings
here at Hopkins.
Non-reflecting film.
making glass of index 1.5
-
"
The novel procedure for
non-reflecting employs
"
.
This substrate
about ?’-ap roximand
ates
2
B
same
its index increases
/
gradually
and
N2 at its outer surface. The substrate
feeble
interference colors and it is
only
presumed an ideal film would show no color. This
affords a surface whose reflection, although much
higher than formerly, can now be effectively elimishows
nated
by
a ~‘
film of index N.
I succeeded in making such a film at first attempt
which reduced the visual reflection of plate glass
f rom 4 per 100 to o.5 per 100. The graded film
was deposited from two separate sources the first
was a quartz source [4] and the last was a titania
source.
The evaporation periods of these sources
were overlapped to effect the necessary gradation
in the film~ f rom pure quartz (N) to pure titania (N2).
Finally the usual magenta film of quartz was
deposited on the titania surface. Although ,I
believe this was the first such evaporated film, the
principle was subsequently found to have been
recognized by Nadeau [5].
REFERENCES.
[1] Astroph. J., 1936, 83,1 and references cited therein.
U. S. Patent, 2, 456, 899.
[2] John STRONG.
[3] COLBERT and WEINRICH.2014 U. S. Patent, 2, 383, 469.
-
film whose index next to the
as that of the glass, N.
film has an indefinite thickness
a graded substrate
glass is nearly the
[4]
[5]
U. S. Patent, 2, 386,
MORGAN.
NADEAU.2014 Canadian Patent, 418,
-
875.
289.

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