Light-weight mirror blank for astronomical purposes and method of making a supporting framework for such blanks

A light-weight mirror for astronomical purposes is disclosed. The mirror consists of a mirror plate, a back plate and a supporting framework disposed between them and consisting of a plurality of rows of tubes. The rows of tubes are staggered one from the other. Each tube in a row has a line or strip of contact with two adjacently disposed tubes of the next row. The tubes are welded together along the contact line or strip. The thickness of the tube wall is reduced in the area of the contact line or strip.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a light-weight mirror blank for astronomical 
purposes, having a mirror plate of transparent vitreous silica or 
high-silica glass, a back plate of transparent vitreous silica, opaque 
vitreous silica or high silica glass and a supporting framework disposed 
between the mirror plate and the back plate and made of transparent 
vitreous silica, opaque vitreous silica or high-silica glass and 
consisting of a plurality of rows of tubes, the rows of tubes being in a 
staggered relationship to one another and the individual tubes being 
permanently joined to the mirror plate and back plate, the tube axes being 
parallel to one another and to the mirror axis. This invention further 
relates to a method of making a supporting framework for mirrors of the 
kind specified above. 
2. Discussion Of Prior Art 
U.S. Pat. No. 1,888,341 discloses a light-weight mirror blank for 
astronomical purposes, which has a supporting framework comprising a 
plurality of rows of tubes, the rows of tubes being in a staggered 
relationship to one another (FIG. 5). The tubes consist of transparent 
vitreous silica. The tube axes are parallel to one another and 
perpendicular to the mirror axis. The tubes are joined to one another at 
their junctions. 
A light-weight mirror blank for astronomical purposes is known from U.S. 
Pat. No. 2,988,959, which has a mirror plate and a back plate between 
which a supporting framework of a plurality of rows of tubes is disposed, 
the tube rows being in a staggered relationship to one another. The tube 
axes are parallel to one another and parallel to the mirror axis. Glass is 
used as the material for this light-weight mirror blank. The individual 
tubes are permanently joined to the mirror plate and the back plate by 
means of a cement. The tubes can have a circular or rectangular or 
triangular cross section. 
Lastly, another light-weight mirror blank for astronomical purposes is 
known from British Pat. No. 968,025, which has a mirror plate and a back 
plate as well as a supporting framework permanently joined to the two 
plates and made of tubes of hexagonal cross section assembled in honeycomb 
form. Transparent vitreous silica, opaque vitreous silica or a glass of 
high silica content is used as the material for the light-weight mirror 
blanks described therein. 
In the known light-weight mirror blanks, either the supporting framework 
weight is very low while having at the same time a low transverse 
stability, or, if the transverse stability is high, the weight reduction 
of the supporting framework is only imperfect. 
SUMMARY OF THE INVENTION 
Accordingly, it is the object of the invention to provide a light-weight 
mirror blank of the kind described in the beginning, which is 
characterized by a weight reduction of the supporting framework such that 
the weight can amount to only 8% of that of a corresponding solid body, 
while at the same time the mirror blank has a high transverse stability 
and stiffness. 
This object is accomplished by the invention in a light-weight mirror blank 
of the kind described above by staggering the rows of tubes such that each 
tube in a row has a line or strip of contact with two adjacent tubes of a 
neighboring row, that the wall thickness of the tubes in the area of the 
contact line or strip is reduced in comparison with the rest of the wall 
thickness of the tube, and the tubes are welded together along the contact 
line or strip. 
When reference is made herein to a high-silica glass, this term is to be 
understood to mean a vitreous material whose silica content amounts to a 
least 90% by weight. 
It has been found practical to reduce the wall thickness of the tubes in 
the area of the contact lines or strips by ten to fifty percent of the 
thickness of the rest of the tube, the rest of the wall thickness ranging 
from 0.8 to 5 mm. 
For making the supporting framework, tubes of square or rectangular cross 
section with rounded corners have been found to be particularly desirable, 
in which the tube wall thickness is reduced in the corner areas. The tubes 
in this case are advantageously disposed in a checkboard pattern, leaving 
spaces between individual tubes of each row. 
If the supporting framework consists of tubes of circular cross section, 
the contact strips of reduced wall thickness of each tube are in the form 
of longitudinal raised ribs. The tubes can have the raised ribs in their 
original configuration, or the raised ribs can be formed when the 
supporting framework is welded together. 
If tubes of triangular cross section are used, especially in the case of 
equilateral triangular cross sections, the arrangement of the tubes is 
such that the tubes of a row are disposed so as to abut one another such 
that the base lines of the triangular cross sections form a straight line. 
The weld along the contact line or strip can be continuous or it can be a 
spot weld like the spot welding known in conjunction with the joining of 
metal parts. 
The reduction of the wall thickness of the tubes along the contact line or 
strip has the advantage that it can be brought rapidly to the welding 
temperature, and the construction of the supporting framework in 
accordance with the invention does not entail any loss of transverse 
stability and stiffness in the supporting framework.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
The lightweight mirror blank represented in FIG. 1 consists of the mirror 
plate 1 and the back plate 2 and of the supporting framework 3, which is 
formed of the tubes 4. In this embodiment, the mirror plate, the back 
plate and the tubes of the supporting framework consists of transparent 
vitreous silica. The tube axes 8 are parallel to one another and parallel 
to the mirror axis 9. 
In FIG. 2, which represents a detail of a horizontal section through the 
supporting frame 3 in the plane A-B of FIG. 1, the supporting framework 
was made of tubes of square cross section with rounded corners, in which 
the wall thickness of the tube is reduced in the area of the corners. In 
the area of the corners, the wall thickness of a tube amounts to 0.5 to 
4.5 millimeters. The rest of the wall thickness of the tube, which is not 
reduced, amounts to 0.8 to 5.0 millimeters. As it can be seen in the 
figure, the tubes are arranged in a checkerboard pattern leaving a space 5 
between the individual tubes of each row. The tubes are welded together at 
the corners forming a narrow plane of contact 6. 
The detail view of a horizontal cross section through a supporting frame 
represented in FIG. 3 differs from the one represented in FIG. 2 in that 
the supporting framework consists of tubes of rectangular cross section. 
The detail view of a horizontal cross section through a supporting 
framework represented in FIG. 4 is of a design in which tubes of 
triangular cross section are used, and in particular the side lengths of 
the triangle are equal. The design is again checkeboard-like, the tubes of 
a row being disposed so as to abut one another such that the base lines of 
the triangle cross sections form a straight line 7. The tubes of 
triangular cross section are welded together along the line of contact 6'. 
In this arrangement, too, a space 5 is present, which contributes to the 
reduction of the weight of the supporting framework, which is also the 
case with the space 5 represented in FIGS. 2 and 3. 
As it is apparent from the detail given in FIG. 5 of a horizontal section 
through a supporting framework, tubes of hexagonal cross section are used 
for the framework, and they are welded together at the contact lines 6', 
and again, spaces 5 are left between them. 
Finally, FIG. 6 shows another embodiment, in which tubes of circular cross 
section are used to form the framework. The wall thickness of these tubes 
is diminished along the contact line or strip 6, and this area of 
diminished wall thickness is in the form of a raised longitudinal rib. The 
tubes are welded together along the contact line 6. 
In Fig. 7, the diagrammatic representation shows an apparatus for making a 
supporting frame for a light-weight mirror blank. It comprises the furnace 
10 having an electrical resistance heater 11, and is provided with lines 
12 for the delivery of inert gas such as nitrogen. The furnace 10 is 
provided with the removable cover 13, which is movable on rollers 14, for 
example. The cover has an opening 15 through which a welding device 16 
consisting of the two gas torches 17 is lowered into the furnace cavity 
and can be raised out of it by means of the motor 18 and the raising and 
lowering apparatus 20 connected to the motor shaft 19. The gas torches 17 
are affixed to the raising and lowering apparatus 20, which in turn is 
mounted on the frame 25 which is disposed so as to be able to travel at 
right angles on the furnace cover. The precise position of the gas torches 
17 within the furnace can be observed through the optical system 21, 21', 
21". 
The making of a supporting framework for a light-weight mirror blank is 
accomplished in the following manner: 
With the furnace chamber cover retracted, first the tubes 4 are lowered in 
rows onto a base plate 23 covered with a thin layer of graphite paper 22 
and consisting of transparent or opaque vitreous silica, the individual 
rows of tubes being staggered such that two portions of reduced wall 
thickness in each tube of a row are in contact with or at a distance of up 
to 1.5 mm from an area of reduced wall thickness of two adjacently 
disposed tubes of the neighboring row. Fillers or graphite, vitreous 
silica or zirconium dioxide are placed in the intermediate spaces 5. Then 
the furnace 10 is closed with the cover 13 and flooded with an inert gas 
through the gas feed line 12, the electrical heating is turned on, and the 
interior of the furnace is heated to a temperature of 800.degree. to 
1000.degree. C. As soon as the tubes disposed within the furnace have 
reached this temperature, the welding apparatus 16 with the two gas 
torches 17 are lowered into the furnace by means of the motor 18 through 
the motor shaft 19 and the raising and lowering apparatus 20, this being 
done in such a manner that only one gas torch enters into one tube. 
While maintaining the temperature and the inert gas atmosphere in the 
furnace, two thinned wall thickness areas of two tubes which are to be 
joined are heated to the welding temperature, and welded together 
beginning from the base plate 23 and moving in the direction of the cover 
13. The welding together of the tubes takes place along the line of 
contact of the two tubes, although the flame gases do not directly play 
upon the line of contact, the flame gases striking only the inner side of 
the thinned wall section. 
After completion of a weld, the cover 13 and/or the frame 25 are shifted 
over to the next welding position and the welding of two tubes along 
another line of contact is performed in the same manner as described 
above. After all tubes have been welded together, the supporting framework 
thus formed is again heated to the transformation temperature of the 
supporting framework material and then, after a slow cooling down, the 
supporting framework is removed from the furnace. The transformation 
temperature for tubes of transparent vitreous silica ranges from 
1050.degree. to 1150.degree. C. 
It has proven desirable before beginning the welding of the individual 
tubes to join them together at their ends facing the cover 13 by spot 
welding at the thinned portions of their walls. During the subsequent 
welding together of the tubes along their lines or planes of contact, this 
prevents them from shifting in relation to one another. 
The production of a mirror blank consisting of the mirror plate 1, the back 
plate 2 and the supporting framework 3 made in accordance with the 
invention can be peformed in a known manner, for example by fusing the 
mirror plate and back plate to the supporting framework as described in 
U.S. Pat. No. 3,728,186 in conjunction with FIG. 7.