Technical Meeting: Conceptual mechanical design and analysis II
Agenda and Discussion Material
- Nasmyth Optical Table
- Tilt of table due to thermal effects
- Conclusions on likely behaviour
- Any further analysis needed?
- Baseplate for FTT optical components
- XS analysis:
- Conclusions on conceptual design of baseplate (dimensions, material etc.)
- Any further analysis needed?
- Mounts for FTT optical components
- XS analysis:
- Conclusions on conceptual design of mounts
- Any further analysis needed?
Nasmyth Table Global Tilt
From DFB email 2010-06-15
A quick calculation to indicate what the thermal drift of the table
tilt might be. Assume that the table is supported by a vertical
support (representing the side of the telescope) and a diagonal
support running from the telescope to the end of the table furthest
from the table. Collapsed into 2 dimensions three items together form
a right-angle triangle, and for simplicity we will assume it is
equilateral, i.e. the diagonal support is at 45 degrees (the final
numbers change according to the tan of this angle, so in this range it
does not make too much difference exactly what angle this is). We will
assume that all three edges of the triangle are perfectly rigid, but
that the joints are flexible, and that the vertical support is
perfectly vertical at all times, i.e. forms a reference surface.
First let us assume that the table and the supports are of carbon
steel. When the system is heated or cooled (uniformly), all the sides
of the triangle change proportionately, so the table remains perfectly
horizontal. Now assume that the table is stainless but the supports
remain as carbon steel. There is approximately 5x10^{-6} difference in
CTE, so the table will not "fit" into the triangle in a perfectly
horizontal position, there being a mismatch of 5 microns per meter of
table per degree C. The only way the system can remain rigidly
attached together is for the triangle to deform. Some simple geometry
will show that the deformation is such that one end of the table will,
for this geometry, rise or fall by the same 5 microns per meter per C
compared to the other end, which means that the table tilts away from
the horizontal at just over 1 arcsecond per degree C.
Over 5 degrees C temperature change the table will tilt by 5 arcsec,
which is way outside the error budget, which requires of order 0.1
arcsec stability. We can compensate for this by measuring the
temperature, this would require knowing the mean temperature of every
member of the structure to 0.1C.
These numbers will change by small factors if we introduce a more
realistic geometry, but unlikely to change the general conclusion...