# Technical Meeting #9: 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: Attach:FTT_mount_analysis_04.pdf
- Conclusions on conceptual design of mounts
- Materials
- How to locate and support optics
- Need for kinematic connection to baseplate?
- Any further analysis needed?

- Transmissive versus Reflective layouts
- Do we still prefer transmissive layouts, given the difficulty of making the system achromatic over 350-1000nm?

## Conclusions

- Suggestions for NMT:
- Redesign table support structure to allow kinematic support of table
- Try to get more uniform thermal inertia for table and support structure
- Consider mild steel surfaced table (but need to prevent corrosion)
- Re-determine TT zero point more often
- Move TT zero point during observation based on thermal model
- Design new system to measure TT zero point frequently or continuously

- Propose building prototype mounts out of aluminium
- No adjustments
- Use 3 point contact for flat-faced optics

- If performance of prototype Al mounts proves inadequate, fallback is invar for mounts, which drive us to use invar for baseplate

## New Tasks

- MF: Ask NMT (again) and/or Newport for details of table design
- MF/XS: Revisit film coefficients to be sure we are considering the worst cases
- ADR: Can we exchange 2nd lens to allow observations with blue light?
- ADR: What is best compromise single lens for both red and blue bands?

## 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...