Technical Meeting #9: Conceptual mechanical design and analysis II

Agenda and Discussion Material

Nasmyth Optical Table
- Tilt of table due to thermal effects
  - XS analysis: Attach:FTT_mount_analysis_05.pdf
  - DFB analysis: see below
- 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?
  - See TechMeet6 for layouts
  - Error budgets

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...