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Mt. Fowlkes DIMM Tower

Mt. Fowlkes DIMM Tower. Concepts – A Second Look 6 August 2002 John Good. What Others Have Done. A world tour of DIMM Tower designs via the World Wide Web and telephone. CTIO ESO National Solar Observatory (NSO) Apache Point. CTIO. CTIO. DIMM Tower. CTIO.

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Mt. Fowlkes DIMM Tower

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  1. Mt. Fowlkes DIMM Tower Concepts – A Second Look 6 August 2002 John Good

  2. What Others Have Done A world tour of DIMM Tower designs via the World Wide Web and telephone. • CTIO • ESO • National Solar Observatory (NSO) • Apache Point

  3. CTIO CTIO DIMM Tower

  4. CTIO • Spy quality image of enclosure. • CCAS-type design except with a concrete center and internal ladder. Door

  5. CTIO •  ”Clam shell cover must be clamped in rotation with a U-shape block on the eastern side. The southern half stays closed during observation (it is fixed anyway): tests have shown that it does not interfere in the seeing measurement and is a good wind barrier.” from the “Operations Manual”, section F – “Other Useful Tricks”.

  6. CTIO • Response to request for design information on the CTIO DIMM tower.

  7. ESO – Telescopio Nazionale Galileo (TNG) ESO

  8. ESO • FEA model of original tower design. • A Serrurier-type truss.

  9. ESO • Study of cable bracing to stiffen the original design. • Original lowest freq. = 7.5 Hz (target was 13 Hz) • New freq. = 15.4 Hz

  10. ESO • First layer of concrete foundation.

  11. ESO • Anchor weldments are positioned for encasement in upper layer of concrete. • 4 much smaller footings for personnel platform ~1 meter away from tower foundation.

  12. ESO • Weldments embedded in concrete.

  13. ESO • Personnel platform is erected 1st.

  14. ESO • Tower base* • *This is what happens when engineers spend too much time doing FEA.

  15. ESO • Top view of DIMM mount.

  16. ESO • Testing the fundamental frequency of the tower.

  17. ESO The Completed System

  18. ESO • Close-up of cloth enclosure. • Fits just around the DIMM. • Appears to be 1 piece custom-made. • Prevailing winds blocked by open shell??? Enhancement of Drive Mechanism

  19. ESO • Duff-Norton type actuator. • Same version as used on the big louvers.

  20. ESO Same basic design at VLT. • Large bottom-less canvass clam shell • Mounted on platform handrail. • Easier access to DIMM • Larger sail area. Big drive motor and transmission. Note box around lower portion of DIMM to supplement weather/wind shielding.

  21. National Solar Observatory (NSO) NSO

  22. NSO

  23. NSO

  24. NSO NOT Stiff Stiff

  25. NSO • This design was implemented at 6 locations • Cost = $13,000 ea. • Designed to minimize on-site work. • 5000 lb. Lift.

  26. NSO • Completed tower at Sacramento Peak.

  27. Apache Point • Trailer mounted. • Enclosed dome.

  28. Apache Point • Interior of Dome

  29. Apache Point • Interior of Dome

  30. Apache Point DIMM

  31. Apache Point

  32. Conclusions From Other Installations • Tower shake DOES matter. • Frequencies > 10 Hz (15.4 Hz) were sought at ESO. • CTIO isn’t happy with theirs. • DIMM wind-shielding appears to have some benefit. • ESO doesn’t mention the problem, but it may not be one due to the shielding effect of the open clam-shell. • CTIO does mention it, but has other problems which might lead them to “finger-pointing”. • NSO is experimenting with wind shielding. • Closed domes have impact on performance. • Apache Point’s conventional dome degrades performance. • ESO’s bottomless canvas clam-shell makes sense and is simple. • Steel is desirable because of its rapid thermal response, but it has to be STIFF.

  33. Conclusions From Other Installations • The “Serrurier-Truss” types are elegant, but • Joints are not simple. • Broader footing is required than the ESO design. • If you make footing broader than the top has to be almost as big (like NSO did). • Access platform has to get bigger to wrap around it, or you have to integrate the platform into the tower (How do you isolate the wind shield?) • Interface to the DIMM gets clumsy, and could easily be the weak link in stiffness. • There are are easier ways to get a stiff structure. • Foundation should have significant mass. • But it helps a LOT to spread the feet out. • Personnel access platform should be well isolated.

  34. Proposed Design Definition • 5-7 meters above ground level. • Optically stable. • Low thermal mass. • Immune to wind shake. • Wind baffling for DIMM. • Safe personnel access. • Automated weather enclosure. • Physical Interface to DIMM (a Pier). • Electricity and conduits for controls. • ~$20k budget.

  35. Tower Concept • Proposed Height 16 Ft.

  36. Tower Concept • Proposed Location

  37. Combination enclosure-wind shield Astrohaven Galactica 360 Custom Canvas Proposed Design Tower Concept Isolated personnel platform • Over all view of whole thing. DIMM Tower

  38. Finite Element Analysis of Proposed Tower (an oil derrick) Tower Concept 3” x 3” Steel Angle (in green) 1.25” x 1.25” Steel Angle (in red) All joints are welded.

  39. Tower Concept First Mode = 42.5 Hz This tower is very easy to make.

  40. Tower Concept

  41. Tower Concept 4 Piers on 10’ centers • Foundation Layout 8’ x 8’ Slab 24” to 30” thick (could also be 4 separate piers)

  42. Tower Concept

  43. Tower Concept

  44. Tower Concept

  45. Tower Concept

  46. Tower Concept

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