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Mt. Fowlkes DIMM Tower

Explore innovative DIMM Tower designs at CTIO, ESO, and NSO, including features like concrete center, wind barriers, and enhanced drive mechanisms. Discover valuable insights from completed systems and conclude with proposed tower concept elements like stability, personnel access, and wind baffling. A detailed breakdown of structural analyses and foundation layouts are also provided.

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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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