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Seeing Control & Turbulence Compensation AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan.

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Presentation on theme: "Seeing Control & Turbulence Compensation AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan."— Presentation transcript:

1 Seeing Control & Turbulence Compensation AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan

2 Local Seeing: Thermal Control One of the major developments: understanding of and reduction in "local seeing" ("dome seeing") The most common sources of local convection are: Large scale convection through entire dome from a floor warmer than the air, or telescope parts colder than the air Mirror seeing from differences in primary or secondary mirror and air Heat sources on telescope or in dome It is useful to maintain the temperature of all systems near the path of the light as close to the ambient air temperature as possible Maintaining thermal control is not only important for reducing local seeing but maintaining proper alignment/figure of optical surfaces (in the infrared: potentially reducing thermal background) Majewski

3 Thermal Control Passive: using coatings, insulation, radiating surfaces, heat pipes to control external heat input or the dumping of internally created waste heat Active: using heaters, coolants, and ventilation (e.g., fans and louvers) Majewski

4 Optical Alignment/Figure Nonuniform axial temperature gradients are a problem for mirror figure The use of low expansion or low thermal inertial glass compensates for this Meniscus mirrors with adaptive optics can correct for figure changes, even if high expansion glass is used Larger monolithic mirrors can be made with honeycomb cells that are ventilated actively Bely Majewski

5 Optical Alignment/Figure Segmented mirrors generally account for errors between segments, but not thermally induced deformations in each segment In this case low-expansion glass is important The telescope mirror will change focal length with temperature changes, and the tube/truss will change length as temperature changes Use of low-expansion rods (like Invar or ceramic) can be used to keep the focal plane a fixed distance from the mirror cell Alternatively, can measure the temperature of the struts and actively alter the focal plane distance according to temperature Majewski

6 Telescope Temperature Traditionally (19th century), it was known that refractors can have better seeing than reflectors because of the stability of air in column inside closed tube For large reflectors, realized that important to get stability of air column in light path too The top of the telescope tube, which has a wide view of the sky, cools down radiatively faster than other parts. Can create downflows of air onto mirror Coat telescope parts with low-emissivity paint, or insulate with, e.g., aluminum foil Ventilation of telescope environment critical Majewski

7 Mirror Seeing Temperature differences between mirror and air create very thin (few mm) but very turbulent convective layer Ventilation (as shown above) helps Keep mirror as free as possible from surrounding structures Natural flushing by wind: wind flushing decreases temperature differences, but increases dynamical effects, thus there is an optimal wind speed Active heating and cooling of mirror Bely Majewski

8 Observatory Enclosure Thermal Control Important to keep all unnecessary heat-generating equipment away from telescope Now put everything possible, including observing room, labs, offices, motors, chillers, etc. in other, insulated rooms or even other buildings Thermal barriers separating rooms Locate air exhausts away from enclosure and downwind. Secondary exhaust in case wind direction changes Important to reject solar heat during the day by insulating and cooling Majewski

9 Observatory Enclosure If the enclosure protects telescope from Sun during day, it can also be a source of degradation at night if it maintains temperature differences with ambient air Many domes/enclosures have been painted with white titanium dioxide paint: low solar absorptivity, reduces daytime heating. However, has high thermal emissivity and quickly cools by radiating to sky at night. Air passing over white paint is cooled and pockets of cold air can fall into dome opening, creating thermal turbulence Bely Now philosophy is to make skin of dome unpainted aluminum or cover with aluminum Mylar tape: Tracks ambient air temperature better. But MUST have good ventilation or cooling in day because aluminum highly heat conductive Majewski

10 Observatory Enclosure Keep daytime telescope room slightly overpressured and rest of building negative pressure to keep daytime airflow out from telescope At Fan Mountain we actively air condition the 40-inch dome during the day Other observatories actively chill the floor with cooling coils filled with something like glycol (e.g., KPNO 4-m) Creates a stratified thermal inversion that can be maintained at night in low wind Works well for older observatories with large thermal inertia But have to guess what the night temperature will be Bely Majewski

11 Observatory Enclosure Another solution, and cheaper, is to have low thermal inertia mirrors and open telescope structures and ventilate aggressively Includes even having fan-forced ventilation This philosophy drives the current design of modern telescope enclosures Bely Majewski

12 Observatory Enclosure In this new philosophy, important then to have a well-flushed enclosure Pockets of air at different temperature cause turbulence Opening up the enclosure as much as possible allows wind flow through enclosure: Not bad if isothermal. It is bad if wind so high that telescope shakes. Need to compromise with variable openings to adjust degree and direction of natural flushing When enclosure open, wind should flow smoothly so as not to excite high frequency modes of telescope Implicit in all of this is that when you observe, important to: Open the dome, dome slit, doors, louvers, mirror/lens cover etc. after sunset (in twilight) and before observing to equilibrate your equipment with the ambient air as soon as possible. Be cognizant of thermal sources in the dome and airflow through dome Majewski

13 Observatory Enclosure Dome size and shape Recent trends are to make the telescope enclosure as small as possible Cheaper Easier to flush: A uniform air-flow of 1 m/s flushes a 30 m enclosure 120 times per hour Bely Majewski

14 Observatory Enclosure Three main types of enclosure Traditional dome (e.g., McCormick, Fan Mountain, KPNO/CTIO 4-m, etc.) Dome clears telescope in all directions Can rotate dome separate from telescope (often useful) But can foster stagnant air pockets and internal vortices depending on wind angle of attack: Many traditional domes now have louvers and fans inserted to fix this problem Better shape for minimizing snow/ice loads Bely Majewski

15 Observatory Enclosure Corotating building (e.g., MMT, LBT) Can tuck stuff closer to telescope, which corotates with it (don't need large "clear space" for telescope to swing through Smaller building possible, but need to move much more mass Can create more pockets and air funneling All electrical lines and fluid pipes become complicated to deal with "wrap- up" Bely Majewski

16 Observatory Enclosure Roll-off roof/hangar or retractable enclosure (e.g., McCormick "doghouse", Sloan telescope) 2005/july/sky_survey.htm Should roll-off far enough to away from scope and on downwind side to prevent wakes Often wind baffles installed to minimize shaking Difficult for large telescopes because wind baffles need to be enormous and movable -- difficult engineering Majewski

17 Observatory Enclosure Dome shapes have been well studied in wind tunnels Best shapes for flushing actually been found to be the two on the right below Bely The octagonal shape was used for WIYN /gallery/gallery/ Majewski

18 Observatory Enclosure In all cases, the use of louvers, openings and windscreens is critical Openings in the walls, with adjustable louvers, can control wind flow and direction Bely Majewski

19 Bely Observatory Enclosure Windscreens are generally a set of panels or canvas covers that are raised along the lower or upper parts of the dome and can control airflow and prevent high winds from rocking telescope An "up and over" shutter can sometimes be used as a windscreen Ventilation on flat surfaces better than curved (air flowing around the curved surfaces creates negative pressure that prevents inward flow) fwalter/ctiopics.html Majewski

20 Dome Sitting Want to make sure atmospheric surface layer does not enter into enclosure or flow over telescope enclosure Dome should be elevated above layer and enclosure not interact with it Dome shape and support can change surface layer flow; some designs "lift" layer over dome Ideal mountain shape is an isolated conical peak: Impinging airflow tends to divide and flow to either side of peak, rather than up and over, Ideal slope angle on windward side is 7-18 degrees Bely Majewski

21 Dome Sitting If peak is flat, observatory should be placed as close to windward ridge as possible to sit in unperturbed flow: WIYN telescope on Kitt Peak has best seeing on mountain, partly for this reason Multiple telescopes should be laid out perpendicular to wind to avoid interference and wakes Ridges not as good as single peaks (disturb air-flow, tend to push it up- slope) Bely Same sitting considerations will also minimize dust getting into dome Majewski

22 Dome Sitting Finally, how telescope attached to mountain is important for minimizing vibrations to telescope Generally, concrete pier attached to bedrock, but isolated from rest of structure Damping layers (sand, lava cinder, loose soil) helpful Fractured bedrock more prone to vibration, so minimize stress on rock during construction Bely Majewski

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