Seeing Control &Turbulence Compensation

AS4100 Astrofisika PengamatanProdi Astronomi 2007/2008

B. Dermawan

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

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

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

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

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

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

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

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

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 windWorks well for older observatories with large thermal inertiaBut have to guess what the night temperature will be

Bely

Majewski

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

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Majewski

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

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

Observatory Enclosure

Three main types of enclosure• Traditional dome (e.g., McCormick, Fan Mountain,

KPNO/CTIO 4-m, etc.)Dome clears telescope in all directionsCan 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

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

Observatory Enclosure

• Roll-off roof/hangar or retractable enclosure (e.g., McCormick "doghouse", Sloan telescope)

http://www.nmsu.edu/~ucomm/Releases/2005/july/sky_survey.htm

Should roll-off far enough to away from scope and on downwind side to prevent wakesOften wind baffles installed to minimize shakingDifficult for large telescopes because wind baffles need to be enormous and movable -- difficult engineering

Majewski

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

http://astrowww.astro.indiana.edu/gallery/gallery/

Majewski

Observatory Enclosure

In all cases, the use of louvers, openings and windscreens is criticalOpenings in the walls, with adjustable louvers, can control wind flow and direction

www.noao.edu/outreach/0.9m/anight.htmlBely

Majewski

Bely

Observatory EnclosureWindscreens 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 telescopeAn "up and over" shutter can sometimes be used as a windscreenVentilation on flat surfaces better than curved (air flowing around the curved surfaces creates negative pressure that prevents inward flow)

www.ess.sunysb.edu/ fwalter/ctiopics.html

Majewski

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

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 reasonMultiple telescopes should be laid out perpendicular to wind to avoid interference and wakesRidges 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

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