Transcript Seeing Control & Turbulence Compensation AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan Majewski Local Seeing: Thermal Control • One of the major developments: understanding of.

Seeing Control &
Turbulence Compensation
AS4100 Astrofisika Pengamatan
Prodi Astronomi 2007/2008
B. Dermawan
Majewski
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
Bely
• Active heating and
cooling of mirror
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
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
Bely
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 wind
Works well for older observatories
with large thermal inertia
But 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
Bely
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 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
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 "wrapup"
Bely
Majewski
Observatory Enclosure
• Roll-off roof/hangar or retractable enclosure (e.g., McCormick
"doghouse", Sloan telescope)
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
http://www.nmsu.edu/~ucomm/Releases/
2005/july/sky_survey.htm
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 critical
Openings in the walls, with adjustable louvers, can control wind flow and
direction
Bely
www.noao.edu/outreach/0.9m/anight.html
Majewski
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)
Bely
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
Bely
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
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 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 upslope)
• Same sitting considerations will also
minimize dust getting into dome
Bely
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