Mid-IR Observation
ASTR 3010
Lecture 18
Textbook N/A
Optical view of the Milky Way
Why mid-IR??
In IR, we can see cooler objects
- forming stars
- interstellar dust
- distant objects
(lower extinction)
- nebulae etc.
InfraRed view of the Milky Way
Interstellar Extinction
At longer wavelength,
we can see through to
further away!
Wien’s Law
• Objects with temperatures in the
range of 100-600°K are brightest at
wavelengths in mid-IR (5-30 μm)
• However, observing environments
(telescope, dome, atmosphere, etc.)
are in the middle of this temperature
range.
This means that mid-IR observations
can be done best in the space with
cooled telescopes.
Ground-based telescope emission peaks
at ~10μm, corresponding to
temperatures of ~270 - 290 K
Comparison to Space Telescope
JWST versus Gemini
Spatial resolution issue  reason for
mid-IR observations from the ground
largest IR space telescope (Spitzer) = 0.85 meters
largest ground-based telescope = 10 meters
Mid-IR Observations from the Ground
• Windows of good/fair transmission between 8-13 and 16-25 μm (N & Q
bands)
• Benefit from cold, high, dry sites (Mauna Kea/Chilean Andes) and low
emissivity, high cleanliness
• Large ground-based telescopes have about 10 times better spatial
resolution than space telescopes
• Suffer huge thermal background which compromises sensitivity compared
to space missions
• Greatest relative gains at
high spatial resolution
Background Signal
• Atmospheric transmission depends primarily on water vapor column above
the site
• Mauna Kea good conditions ~1mm PWV, but can be much higher, and
generally higher at other sites
• Sky Noise - unstable weather, thin cirrus and other structured cloud, windborn dust, etc.
• Need a stable telescope, uniform clean mirrors,
• Major sources of background : Sky, Telescope Mirrors + support structures,
instrument window
• Background cancellation via chopping secondary, want small stable residual
offset signals
• Best case is to keep everything cold, but it’s impossible.
 try to minimize the thermal emission from the telescope (low emissivity) +
a special observing technique
Chopping observation with the Secondary
• chopping the secondary mirror at ~3Hz to subtract out the background
signal
chop B
chop A
The telescope secondary mirror rocks in a quasi-square
wave pattern at a few Hz, displacing the image of the
object by typically ~20 arcsec on the detector. This
allows the weak emission from the astronomical object
to be detected differentially on top of the large thermal
background.
The mirror position is stabilized with fast guiding at one
or both chop positions
Chopping and Nodding (“Beam Switching”)
• Motion of the secondary mirror, means that the detector beam falls on
slightly different parts of the primary mirror, which have different defects,
dust etc, leading to a radiative offset between the two chop positions.
• This is compensated by Nodding the telescope so that the object and
reference positions are switched
• Beamswitching :
o Nod the telescope by a distance equal to the chop throw along the chop axis
Standard Chop-Nod Observation (“Beam-Switching”)
Compact objects: chop on-chip  maximize
detected source signal.
Standard beam switching : 4-point chop – nod
Two best mid-IR telescopes
VLT
Gemini
• 30 arcsec chop throw (20” if
guiding on both beams) at ~5Hz
• Beryllium secondary : Al coating,
retractable baffle
• Altitude 2635m
• 15 arcsec chop throw, guide on 1
beam
• Glass secondary with Ag coating,
central hole, retractable baffle
• Altitude 2715, 4214m
Beating the huge background
8x108 e-
3x106 e-
5 minutes exposures
 ~15,000 frames total
104 ethe effective background subtraction
is nearly five orders of magnitude
below the raw background!
T-ReCS Sky frame (Gemini South mid-IR instrument)
• T-ReCS (Thermal Region Camera Spectrograph)
Extremely Bright object in the sky (chop A and nod A)
Chop differenced image (chop A – chop B)
Spectroscopy : object
• Chop-Nod double
differenced image
Spectroscopy – wavelength calibration
• Use night sky emission lines
AO at long wavelengths
• Need to decrease the number of warm optics
o primary mirror + secondary mirror + instrument window + instrument
 no room for fancy image correction (AO)
o Adaptive secondary mirror is the future
Large Binocular Telescope
adaptive secondary mirror
LBT M2 = deformable mirror of 672actuators
correcting at ~1000Hz
Adaptive Secondary Mirror
• Real Example:
N-band AO image
from Multi-Mirror Telescope (6.9m)
Strehl ratio > 98% nearly par to that of extreme AO (~99%)
In summary…
Important Concepts
Important Terms
• Difficulty of mid-IR observations
• Advantage of adaptive secondary
• Beam-switching observation
• Chopping
Chapter/sections covered in this lecture : N/A