1. ASTR 3010 Lecture 18 Textbook N/A
Mid-IR Observation
ASTR 3010
Lecture 18
Textbook N/A

2. Why mid-IR?? Optical view of the Milky Way
In IR, we can see cooler objects
- forming stars
- interstellar dust
- distant objects
(lower extinction)
- nebulae etc.
InfraRed view of the Milky Way

3. Interstellar Extinction
At longer wavelength, we can see through to further away!
Extinction to the center of Milky Way : A(V)~30mag, A(K)~3mag

4. Wien’s Law
Objects with temperatures in the range of °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 ~ K

5. 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

6. Mid-IR Observations from the Ground
Windows of good/fair transmission between 8-13 and 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

7. 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, wind-born 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

8. Chopping observation with the Secondary
chopping the secondary mirror at ~3Hz to subtract out the background signal
chop A
chop B
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

9. 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 :
Nod the telescope by a distance equal to the chop throw along the chop axis

10. Standard Chop-Nod Observation (“Beam-Switching”)
The combined chop/nod situation is shown above where the blue and green arrows show the two telescope pointings in the nod positions while the black lines show the positions of the telescope in the different chop pointings. The three fields of view that result are labeled on the figure.
Compact objects: chop on-chip  maximize detected source signal.
Standard beam switching : 4-point chop – nod

11. 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

12. Big telescopes are better for chopping
Beams separate higher in the atmosphere, and have more overlap on primary mirror
Chop throw of 20 arcsec corresponds to ~14mm in telescope focal plane and a motion of ~11mm on the Primary c.f. diameter of 7.9m

14. Beating the huge background
8x108 e-
3x106 e-
Poissionian noise of the background: sqrt(109) = 104-5
5 minutes exposures
 ~15,000 frames total
104 e-
the effective background subtraction is nearly five orders of magnitude below the raw background!

15. T-ReCS Sky frame (Gemini South mid-IR instrument)
T-ReCS (Thermal Region Camera Spectrograph)
Fixed-pattern offsets due to pixel-to-pixel variations and offsets between the 16 channels of the acquisition electronics

16. Extremely Bright object in the sky (chop A and nod A)

17. Chop differenced image (chop A – chop B)

18. Spectroscopy : object
Chop-Nod double
differenced image

19. Spectroscopy – wavelength calibration
Use night sky emission lines

20. AO at long wavelengths Need to decrease the number of warm optics
primary mirror + secondary mirror + instrument window + instrument
 no room for fancy image correction (AO)
Adaptive secondary mirror is the future
Large Binocular Telescope
adaptive secondary mirror
LBT M2 = deformable mirror of 672actuators correcting at ~1000Hz

21. 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%)

22. Chapter/sections covered in this lecture : N/A
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