Mid-IR Observation

ASTR 3010

Lecture 18

Textbook N/A

Optical view of the Milky Way

InfraRed view of the Milky Way

In IR, we can see cooler objects- forming stars- interstellar dust- distant objects (lower extinction)- nebulae etc.

Why mid-IR??

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 meterslargest 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, 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

Chopping observation with the Secondary• chopping the secondary mirror at ~3Hz to subtract out the background

signal

chop Achop 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

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• 30 arcsec chop throw (20” if

guiding on both beams) at ~5Hz• Beryllium secondary : Al coating,

retractable baffle• Altitude 2635m

Gemini• 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-

104 e-

3x106 e-

5 minutes exposures ~15,000 frames total

the 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 opticso primary mirror + secondary mirror + instrument window + instrument

no room for fancy image correction (AO)o Adaptive secondary mirror is the future

Large Binocular Telescopeadaptive 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• Difficulty of mid-IR observations• Advantage of adaptive secondary

Important Terms• Beam-switching observation• Chopping

Chapter/sections covered in this lecture : N/A