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

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

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

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, 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 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

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

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

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

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

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!

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

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 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

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%)

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