near-infrared astronomy

Near-Infrared AstronomyNear-Infrared AstronomyNear-Infrared AstronomyNear-Infrared Astronomy

• 0.8-50.8-5mm• How is it different from the optical?How is it different from the optical?

– Night sky & thermal emission– Atmospheric absorption– Detectors– Instruments

• What does this mean?What does this mean?– Beam switching/nodding/chopping– Careful calibration of data

• Some dataSome data– Imaging– Spectroscopy

Why?Why?Why?Why?

J,H,KH,NII,SII

Optical Near-IR Far-IR

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.5 m

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s!

Near-Infrared SkyNear-Infrared SkyNear-Infrared SkyNear-Infrared Sky• As wavelengths lengthen the sky gets brighterAs wavelengths lengthen the sky gets brighter

– Combined effects of Moon/zodiacal (ie scattered solar spectrum) OH airglow (forest of lines from 0.8mup) Thermal emission (from sky dominates 2.4 mup)

• The sky also absorbs a lot more light.The sky also absorbs a lot more light.– H20 absorption bands block regions from 0.8mup

– The “gaps” between these bands define the “windows” or “bands” of IR astronomy.

Z,J,H,K,L,M,….

• Your instrument contributesYour instrument contributes– Without a cryogenic pupil stop, you get to “see” the

thermal emission from your telescope, dome, and instrument.

The sky “brightness” of IR The sky “brightness” of IR bandsbandsThe sky “brightness” of IR The sky “brightness” of IR bandsbands

V(dark) 0.5 21.5

V(bright) 0.5 18.5

I (dark) 0.8 19.3

I (bright) 0.8 18.2

Dark Sky

The near near-IR & the The near near-IR & the MoonMoonThe near near-IR & the The near near-IR & the MoonMoon

ThermalBright Sky

Night Sky emission

1.E

-01

1.E

+01

1.E

+03

1.E

+05

1.E

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600 1600 2600 3600 4600

Wavelength(nm)

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Atmospheric Transmission

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0.2

0.4

0.6

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Wavelength(nm)

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z’

J

H

KsL M

z’

J

H

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When the Moon is When the Moon is notnot irrelevant in the IRirrelevant in the IRWhen the Moon is When the Moon is notnot irrelevant in the IRirrelevant in the IR

Expanded Night Sky emission (R~1600nm/1nm)

1.E+00

1.E+01

1.E+02

1.E+03

1550 1570 1590 1610 1630 1650

Wavelength(nm)

Flu

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/nm

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• At high resolution - At high resolution - 3000 - we will 3000 - we will see continuum sky see continuum sky “between” the OH “between” the OH lines.lines.

• Spectroscopy in Spectroscopy in near-IR will near-IR will become a become a darkdark time observationtime observation

The sky is bright The sky is bright andand highly variable.highly variable.The sky is bright The sky is bright andand highly variable.highly variable.

• If the sky was only bright, it would add to our S/N, If the sky was only bright, it would add to our S/N, but observing would be no different to the optical.but observing would be no different to the optical.

• OH in z,J,H,KOH in z,J,H,Kshortshort bands. Thermal in K bands. Thermal in Klonglong,L,M bands.,L,M bands.

• 2MASS prototype camera2MASS prototype camera– H-band filter– 120”/pix, 256x256 pix– 30 frames over 45 minutes– Camera fixed– Moon & stars track across field– OH night-sky emission is very

variable!

The sky is bright The sky is bright andand highly variable.highly variable.The sky is bright The sky is bright andand highly variable.highly variable.• In imaging In imaging andand spectroscopy you need to spectroscopy you need to

“measure” the sky regularly, so you can subtract it “measure” the sky regularly, so you can subtract it – every few minute (imaging) to every 10-15 minutes

(spectroscopy)

• In imaging you achieve this by ‘dithering’ every few In imaging you achieve this by ‘dithering’ every few minutesminutes– Because the sky is so bright there is rarely a read-noise

penalty (typically 10-15e per read) in doing this

• In spectroscopy you achieve this by ‘nodding’ your In spectroscopy you achieve this by ‘nodding’ your target on the slit (or nodding to sky) every 5-15 target on the slit (or nodding to sky) every 5-15 minutesminutes– In spectroscopy you have to trade off between read noise

(~10e) and sky brightness, to ensure you don’t get read noise dominated.

IRIS2 - IR imager/spectrographIRIS2 - IR imager/spectrograph

INCOMINGRADIATION

FILTER WHEEL

COLD STOP WHEEL

GRISM WHEEL

TELESCOPECENTRE LINE

FLOOR OFCAGE

ANGLO-AUSTRALIAN OBSERVATORY

REV. D

IRIS2 LAYOUT - F/2.2 CAMERA

3RD. ANGLE PROJ.ASS`Y DRAWING No.

22MAR99

GRASEBY SPECAC OPTICS

A-2IRIS2 PROJECT

DRAWINGNo.MILLIMETRES

0 20 40 80 160

E4470

E4470

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SLIT WHEEL

CRYO COOLER

TELESCOPE BACK FOCUS

FIELD LENS

COLLIMATORLENS ASEMBLY

CAMERALENS ASSEMBLY

FIELD FLATTENER

DETECTOR

PUPIL IMAGER

80

STEPPER MOTOR

110

1370

UNSWIRF(Upgrade)

DETECTOR TRANSLATOR

TWO STAGECRYO COOLER

WINDOW

MOUNTINGPLANEOF A&G BOX

INCREASED 50 MM

Fore-dewarMain-dewar

Cold StopGrism

SlitMultiSlit

Cold Stops and Cold Stops and CryogenicsCryogenicsCold Stops and Cold Stops and CryogenicsCryogenics• Infrared detectors need to be kept cold (like Infrared detectors need to be kept cold (like

optical detectors)optical detectors)– To control their own dark current and thermal emission

• They also need cold stopsThey also need cold stops– To control the telescope’s thermal emission

• They also need the instrument and optical They also need the instrument and optical components held coldcomponents held cold– To control the instrument’s thermal emission

• As a result they are expensive and difficult to As a result they are expensive and difficult to build.build.– Elements must be kept small. Elements can’t be

interchanged. Everything is more difficult.

DetectorsDetectorsDetectorsDetectors• HgCdTe (0.8-2.5HgCdTe (0.8-2.5m) and InSb (1-5m) and InSb (1-5m). m).

– Read by multiplexers rather than charge shifting. Each pixel is addressed in sequence.

• Must be read faster than CCDs. Must be read faster than CCDs. – Typically an array (1Kx1K or 2Kx2K) needs to be read in ~1s– Compare with 2Kx4K CCDs read speeds of 60s

• Read noise is higherRead noise is higher– Typically 10e- per read, or 14e- per double correlated

sample– Multiple reads can lower this to 4-5e- for some applications.– Compare with <2e- for CCDs

• Dark current is higherDark current is higher– Typically 1 to 0.1 e/pix/s.

DetectorsDetectorsDetectorsDetectors• HgCdTe arrays are sometimes subject to HgCdTe arrays are sometimes subject to

“residual images”“residual images”– Heavily saturated pixels may retain high dark current for

some time– Effect is less in late model (HAWAII) detectors.

• Read-out amplifiers can glowRead-out amplifiers can glow– This sets a limit to the number of reads which can be

used to beat down read-noise.

• Usually have significant non-linearity.Usually have significant non-linearity.– Can be as much as 5% near full-well.

Detector Read ModesDetector Read ModesDetector Read ModesDetector Read Modes• Double-correlated sampling (Flux=Read2-Read1).Double-correlated sampling (Flux=Read2-Read1).• Multiple Read modesMultiple Read modes

– Fowler (Mean of n reads at end - mean of n reads at end)

– “Up the ramp” (Least squares fit to n reads)– Total reads usually limited < 60-100 by read-out glow.

Time

Flu x

Reset Read1-n

Read (n+1) - (2n)

Fowler Sample

Time

Flu x

Reset Read1

Read2

Double Correlated Sample

Time

Flu x

Reset

Read 1…n

“Up the ramp” Sample

Some Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTeSome Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTe• DarkDark

Some Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTeSome Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTe

• Dark - vertical cutDark - vertical cut

Some Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTeSome Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTe• Dark - read-out glow & bad pixels.Dark - read-out glow & bad pixels.

Some Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTeSome Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTe• A Flat field in JA Flat field in J

Some Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTeSome Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTe• Raw Data in JRaw Data in J

~500adu in sky~500adu in sky10adu/e-10adu/e-

• Quick-look atQuick-look attelescope istelescope ismost easily most easily done by pair-done by pair-wise subtractionwise subtraction

Some Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTeSome Data … from ESO Some Data … from ESO SOFI 1Kx1K HgCdTeSOFI 1Kx1K HgCdTe• HK R~1000 spectraHK R~1000 spectra Blue

RedThermal at red end of K

OH sky emission lines


• HK R~1000 spectraHK R~1000 spectravertical cutvertical cut


• HK R~1000 spectraHK R~1000 spectrahorizontal cuthorizontal cut


• HK R~1000 spectraHK R~1000 spectrapair-wise subtractionpair-wise subtraction

Planning your Planning your observationsobservationsPlanning your Planning your observationsobservations• Sky brightness and target brightness.Sky brightness and target brightness.

– You have to be very aware of how bright your sky will be, how bright your target will be. Keep sky below half-full-well (or lower if non-linearity is significant). Keep target below full-well!

• Read-noise and read-mode. Read-noise and read-mode. – Make sure you are sky limited. You can make as many

reads as you like, once you are sky limited.

• Dark currentsDark currents– Be aware of the dark current and associated noise.

• Dead-times.Dead-times.– Be very aware of time to nod & settle telescope, time for

a single, time for a detector read, and other overheads. These can be much more significant than in the optical.

Beam-switching Thought Beam-switching Thought ExperimentsExperimentsBeam-switching Thought Beam-switching Thought ExperimentsExperiments• Should be done as often as possible, without adding a Should be done as often as possible, without adding a

read-noise or dead-time penalty.read-noise or dead-time penalty.– Imaging in DCS, 14e- read noise, 0.5e-/s/pix dark current.

100000e- full well. Array read time 1.5s. Time to nod 10s. J : Sky=3000e- in 10s. So sky noise=54e- in 10s. There is no

read noise penalty in many reads. Dither ~ 120s to keep dead time down.

K : Sky=50000e- in 10s. Similar situation. Dither ~ 120s.– Spectroscopy in Fowler mode. 7e- read noise for 50 reads. So

we must stay on target for at LEAST 75s. After 300s, dark current=150e/pix, dark noise=12e- so we are dark limited.

J : Sky in lines is 3000e- in 300s, but sky in gaps is 300e-, so we are just sky limited and can nod every 5 minutes.

K : Sky will be brighter than J. We can certainly nod every 5 minutes or faster, but should look at the bright thermal end to ensure we don’t hit full well.

CalibrationsCalibrationsCalibrationsCalibrationsIf you cheat on your calibrations, you will get all the If you cheat on your calibrations, you will get all the

trouble you deserve.trouble you deserve.• Flat fieldsFlat fields

– usually best constructed from dome, but taken in pairs with lamps on, and lamps off, to remove thermal effects of dome and instrument.

• Darks/biasDarks/bias– usually best acquired for every exposure time and read-

mode you use. You will also use these (and the flats) to determine which pixels are bad.

• Arcs (ie wavelength calibration for spectra)Arcs (ie wavelength calibration for spectra)– straightforward, but beware not to use bright arc lines on

detectors with residual images. Arcs may be best taken at end of night. OH sky lines may provide all the calibration you need.

CalibrationsCalibrationsCalibrationsCalibrations• Atmospheric transmission (spectra)Atmospheric transmission (spectra)

– usually almost featureless F and G star spectra. The combination will allow you to remove features from the star spectra. Everything else is due to the atmosphere. Take these frequently and close to your target on the sky - especially if you want to trust data at the edges of the atmospheric bands.

• Photometric standards (imaging)Photometric standards (imaging)– are self explanatory. However, you may want to take at

least one rastered in a dense grid across your field of view, if your instrument has light concentration problems (most IR cameras do).

Light Light concentrationconcentrationLight Light concentrationconcentration

Data processingData processingData processingData processing• Dark/bias subtraction : obvious.Dark/bias subtraction : obvious.• Flat fieldingFlat fielding

– Use pixels which won’t flat-field/dark subtract to create a bad-pixel mask. IRAF will allegedly handle bad pixel masks now. Figaro and many Starlink packages certainly do.

• Sky subtractionSky subtraction– Imaging : usually median (or median with sigma clipping,

or average with sigma clipping) 5-7 normalized images taken near your target in time and on the sky. Then re-normalize this “sky” to your frame and subtract.

– Spectroscopy : can do the above, but more usually simply subtract other frame from an object sky pair AB BA (ie A is sky for B) or the average of the spanning pair in an ABAB sequence.

Data ProcessingData ProcessingData ProcessingData Processing• If you are going to linearity correct your data you If you are going to linearity correct your data you

should do this after dark subtraction, but before should do this after dark subtraction, but before flat fielding.flat fielding.

• Once you have sky subtracted your data,Once you have sky subtracted your data,– Imaging : you can use your favourite combination

algorithm, or photometer all images separately. Combination has the advantage you can ‘fill in’ bad

pixels in a dithered data set. However, it can be a lot of work, and may actually give worse photometry than processing all data as separate images.

– Spectroscopy: by this point your data should look much like optical data and further steps are pretty much the same.

ConclusionsConclusionsConclusionsConclusions• Short of 2.3Short of 2.3m consider the near-IR an extension m consider the near-IR an extension

of the optical.of the optical.– Night sky emission makes life harder, but only slightly

harder than the 0.8-1.0 m optical range.– Detectors are smaller and need to be more carefully

processed– But this will change over your careers.

• 2.3 2.3 m - 5 m - 5 m is really “thermal” IR and quite m is really “thermal” IR and quite different.different.– Backgrounds are high and variable. H2O absorption is

strong and variable. Really only suited to high, dry sites like Chile & Mauna Kea.

near-infrared astronomy

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