MID INFRARED ASTRONOMY TECHNIQUES, AND DATA

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1 MID INFRARED ASTRONOMY WITH T RECS: THEORY, TECHNIQUES, AND DATA James Radomski Gemini South Observatory UFGRS March 19, 2009

2 Acknowledgments Gemini North (MICHELLE) Gemini South (T-ReCS) Kevin Volk Rachel Mason Tom Geballe James Radomski Michelle Edwards Fredrik Rantakyro R. Scott Fisher Tom Hayward Also Thanks to Chris Packham (UF) and Jim De Buizer (SOFIA)

3 Mid IR Astronomy (See Mid IR Resources on Gemini web page)

4 Why Mid IR Astronomy? Mid-IR (8-25µm) suffers 25-75X less extinction than optical and traces many features including: Dust re-radiationradiation Fine structure Lines ([ArIII], [SIV], [Ne II] Silicate emission/absorption PAH features Image Courtesy Charles Telesco

5 The Atmosphere Strongly affected by WV and clouds (water emits AND absorbs mid-ir photons) Overwhelmed by emission from the sky and telescope (Background limited regime S/N~sqrt(time)) Not affected by Moon (or Sun )

$35 at N, diffraction limited at Q (~0.$

6 MIR Seeing Seeing and N and Q band much more stable than visible λ (~1/2 of visible) Typically obtain images ~0.35 at N, diffraction limited at Q (~0.5 ) In good seeing and telescope image quality, 2 3 Airy rings around a point source can be observed

7 Chopping & Nodding T ReCS Source Frame (typically can t see object due to background) Off source beam unguided (small affect of 15% in S/N)

8 Chopping & Nodding Redux Chopping & Nodding Redux T ReCS data structure 6 dimensional fits [320, 240, 2, S, 2, N]

9 Why do we chop & Nod? Because of the sky variation, of course! Time variable sky background Telescope thermal emission So called 1/f detector noise Image Courtesy of TIMMI2 team

10 Array Noise Inputs Note the closed cycle cooler noise For T ReCS it s ~1.2Hz Note well the 1/f noise For T ReCS RCSthi this necessitates chopping to avoid resonance with this frequency The chop frequency enc is (typically) determined by the 1/f noise of the array

11 T ReCS (Thermal Region Camera Spectrometer)

12 T ReCS Imaging - Filters 320 x 240 pixels Pixel size = 0.089" (fixed) Field of view = 28.8" " 21 Diff. Limit ~0.3 (10um) Spectroscopy - Modes R~ 100 (Low) R~1000 (Med)

13 T ReCS Almost all data taken in chop nod mode T ReCS can only nod ABAB:ABAB: Only guide in one beam, (off source beam unguided) Chop and Nod Throw ~15 (hope to improve to 30 )

14 T ReCS Exposure Times 4 exposure times Frame time optimized in software (typical is 25 ms) Saveset time optimized in software, (default is 10 s) Nod Time optimized in software (~40 sec) Total exposure time user selected, tweaked in software (300 sec ~ sec) Total exposure time is defined parameter Clock time takes account of efficiency losses (factor of 3~4) 60s of exposure time is ~200s of clock time

$Diffraction limit Source color T-ReCS$ Sensitivity ii i N band very wide good for

highly reddened objects Si1 & all Q band

15 Filter Selection Need to consider Sensitivity Diffraction limit Source color T-ReCS Sensitivity ii i N band very wide good for detection (but other problems) Qa best for 20um imaging Si2 best combination of resolution and sensitivity Si5 great for highly reddened objects Si1 & all Q band filters are highly dependant on water vapor Si3 & Si4 strongly affected by O 3

16 Imaging Calibration

17 Calibration (Imaging) Flux standard d ~15% uncertainty typical PSF standard May need to take separate flux and PSF standard as Flux standard too bright or radically different flux to object Flux standard could to too distant from object Change of gravity vector to telescope can significantly affect delivered PSF Large telescope slew forces large pupil rotation

imaging) g) relatively eat eysimple pe Aperture on star to determine ADU

18 Flux Standard (Imaging) Flux standards d are often drawn from Cohen standards TIMMI2/MIRAC lists Flux calibrationin at mid IR (for (o imaging) g) relatively eat eysimple pe Aperture on star to determine ADU s Ratio by flux of star in mjy (cal value 10um ~0.02, 18um ~0.2) Final image in mjy/pixel

19 Cohen Standards (Imaging) Cohen has modeled a continuous spectra of many stars carefully tied to observational data Spectra can be used for imaging [and spectroscopic flux calibration] by integrating over the filter s bandpass [or smoothing to the appropriate resolution] Best reference is Cohen et al. 1999, AJ, 117, 1864 Calibration anchored to two primary standards d Alpha Lyr (AO V) Alpha CMa (A1 V)

20 Flux Standard Selection (Imaging) g) Gemini has a tool Gemini has a tool to facilitate this search

22 PSF Calibration (Imaging) Some flux standards can also be used for PSF calibration Often not the case as flux standards are too bright Best PSF standards are stars of spectral type K or M giants (<10 deg from source if possible) M supergiants should be avoided due to possible extended dust shell emission Hipparcos provides ideal catalogue tl for PSF searches Stars that are very bright should be avoided due to array effects

23 Spectroscopy Calibration

24 Calibration (Spectroscopy) Can be a star or asteroid Asteroids preferable if in high spectral resolution as stars can show resolvable spectral details at hires Stars preferable at low res as can be used as flux calibrators

25 Telluric Standards (Spectra) For object observations lasting longer than ~30 minutes (2hrs clock time), Gemini suggest two standards, prior and post observing For Gemini in queue mode, both are free At low resolution, B, A, F and G stars have a smooth spectrum in MIR region Early K maybe O.K., but Late K and M stars should be avoided

26 Gemini List of B G stars [not well flux calibrated]

27 Cohen Standards (Spectra) Many Cohen spectrophotometic standards are early K dwarfs Fine for Low Res spectra but not High res mode Many have accurate IRAS mid infrared flux densities, making them potentially decent spectrophotometric calibrators Providing telluric line removal and flux calibration At low resolution the fundamental vibration rotation band of SiO significantly depresses the spectrum at microns in stars later than K0III K2III Affects ratio ing Can use Cohen model template specta to correct effect Accounted for by Gemini IRAF spectral reduction task msabsflux, which can also be called from within msreduce

28 Other Calibration

29 Airmass Correction Airmass calibration difficult due to rapid changes in Sky transmission Sky emission Flux calibrators should be at similar airmass to source If long source observation and high photometric required, may want to have pre and post flux standard observations where pre has <RA than object, post with >RA Airmass correction that is analogous to NIR world is unusual due to intrinsic photometric uncertainty

30 Seeing Changes vs. time (& airmass) (Radomski et al. 2008)

31 Diffraction & Temperature Effects

33 Flat Fielding When chopping, flats, biases, bad pixel masks are not needed Act of chopping and fullness of pixel wells effectively removes need to flat field as multiplictive gain change across array minimal and well accounted by chopping Bias is removed through chopping process Array has very few (~2) bad pixels, therefore bad pixel masks irrelevant

34 DATA

35 DATA Reduction IRAF/IDL The Gemini i supported td IRAF DR package is the official one Other methods possible include IDL (meftools provided by Jim De Buizer) IRAF DR provided and continually supported by Gemini Will be evolved to pyraf

36 IRAF Tasks IDL Tasks TBACKGROUND TPREPARE TVIEW MISTACK or MIREGISTER MEFHEAD MEFGET Or in one command, MEFREDUCE Or in one command, MIREDUCE Other tasks for removing noise miclean cl Other tasks for removing noise miclean.cl (IRAF), noise_mask (IDL)

37 T ReCS Source Image

38 T ReCS Bright Object

39 T ReCS Chop Correction

40 T ReCS Chop Correction: With Cross Talk

41 T ReCS Spectra: Object

42 T ReCS Spectra: Sky

43 T ReCS Spectra: Object

44 Typical Noise (OK) Telescope NOD (Dif1, Dif 2) Window (Src1, Src2) Crosstalk (Any)

45 Typical Noise (BAD) Clouds (Sig) High Frequency Striping (Any) Chop elongation (Any)

46 Questions?

Gemini. Marie Lemoine-Busserolle Gemini Observatory. NOAO/Gemini Data workshop Tucson,

Gemini. Marie Lemoine-Busserolle Gemini Observatory. NOAO/Gemini Data workshop Tucson, Mid-IR @ Gemini Marie Lemoine-Busserolle Gemini Observatory NOAO/Gemini Data workshop Tucson, 2010-07-22 Overview Part 1 : general intro to mid-ir Science Part 2 : general intro to mid-ir observing Part