Data reduction for Optical Imaging and Spectroscopy Stefano Benetti, 8 Feb 2005

Focal reducer instruments Afosc (D, Al, B etc..) EFOSC2 EMMI Dolores FORS1-2 SOFI NICS ISAAC etc.. Simple but powerful: imaging + spec. almost simultaneously

, 8 Feb 2005 Optical –mechanical scheme

, 8 Feb 2005 stable instruments! (if the site is good…)

, 8 Feb 2005 …and spectroscopy Averages of ~12 curves each: scatter around zero point: ~10% once normalised: ~3%!

, 8 Feb 2005 Calibrations are also stable: If instrument and/or single elements (slits/filters/grisms) have NOT been removed/re-mounted from the telescope/ Instrument  you may use calibrations (BIAS, FF, arcs) which are weeks-to-months old! REMEMBER: take out always the overscan: the zero point of the BIAS level may change quit a lot also during a single night!

, 8 Feb 2005 In summary: versatiles powerful:spectroscopy of faint object stable Main problem: Central light condensation ( light reflected back from the detector to the optics)  prevent accurate photometry The effect can be as high as 5% in EFOSC-DFOSC FORS1-2 as low as 0.7%!

, 8 Feb 2005 …and surely much more! Data reduction: the SN case Asking an SN observation you get the unknown (ToO)!!! You have to deal with lots of configurations ToO observations: you reduce what you get You need color equations and response curve database You get observations with no parall. angle used: what to do Normally you don’t observe an objet with ZD~70 o, but for observing SN you must sometime do! You observe trough clouds (rain?!?) and you must be able to deal with it

, 8 Feb 2005 Data reduction general: overscan, BIAS and FF correction FIRST inspect visually all the files to look for problems, defects, etc… I suggest the use of standard IRAF procedure: noao.imred.ccdred.ccdproc where for overscan I normally fit a legendre with order 1 (a constant!) to all images

, 8 Feb 2005 Combine the biases with the task: noao.imred.ccdred.zerocombine

, 8 Feb 2005 BE AWARE: CCD windowing might produce slopes in BIASes!  If a box is used, ask BIAS with the same CCD window!

, 8 Feb 2005 dome sky dome/sky sky FFs are better! trace Differences up to 2%  error in mag of ~0.02

, 8 Feb 2005 Imaging Flatfielding: Combine the FFs with the task: noao.imred.ccdred.flatcombine

, 8 Feb 2005 with thin CCDs you get fringing in I: …and you do not correct it with dome/sky FF!!! because….

, 8 Feb 2005 …then how to deal with fringing? We need a master (super) flat! How to make it: take I-band images of different fields (spread over one night) and corrected for bias and flatfield. flatcombined the images. compute the average value and subtracted it from the image. How to correct? Fringing is additive: then it has to be subtracted from the images after the pattern has been scaled to the actual value of the fringing in the image! not known how variable is the pattern: the intensity might varies a lot, but the shape could be quite stable, e.g. instrument has little flexures (EFOSC-FORS-EMMI…); problems with CAFOS, DFOSC-like… solve only FF with telescope on target position!  impossible for imaging!!!

Measuring SN magnitudes only Aperture photometry: to be used as quick look only NOT PSF technique: reliable up to mag~16-17 – NOT need to wait the fading of the target Template subtraction: gives the better result (you REAL subtract the REAL background) but DO need to WAIT you DO need to WAIT the fading (few years), 8 Feb 2005

Differences between methods PSF template Aperture (Pastorello et al. 2005)

We use snoopy (Cappellaro & Patat-Ledan software) based on daophot (noao.digiphot.daophot), 8 Feb 2005 SN98A a) PSF definition: stars close to SN PSF method

, 8 Feb 2005 b) creation and check

, 8 Feb 2005 SN98A HII c) Background definition

, 8 Feb 2005 Too much!!! ~0.5mag brighter d) background and PSF subtraction

SN2002cm working imagereferencesubtraction The steps to follow are:, 8 Feb 2005 Template subtraction

a)Align the two images: x,y translation and rotation b)Flux normalisation of template to science c)Degrade the image with better seeing! d)Subtraction e)If object is faint (peak signal~3-5 bkg noise): better do the measure with PSF (not aperture) Steps to follow:, 8 Feb 2005 see Stefano contribution!!!

, 8 Feb 2005 Photometric calibration a) Photometric night You have the color equations system: U-u = K U +S U x(U-B)+A U z B-b = K B +S B x(B-V)+A B z V-v = K V +S V x(B-V)+A V z R-r = K R +S R x(V-R)+A R z I-i = K I +S I x(R-I)+A I z where: A i atm. absorption (table or fitting), z airmass solve in K i and S i : through l.s. fitting of std stars measurements having a big spread in colors!!!

, 8 Feb 2005 why large colours OK! If B-V SN is ~0  error ~0.08 mag!

, 8 Feb 2005 Having K i and S i  solve previous system of equations for SN and local sequence  UBVRI b) Not photometric night get color equations from archive  solve system for local sequence  U’B’V’R’I’ First you must have the local sequence calibrated You compare  band and star m’ with true mag  Δm i   band take the average of Δm i You add Δm i to K i of c.eq.  solve the system again and get UBVRI of SN!!!

, 8 Feb 2005 After that you have to homogenize all measurements taken with different instruments: From previous experiences: Asiago-ESO-TNG are in the same photometric system SNe 2002bo-2002er told us that : ING+JKT+NOT are also in the same system Some bands of WFI and CAFOS are instead off …and here comes in our help the S-correction see Max talk!

, 8 Feb 2005 FFs in spectroscopy If I had not a good FF in the blue I couldn’t say it was real! SN 2002bo 14d before maximum FλFλ watch the blue!

Steep gradient blue red trace | ~3800 Å | ~8000 Å lots of FFs to improve S/N!!!, 8 Feb 2005

How to get rid of FF continuum? with task noao.imred.ctioslit.response

Be aware to properly fit the continuum:  interpolate only higher frequencies!, 8 Feb 2005 TNG+Dolores LR-R

, 8 Feb 2005 Result of division: reflectionFringing up to 20%! CaII – IR Do flats on target position!!!

, 8 Feb 2005 zone with reflections should be avoided! To be specified in ToO observations!

or “How to write two papers on an inexistent Hα” The importance of the background subtraction also in spectroscopy:, 8 Feb 2005

Polcaro & Viotti 1991: Hydrogen in a Ia! SN 1990M Della Valle, Benetti et al 1996: No No Hydrogen!!! If background removed!, 8 Feb 2005

Fitted background & Optimised extraction bkg well removed bkg HαHα bkg not well removed HαHα Importance of background

, 8 Feb 2005 Extraction of the spectrum: noao.imred.ctioslit.apall A recent example: the classification of SN 2005H (mag~15.9; offsets: 0.3”E; 1.8”N) how difficult is to find the object and define bkg even with a relatively bright SN. be aware always of what is doing “trace”!!!

, 8 Feb 2005 In fact, here is a nice spectrum…. btw, they didn’t align the grism very carefully… we start apall…

, 8 Feb 2005 and background:

, 8 Feb 2005 checked the trace!

, 8 Feb 2005 Result: not a SN!?!?!?!

, 8 Feb 2005 we went back to have a closer look to the place where the SN should be: SN?

, 8 Feb 2005 Back to the spectrum:

, 8 Feb 2005 Apall again, but mashed rows were the contrast stellar tracing to galaxy bkg is higher: Here is where to define the SN position

, 8 Feb 2005 And the bkg: Very close to SN limits!

, 8 Feb 2005 Finally we got the spectrum of a nice SNII!!!!

, 8 Feb 2005 NOT If target is faint and grism well align better NOT trace spect.! SN 2003cg with VLT (1800s)

, 8 Feb 2005 result (avg of 2)!!!:

Godness expected: with a 300gr/mm (2-3 Å/px) grism; rms<1Å range:~4500Å; used line  rms<1Å if 4-5 degree polynomial, 8 Feb 2005 Spectrum in wavelength: noao.imred.ctioslit.identify noao.imred.ctioslit.refspectra noao.imred.ctioslit.dispcor goal: correlate pixels with wavelength with an arc spectrum

, 8 Feb 2005 Suggestions: Each instrument/grism needs a table (wavelength of lines may change with resolution)  keep vast archive of arc tables! Often with ToO you don’t get what you ask  be prepared to calibrate spectra with skylines: (see above, especially in the red!) Alternatively: use previous solutions  anchor zero point with skylines Use bright skylines (e.g. [OI] 5577, 6300, etc…) to get spectral resolution

, 8 Feb 2005 Spectrum in flux: Spectrum in flux: noao.imred.ctioslit.standard noao.imred.ctioslit.sensfunc noao.imred.ctioslit.calibrate (see previous slides for godness (10% - <3%) and stability (yes) of response cur.) Suggestions: From above  ToO NOT need always spectrophot. standard! I prefer to use standards from Hamy (1992, 1994) or Oke 1990 useful link: Check the header parameters! (exptime and airmass)

, 8 Feb 2005 Check the spectrophotometry U B V R I Convolving passbands to a spectrum a nice IRAF task is: stsdas.hst_calib.synphot. calcphot

, 8 Feb 2005 spectrophotometry vs. photometry If everything went well  ∆U=∆B= ∆V=∆R=∆I=0 If night not photometric  ∆U=∆B= ∆V=∆R=∆I≠0 If you get ∆U≠∆B≠∆V≠∆R≠∆I you are in trouble!!!! Did you use parallactic both for target and standard? … probably not!

, 8 Feb 2005 What is parallactic angle? To apply if dz>30 o N P.A. See Filippenko 1982, PASP 94, 715

, 8 Feb 2005 its amplitude: At z=1.1  point source at 6500Å is displaced by 0.75” at 3500Å At z=1.3  point source at 6500Å is displaced by 1.40” at 3500Å as function of H.A. and δ:

, 8 Feb 2005 How losses may change derived parameters: Tbb~14000 o K Tbb~11000 o K (~20% at 4800 Å) Tbb~18000 o K Watch to differential losses: failing of spectra modelling!

, 8 Feb 2005 spectrophotometry vs. photometry If everything went well  ∆U=∆B= ∆V=∆R=∆I=0 If night not photometric  ∆U=∆B= ∆V=∆R=∆I≠0 If you get ∆U≠∆B≠∆V≠∆R≠∆I you are in trouble!!!! Did you use parallactic both for target and standard? … probably not! probably because it was a ToO… My suggestion is NOT to use this spectrum for measuring line fluxes, only positions (wavelengths)! If you desperately need it, I may teach you (privately) a way to “massage” it!!!! So, please, use (ask) always p.a. or few minutes trough a wide (5”-10”) slit!

You divide the zapped spectrum to the unzapped one, 8 Feb 2005 Modelling atmospheric absorptions To be done AFTER normalization to photometry! You need a flux calibrated spectrum of a hot star obtained at a similar airmass!

, 8 Feb 2005 You get:

, 8 Feb 2005 Then you apply to SN spectrum: before (SNIa z~0.30)   

, 8 Feb 2005 After correction:    you may adjust the abs. intensity with: (atm.fits+k)/(1+k) and k>0;<0

, 8 Feb 2005 Conclusions What I would like you keep from this long lesson is that when starting a data reduction of a SN you must be prepared to face always new unknowns and challenges. Moreover I would suggest NOT to stop to the first result you get, but keep trying to see if you get a better result (S/N), this is True also for an expert reducer. Curiosity is our main drive in science, let’s be curious also when reducing!!!