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NIRSpec pipeline concept Rectifying NIRSpec Spectra Slit is curved (function of field angle) Guido De Marchi & Torsten Böker ESA Space Science Department Lines of constant l spread over multiple pixel columns Instrument characteristics: l1 l2 li Use “Drizzle” technique as possible approach for coordinate transformation Need re-binning before final spectrum is extracted l1 l2 1) 2) 3) 4) 5) Defining the extraction windows used on a diffraction-limited telescope --> PSF varies with l wide wavelength range (0.6 - 5 mm) --> chromatic slit losses off-axis telescope and wide field of view --> significant distortion reflective optics (including dispersive elements) --> large, variable slit curvature multi-object spectrograph --> every detector pixel sees every wavelength MSA mask li each spectrum has an “extraction box” on the detector Red: object, green: background Extraction boxes overlap, possible “spill-over” An outline of the NIRSpec pipeline RAW DATA - depends on source shape and position within shutter - must be user-controlled BIAS & DARK SUBTRACTION LINEARITY REF. FRAME LINEARITY CORR. “P-FLAT” REF. FRAME PIXEL-TO-PIXEL DQE CORR. MSA CONFIG. & DISTORTION MAP LOCATE EXTR. WINDOW FLATFIELD REF. CUBE GRATING EQUATION GW TELEMETRY l1 l2 li l1 l2 li } (Assuming no l-dependence) One window for every open shutter... THROUGHPUT CORRECTION (incl. L-flat, blaze function, transmission of optics, & “default” chromatic slit loss) GEOMETRIC DISTORTION MAP GEOMETRIC DISTORTION (spatial) PHOTFLAM KEYWORD ABSOLUTE FLUX CALIBRATION Collapse to 1-D spectrum ^ Throughput correction of (output is re-sampled grid) Need a “throughput” data cube (for each filter/grating combination) EXTRACT 1-D SPECTRUM (one spectrum for every open shutter) SUBTRACT BACKGROUND (defined by user or average) “DELTA” CHROMATIC SLIT LOSS CORR. - depends on source extent and background subtraction - must be user-controlled “Flat-fielding” NIRSpec spectra FINAL l CALIBRATION (dispersion solution) FINAL 1D SPECTRUM (defined by user) y ---> “Delta” correction for chromatic slit loss BIAS/DARK REF. FRAMES (erg/cm2/s/Å, sampled within variable slit aperture) Output: (assuming a source with flat spectrum) l ---> x ---> For quick-look analysis, pipeline subtracts TBD “default” background Goal: to correct for the total instrumental throughput variations, both as a function of wavelength (e.g. optics transmission, blaze function) and field angle (e.g. DQE, vignetting). Contributions to the “Throughput correction” reflection curves of all mirrors: f(l), f(x,y) The bummer: chromatic slit loss Fixed slit size, but variable PSF width… transmission curves of filters: f(l) blaze function of grating: f(l), f(x,y) large-scale response of detector (L-flat): f(l), f(x,y) All of these contributions need to be measured at component level and built into a physical/optical instrument model. Once NIRSpec is assembled, they cannot be measured individually. However, once a shutter has been specified, all of them are in principle - deterministic, and can be accurately modeled. Using the instrument model, all these effects will be corrected simultaneously. Protected Silver 0° Measured on SiC substrates 1 mm 5 mm … causes “flaring” and intensity gradient l ---> However… 3 mm A “default” correction for e.g. a perfectly centered point source can be included in throughput correction. The user needs to optimise this correction at a later stage. l --->