nirspec pipeline concept guido de marchi, tracy beck, torsten böker
DESCRIPTION
NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker. Instrument characteristics multi-object spectrograph --> every detector pixel sees every wavelength reflective optics (incl. dispersive elements) --> large, variable slit curvature - PowerPoint PPT PresentationTRANSCRIPT
NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker
Instrument characteristics
1) multi-object spectrograph --> every detector pixel sees every wavelength 2) reflective optics (incl. dispersive elements) --> large, variable slit curvature 3) used on a diffraction-limited telescope --> PSF varies with 4) wide wavelength range (0.6 - 5 m) --> chromatic slit losses5) off-axis telescope and wide field of view --> significant distortion
NIRSpec pipeline concept
Mounting frame Active MSA area
Fixed slits and IFU aperture
Detector array
Direction of dispersion
NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker
Instrument characteristics
1) multi-object spectrograph --> every detector pixel sees every wavelength 2) reflective optics (incl. dispersive elements) --> large, variable slit curvature 3) used on a diffraction-limited telescope --> PSF varies with 4) wide wavelength range (0.6 - 5 m) --> chromatic slit losses5) off-axis telescope and wide field of view --> significant distortion
A first outline of the NIRSpec pipeline
RAW DATA
BIAS & DARKSUBTRACTION
PIXEL-TO-PIXELDQE CORR.
LOCATEEXTR. WINDOW
THROUGHPUT CORRECTION(incl. L-flat, blaze function,
transmission of optics, & “default” chromatic slit loss)
MSA CONFIG. &DISTORTION MAP
GW TELEMETRY
GEOMETRICDISTORTION
(spatial)
FINAL CALIBRATION
(dispersion solution)
“DELTA” CHROMATICSLIT LOSS CORR.
ABSOLUTEFLUX CALIBRATION
EXTRACT1-D SPECTRUM
SUBTRACTBACKGROUND
FINAL 1D SPECTRUM
FLATFIELD REF. CUBE
GRATINGEQUATION
GEOMETRICDISTORTION MAP
PHOTFLAMKEYWORD
BIAS/DARKREF. FRAMES
“P-FLAT”REF. FRAME
LINEARITY CORR.LINEARITY
REF. FRAME (Assuming no -dependence)
One window for every open shutter….
Each spectrum has “extraction box” on FPA
Defining the extraction windowsMSA mask
Red: object, green: background
Extraction boxes overlap, possible “spill-over”
RAW DATA
BIAS & DARKSUBTRACTION
PIXEL-TO-PIXELDQE CORR.
LOCATEEXTR. WINDOW
THROUGHPUT CORRECTION(incl. L-flat, blaze function,
transmission of optics, & “default” chromatic slit loss)
MSA CONFIG. &DISTORTION MAP
GW TELEMETRY
GEOMETRICDISTORTION
(spatial)
FINAL CALIBRATION
(dispersion solution)
“DELTA” CHROMATICSLIT LOSS CORR.
ABSOLUTEFLUX CALIBRATION
EXTRACT1-D SPECTRUM
SUBTRACTBACKGROUND
FINAL 1D SPECTRUM
FLATFIELD REF. CUBE
GRATINGEQUATION
GEOMETRICDISTORTION MAP
PHOTFLAMKEYWORD
BIAS/DARKREF. FRAMES
“P-FLAT”REF. FRAME
LINEARITY CORR.LINEARITY
REF. FRAME
One window for every open shutter….
An outline of the NIRSpec pipeline
(Assuming no -dependence)
Operations on the extraction windows
Same as those carried out for traditional ground-based MOS flat-field (uniformity of detector response): f(), f(x,y)
tracing the spectrum and rectifying it: f(), f(x,y)
wavelength calibration: f(), f(x,y)
flux calibration (throughput of optics and gratings): f(), f(x,y)
absolute flux calibration
But wait…
Because of NIRSpec’s design (e.g. MSA always in the way), some of the steps above must be carried out simultaneously, require calibration measurements that are intertwined
--->
x --->
y --
->
Need a “throughput” data cube(for each filter/grating combination)
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).
“Flat-fielding” NIRSpec spectra
---> --->
Output (assuming a source with flat spectrum)
Throughput correction of^
Contributions to the “Throughput correction” reflectivity of all mirrors: f(), f(x,y)
transmission curves of filters: f()
blaze function of gratings: f(), f(x,y)
large-scale response of detector (L-flat): f(), f(x,y)
All contributions are 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.
But wait……
Fixed slit size, but variable PSF width…….
…… causes “flaring” and intensity gradient:
The bummer: chromatic slit loss
--->
1 m 3 m 5 m
A “default” correction for e.g. a perfectly centered point source can be included in throughput correction.The user needs to optimize this correction later…..
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
RAW DATA
BIAS & DARKSUBTRACTION
PIXEL-TO-PIXELDQE CORR.
LOCATEEXTR. WINDOW
THROUGHPUT CORRECTION(incl. L-flat, blaze function,
transmission of optics, & “default” chromatic slit loss)
MSA CONFIG. &DISTORTION MAP
GW TELEMETRY
GEOMETRICDISTORTION
(spatial)
FINAL CALIBRATION
(dispersion solution)
“DELTA” CHROMATICSLIT LOSS CORR.
ABSOLUTEFLUX CALIBRATION
EXTRACT1-D SPECTRUM
SUBTRACTBACKGROUND
FINAL 1D SPECTRUM
FLATFIELD REF. CUBE
GRATINGEQUATION
GEOMETRICDISTORTION MAP
PHOTFLAMKEYWORD
BIAS/DARKREF. FRAMES
“P-FLAT”REF. FRAME
LINEARITY CORR.LINEARITY
REF. FRAME
One window for every open shutter….
An outline of the NIRSpec pipeline
(Assuming no -dependence)
(output is re-sampled grid)
1 2 i
Slit is curved (function of field angle)
Need rebinning before final spectrum is extracted
Lines of constant l spread over multiple pixel columns
1 2 i
Use “Drizzle” technique as possible approach for coordinate transform
Rectifying NIRSpec Spectra
1 2 i
- depends on source shape and position within shutter- must be user-controlled
1 2 i
“Delta” correction for chromatic slit loss
Collapse to 1-d spectrum
- depends on source extent and background subtraction - must be user-controlled
For quick-look analysis, pipeline subtracts TBD “default” background
RAW DATA
BIAS & DARKSUBTRACTION
PIXEL-TO-PIXELDQE CORR.
LOCATEEXTR. WINDOW
THROUGHPUT CORRECTION(incl. L-flat, blaze function,
transmission of optics, & “default” chromatic slit loss)
MSA CONFIG. &DISTORTION MAP
GW TELEMETRY
GEOMETRICDISTORTION
(spatial)
FINAL CALIBRATION
(dispersion solution)
“DELTA” CHROMATICSLIT LOSS CORR.
ABSOLUTEFLUX CALIBRATION
EXTRACT1-D SPECTRUM
SUBTRACTBACKGROUND
FINAL 1D SPECTRUM
FLATFIELD REF. CUBE
GRATINGEQUATION
GEOMETRICDISTORTION MAP
PHOTFLAMKEYWORD
BIAS/DARKREF. FRAMES
“P-FLAT”REF. FRAME
LINEARITY CORR.LINEARITY
REF. FRAME
(erg/cm2/s/Å, sampled within variable slit aperture)
(one window for every open shutter…. )
An outline of the NIRSpec pipeline
(output is re-sampled grid)
(Assuming no -dependence)
(one spectrum for every open shutter)
(defined by user or average)
(defined by user)
“CALWEBB”
“CALNIRSpecA”
“CALNIRSpecB”