NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker ment characteristics ti-object spectrograph --> every detector pixel sees every wavelengt lective optics (incl. dispersive elements) --> large, variable slit d on a diffraction-limited telescope --> PSF varies with e wavelength range (0.6 - 5 m) --> chromatic slit losses -axis telescope and wide field of view --> significant distortion
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NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker
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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
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…..