nirspec pipeline concept guido de marchi, tracy beck, torsten böker

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



LOCATEEXTR. WINDOW




GW TELEMETRY

GEOMETRICDISTORTION

(spatial)

FINAL CALIBRATION




EXTRACT1-D SPECTRUM

SUBTRACTBACKGROUND

FINAL 1D SPECTRUM

FLATFIELD REF. CUBE

GRATINGEQUATION


PHOTFLAMKEYWORD




REF. FRAME


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



LOCATEEXTR. WINDOW




GW TELEMETRY

GEOMETRICDISTORTION

(spatial)

FINAL CALIBRATION




EXTRACT1-D SPECTRUM

SUBTRACTBACKGROUND

FINAL 1D SPECTRUM

FLATFIELD REF. CUBE

GRATINGEQUATION


PHOTFLAMKEYWORD




REF. FRAME




(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



LOCATEEXTR. WINDOW




GW TELEMETRY

GEOMETRICDISTORTION

(spatial)

FINAL CALIBRATION




EXTRACT1-D SPECTRUM

SUBTRACTBACKGROUND

FINAL 1D SPECTRUM

FLATFIELD REF. CUBE

GRATINGEQUATION


PHOTFLAMKEYWORD




REF. FRAME

(erg/cm2/s/Å, sampled within variable slit aperture)

(one window for every open shutter…. )


(output is re-sampled grid)


(one spectrum for every open shutter)

(defined by user or average)

(defined by user)

“CALWEBB”

“CALNIRSpecA”

“CALNIRSpecB”

nirspec pipeline concept guido de marchi, tracy beck, torsten böker

Documents

function of wavelength

y wavelength calibration

nirspec pipelineassuming

fl blaze function of

wavelength reflective

nirspec pipelineeach

variable slit curvature

detector pixel