Transcript Poster

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 --->