NIRSpec Data reduction Pipeline
“Final” End Products
Guido De Marchi & Tracy Beck
Reminder on NIRSpec Instrument Characteristics:
1)
2)
3)
4)
1)
used on a diffraction-limited telescope --> PSF varies with λ !
wide wavelength range (0.6 - 5 µm) --> chromatic slit losses!
off-axis telescope and wide field of view --> significant distortion!
reflective optics (incl. dispersive elements) --> large, variable slit curvature!
MSA and Fixed Slit spectra are tilted, curved and flared by differing
•
amounts
MOS Complexity --> every detector pixel sees every wavelength!
NIRSpec IFU Data
NIRSpec IFU Data
1)
2)
R=2700, 1000 or 100 0.6 - 5µm spectral data over a 3” x 3” field of view
Spectral pixel elements of 0.”1 x 0.”1 in size
IFU Data “End” Products
IFU Data End
product = 3-D
datacube with
corresponding
variance and
DQ extentions
“Next Level” End products for IFU data - Mosaiced datacubes from
dithered sets, improve spatial sampling, enlarge spatial FOV
NIRSpec MOS Data
MSA mask
Each Open shutter will result in Science or Background Spectra
Red: object, green: background
Using ~0.”2 x ~0.”4 slitlets, 2-D Spectra acquired from 0.6-5µm with:
• R=2700, R=1000 or R=100 resolution
• up to 100-150 science objects with each MSA configuration
MOS and FS “End” Products
FS/MOS Data End
product = 2-D
background
subtracted,
combined,
rectified,
spectral image
with
corresponding
variance and
DQ
extensions,
AND a
collapsed /
extracted 1-D
spectrum with
Var & DQ
Further Complication: chromatic slit loss
Fixed slit size, but variable PSF width…
1 µm
3 µm
5 µm
… causes “flaring” and intensity gradient:
QuickTimeﾪ and a
TIFF (LZW) decompressor
are needed to see this picture.
λ --->
A “default” correction for e.g.
a perfectly centered point
source can be included in
throughput correction.
The user needs to optimise
this correction later…..
More accurate slit correction (user call)
Wavelength-dependent slit transmission
correction strongly depends on source
location in shutter
- Link to Target Acquisition
Also strongly dependent on spatial
shape of source
- Point source
- Extended source
This step of reduction cannot be automated!
More accurate slit correction (user call)
Exponential profile galaxies, 2 µm wavelength
Typically more than 1 mag difference w.r.t. equivalent point source
Similar results for other assumed profiles
Limit to what automatic pipeline reduction can do with MOS
data… We will need post-processing tools for the user to
optimise spectral extraction and flux correction of spectra!
“Next Level”Considerations for MSA
Pipeline Reduction
•
•
•
•
For each NIRSpec configuration of the MSA shutters, we
could be acquiring simultaneous data on 100-150 science
targets
Hence, NIRSpec MOS programmes with multiple MSA
configurations could be obtaining spectra on hundreds,
perhaps even thousands of targets!
For the faintest sources, we could acquire data on a
single target using multiple MSA configurations.
For very deep fields, spectra on a given science target
could also be acquired in different visits, at different times
Ultimately, we need to be able to track, “associate” and
even combine spectral data acquired on hundreds of
targets, over different MSA configurations, different
visits… etc!
“Delta” correction for chromatic slit loss
- depends on source shape and position within shutter
- must be user-controlled
λ1
λ2
λi
λ1
λ2
λi
Optimized Collapse to 1-d spectrum
- depends on source extent and
background subtraction
- must be user-controlled
Limit to what automatic pipeline reduction can do with MOS data…
We will need post-processing tools for the user to optimise spectral
extraction and flux correction of spectra!
MOS and FS “End” Products
Remember those “Headaches”…??
Each Spectrum is curved, tilted and flared on detector!!
2-D Extraction and processing of data from raw image is complicated.
Reduction to spatially & spectrally rectified 2-D image will require
“drizzle”-like approach.
Open issues to consider when combining multiple spectra:
Can we easily combine dithered datasets in 2-D?
•
Or, will it be better to combine spectra in 1-D using collapsed data?
•