Goals
- Retire aXe
- Retire aXeSIM
- Leverage the JWST infrastructure
- just crds, gwcs and asdf?
- Enable common "Grismconf" infrastructure for:
- pyLINEAR
- grizli
- EM2D, and Nor's other modules
- JWST pipeline
- (WFIRST)
- Ensure that users can do all the flavors of geometric transformations (and can understand them)
- Make sure calibration (files) are consistent with the approach to geometry
- Support the following HST modes:
- WFC3/IR
- ACS
- WFC3/UVIS?
- Compatible with JWST pipeline outputs of WFSS modes (NIRISS and NIRCam)
Use-Cases / Workflows
1D spectral extraction for perfectly registered & perfectly calibrated data
Story: As an impatient astronomer, I just want to extract a 1D spectrum for a single source given perfectly registered images and assuming calibrations are correct. I want it fast and I don't want to think about forward modeling.
What do I need as inputs?
- Direct and slitless images with perfect WCSs
- catalog information
- Exposure times?
- Noise model? Or components of the noise model (gain, exptime, read noise, subtracted background)?
- Or is this already in an uncertainty array?
- Detector signatures already removed (e.g. CALWEBB_DETECTOR1)
- Identification of the pixels in the direct image corresponding to "the source" (e.g. from a segmentation map)
- Identification of any pixels in the direct image corresponding to contaminating sources
What does this step do for me?
- Maps pixels in the direct image to x,y, wavelength
- Accumulates the flux at each wavelength (onto some wavelength grid)
- Perhaps weighted by the flux in the direct image at this cross-dispersion location
- Accumulates uncertainties at each wavelength
- Accumulates some kind of contamination flag at each wavelength
Why would I not want to do this?
- This doesn't do much to address contamination
- This mixes together wavelength and position more than a forward-modeling approach.
- Uncertainties are likely to be quite misleading
Epics
Identify and match data sets
- Should this be part of Astrogrism, or outside the package (e.g. in a notebook, as a tutorial)?
- Archive queries via API for both direct and slitless observations
- Build associations
- Can/should we use the JWST pipeline machinery for such associations?
Geometric transformations
Direct image: x,y ↔ RA, DEC
Slitless image:
ra, dec, wavelength → x, y
x,y | RA, dec → wavelength
- x,y | wavelength → ra, dec
Astrometric registration
Simulations
aXeSIM - https://www.stsci.edu/scientific-community/software/axe
Contamination
Background Subtraction
HST ACS sky background
ACS uses the original aXe implementation of 'Master sky image' (global background) and local background subtraction approach.
Basic method is outlined in aXe paper at
https://iopscience.iop.org/article/10.1086/596715/pdf (section 2.6)
HST WFC3 sky background
Master sky images have been provided for both G102 and G141, however the single images used by aXe do not take into account the full complexity of the sky background of grism observations. A more accurate background subtraction can be achieved by using separate images for each of the background components: zodiacal light, He I emission and scattered light (for G141).
More info at
https://www.stsci.edu/hst/instrumentation/wfc3/documentation/grism-resources/ir-grism-master-sky-images
Relevant paper
https://iopscience.iop.org/article/10.3847/1538-4357/aa81cc/pdf (section 3.2.6)
Most recent 'WFC3_Back' (Nor's slides)
Flatfielding
1D Spectral Extraction
1-D extraction
non-weighted extraction (section 3.3.2, figure 11)
https://iopscience.iop.org/article/10.3847/1538-4357/aa81cc/pdf
and
optimal extraction (section 3.3.2, figure 12)
https://iopscience.iop.org/article/10.3847/1538-4357/aa81cc/pdf
- original concept from Horne 1986
https://ui.adsabs.harvard.edu/abs/1986PASP...98..609H/abstract
2D Spectral Extraction
2-D extraction
Simulation based extraction (SBE) details in section 3.2.4 (figure 10)
https://iopscience.iop.org/article/10.3847/1538-4357/aa81cc/pdf
1D Forward modeling
2D Forward modeling
Combine spectra single PA
Combine spectra multiple orients
Documentation
- Getting Started (landing page)
- Installation
- Overview
- Input/Output
- images
- spectra
- catalogs and tables
- reference files
- Matching direct and slitless observations
- Astrometric registration
- Background subtraction
- Spectral extraction
- Contamination
- Combining spectra
- Simulations and forward modeling
- Using Astrogrism with other packages
- specutils
- grizli
- pylinear
- EM2D
- MIRAGE?
- Terminology:
- Use "slitless" instead of grism?
