"redshift versus blueshift"
Quasar and emission line precise redshift fitting from prior

This package aims at measuring a precise redshift given a broad redshift prior. To do so the package, for each emission line or the full spectrum:

• Runs a coarse chi2 scan as a function of redshift, using the input PCA+boradband Legendre polynomials

• Finds three local minima

• Does a finer chi2 scan in each minima

• Defines the global PCA redshift, "ZPCA" from the best minimum of the three. ZPCA is a redshift estimator biased toward the computation of the PCA.

• The redshift of the line, "ZLINE", is defined from the maximum of the best-fit model of the line. ZLINE is a redshift estimator un-biased toward the velocity of the line, but can be biased with respect to the cosmological redshift.

Chi2 scan vs. redshift	Best fit

To install simply clone the repository, add to your PATH and to your PYTHONPATH

git clone https://github.com/londumas/redvsblue
PYTHONPATH=$PYTHONPATH:<path_to_folder>/redvsblue/py/
PATH=$PATH:<path_to_folder>/redvsblue/bin/

To get all available options:

redvsblue_lineFitter.py --help

To run on SDSS data, do:

redvsblue_lineFitter.py
--out <path_to_write_output>.fits
--in-dir $BOSS_SPECTRO_REDUX/v5_13_0/
--drq DR12Q_v2_10.fits
--z-key Z_VI
--qso-pca <path_to_folder>/redvsblue/etc/rrtemplate-qso.fits

To run on DESI data, for example:

redvsblue_lineFitter.py
--out <path_to_write_output>.fits
--in-dir /project/projectdirs/desi/datachallenge/redwood/spectro/redux/redwood/spectra-64/
--drq /project/projectdirs/desi/datachallenge/redwood/spectro/redux/redwood/zcatalog-redwood-target-truth.fits
--z-key Z
--qso-pca <path_to_folder>/redvsblue/etc/rrtemplate-qso.fits
--data-format DESI
--no-extinction-correction

The output is a FITS file, with one HDU per redshift type:

• CAT, HDU=1: redshift prior
• PCA, HDU=2: PCA redshift
• HALPHA, HDU=3: redshift of the HALPHA line
• HBETA, HDU=4: redshift of the HBETA line
• MGII, HDU=5: redshift of the MGII line
• CIII, HDU=6: redshift of the CIII line
• CIV, HDU=7: redshift of the CIV line
• LYA, HDU=8: redshift of the LYA line

For each best fit readshift, the code gives the following quantities:

• ZLINE: best fit redshift of the line according to maximum of PCA

• ZPCA: best fit redshift according to PCA

• ZERR: redshift error

• ZWARN: redshift warning, use ZWARN=0 for reliable redshifts

• CHI2: chi^2 of the best fit

• DCHI2: Delta chi^2 against a Legendre polynomial, i.e. gives the significance of the redshift

• NPIXBLUE: number of pixel on the blue side around the prior redshift

• NPIXRED: number of pixel on the red side around the prior redshift

• NPIX: total number of pixels around the prior redshift

• NPIXBLUEBEST: number of pixel on the blue side around the best redshift

• NPIXREDBEST: number of pixel on the red side around the best redshift

• NPIXBEST: total number of pixels around the best redshift

• SNR: mean signal-to-noise ratio per pixel in the spectral region used for measuring the redshift

RED vs BLUE, from Corridor: https://www.youtube.com/watch?v=arg_aHzviQw

https://fr.wikipedia.org/wiki/Liste_des_reines_et_imp%C3%A9ratrices_de_France