Description
Issue JP-645 was created by Brad Whitmore:
Elaboration of Requirements for Aperture Photometry
in the JWST Pipeline Source Lists.
BACKGROUND:
Basic source lists requirements for the JWST pipeline were defined in:
https://outerspace.stsci.edu/display/JWSTCC/Vanilla+Point+Source+Catalog
This is sometimes referred to as the "Vanilla Source List". An
"Optimal Source List" is also being discussed within the JWST
Photometry Working Group and will be presented to the
Calibration Working Group at a future date.
Below is an elaboration of the requirements for aperture photometry, as
discussed within the JWST Photometry Working Group >>> webpage:
https://outerspace.stsci.edu/display/JWSTCC/JWST+Photometry+Working+Group
REQUIREMENTS:
- Circular aperture photometry should be performed using
the Photutils package.
This can be in addition to the detection and photometry algorithms
that are currently included in the pipeline code using an image
segmentation approach, as long as this approach is able to identify
point-like sources in different environments (e.g., high background,
crowded regions) at a level that at least looks as good as with DAOPHOT.
Early results from Larry Bradley's Jupyter handbook suggest this is true
but this should be evaluated in more detail as we go along.
-
Aperture sizes in pixels should come from reference files developed
by the NIRCAM, MIRI, and NIRISS instrument teams providing the pixel
values for each instrument/filter combination that encircle 20 % - 50
% - 70 % of the energy in a PSF. -
A local background should be used for sky subtraction based on the
median value in a circular aperture taken between the 80 and 85 %
encircled energy apertures, as defined in pixel values by the
reference file described in item 2.
(NOTE: it is possible that the percentages defined in items 2 and 3
will be changed in the future, following a study of the optimal apertures.)
- The format for the reference files should include aperture radii
(in pixels as a function of detector/filter), and aperture corrections
(to infinity) for each detector/filter combination. The aperture
corrections should take into account the small contribution from the
wings of the PSF in the sky annulus. A separate row should be used
for each filter.
Clarification from Larry Bradley - " Right now, I simply need the aperture radii (in
pixels) as a function of detector/filter. Once we have defined how
many apertures to use in the output catalog, etc., I will
define/deliver the initial version of the references files (one per
instrument). The instrument teams can then redeliver updates as
needed."
- The source list should be optimized for point source detection, but
should also do a reasonable job on somewhat extended sources. The
finding algorithm should do reasonably well on all fields (e.g.,
stars, small galaxies like HDF, nearby galaxies with high backgrounds,
... ), rather than being tuned for a specific type of field.
(NOTE: A study may be needed to determine what the best values are for
search parameters, e.g., FWHM, S/N, ... , and how well they work for different
types of images).
-
The number of artifacts should be minimal. Typical artifacts include
close pairs of stars, diffraction spikes, multiple detections of saturated
cores, Airy rings, ... -
Photometry with three units will be included in the catalog
1 - flux densities in Jy
2 - Vegamag in mag
3 - Abmags in mag -
A single source list file should be produced providing the following information:
c1: X-position
c2: Y-position
c3: ID-number
c4: RA
c5: DEC
c6: fluxdens_20 (i.e., the flux density for the 20 % encircled energy aperture)
c7: err_fluxdens_20 (error estimate in fluxdens_20)
c8: fluxdens_50
c9: err_fluxdens_50
c10: fluxdens_70
c11: err_fluxdens_70
c12: back_fluxdens (median value in background annulus, tentatively planned for 80 - 85 % encircled energy aperture)
c13: err_back_fluxdens (rms error estimate in back_fluxdens)
c14: fluxdens_tot (total flux density if a star - set to NA if star? = N)
c15: err_fluxdens_tot
c16: abmag_20 (i.e., the magnitude for the 20 % encircled energy aperture)
c17: err_abmag_20 (error estimate in mag_20)
c18: abmag_50
c19: err_abmag_50
c20: abmag_70
c21: err_abmag_70
c22: abmag_tot (total magnitude if a star - set to NA if star? = N)
c23: err_abmag_tot
c24: vegamag_20 (i.e., the magnitude for the 20 % encircled energy aperture)
c25: err_vegamag_20 (error estimate in mag_20)
c26: vegamag_50
c27: err_vegamag_50
c28: vegamag_70
c29: err_vegamag_70
c30: vegamag_tot (total magnitude if a star - set to NA if star? = N)
c31: err_vegamag_tot
c32: CI_20_80 (concentration index, i.e., the difference between mag_20 and mag_80,
Note: other CIs can be determined based on other columns above)
c33: star? (Is this a star? Y/N. e.g., . using a combination of the CI_ 20_50 and CI_50_80 and
CI_20_80 values)
c26: sat_flag (saturation flag - Y/N - based on whether any pixels are saturated in full readout
from the DQ array)
c27: sharpness
c28: roundness
c29: major_axis
c30: minor_axis
c31: isolation metric (e.g., how roundness changes as function of radius )
Might use Leonardo's - nearest neighbor algorithm for this values
c32: sky_bbox_ll (RA/DEC for source bounding box corners for WFSS spectral pipeline)
c33: sky_bbox_ul
c34: sky_bbox_lr
c35: sky_bbox_ur
NOTE: Other columns based on the image segmentation of the code can also be included.
(e.g., mag_auto, Kron_radius, ...)
Question under consideration - If star? = no, should there be an estimate for total magnitude, since it
could be misleading. At present the answer is no, with a NA = Not Applicable used for the relevant columns above.
There was not a consensus on this item so it is still under discussion.
- The catalog file should include a header that includes the information that does not vary
(e.g., dataset used to obtain source list, date taken, aperture sizes
used, filter used, ...) and that also explains the columns.