DB_MAPCOR Very Bright Parent Processing Algorithms and Input Parameters

T. Evans - IPAC

DB_MAPCOR works outside the 2MASS processing pipeline, on the database of point sources, to do final cleanup of diffraction spike and confusion artifacts only. Therefore, not only can it work on all three band's data at once, it also has access to "parent" sources which are not found in the same scan as their artifacts, and data on very bright Read1-saturated stars compiled from published lists of IR photometry. This page discusses the "very bright parent" processing, which uses the compiled "parent" list of data and slightly different algorithms and input parameter values than MAPCOR or "edge parent" DB_MAPCOR processing.

These possible "parent" sources (sources which cause artifacts) are the stars in the compiled list with J, H, or K_s mag <= 5.0; thus they are heavily saturated in the R1 exposure and affect more than one scan. (MAPCOR reliably finds artifacts from fainter "parent" sources.) DB_MAPCOR, using algorithms dependent on the position and brightness of the "parents", looks for sources in any scan caused or affected by diffraction spikes and confusion (for artifact definitions see this page). These sources are then flagged as either artifacts or real objects on the sky with positional or brightness measurements affected by nearby artifacts.

The positional parameters below are all given in units of arcsec (which is the size of the Atlas image pixels), except where noted.

Diffraction Spikes

Diffraction spikes from very bright stars in both hemispheres, especially the E-W spikes, spread out and separate into approximately 6 (or more) total components as they extend farther from the "parent" source. Because they are not yet well-characterized, extra-large diffraction spike masks are presently used. The 6 diffraction spikes are at these position angles (E of N, in degrees):

i =	1	2	3	4	5	6
J band	0.0	89.05	91.48	180.0	269.47	271.63
H band	0.0	89.12	91.46	180.0	269.49	271.60
K_s band	0.0	89.10	91.49	180.0	269.52	271.65

The diffraction spike widths are set at 1.0 arcmin (full width). The calculated length in arcsec of each spike (from the center of the "parent" source) is:

l = l_0 { 10^[ a ( m_0 - mag_parent ) ] }

where the l_0, a, and m_0 parameters, at present, are the same for all spikes in all bands, north and south:

l_0	a	m_0
3600.0	0.27	0.0

However, since MAPCOR cannot reliably find the persistence sources from these very bright "parents", DB_MAPCOR must also remove the persistence artifacts. Thus, if a N-S spike (i=1 or 4) is shorter than 675 arcsec (11.25 arcmin), it is set to 675 arcsec to make sure at least 8 possible persistence sources are contained in the spike area. The maximum allowed spike length is 67500 arcsec (1125 arcmin or 18.75 deg). These long spike lengths, and large confusion radii below, also get rid of the NICMOS3 horizontal stripes. Example values, in arcmin, are:

mag	l[i=1,4] arcmin	l[i=2,3,5,6] arcmin
-3.54	541.9	541.9
-1.5	152.5	152.5
0.0	60.0	60.0
2.5	12.7	12.7
5.0	11.25	2.68

Any sources extracted within the diffraction spike area are either part of the spike itself or are contaminated by the spike. If a spike source is brighter than mag_parent + diff_dm, it is considered contaminated by the spike; if it is fainter, it is considered part of the spike (persistence sources will also fall into this category). The values of diff_dm for both telescopes are:

5.0 for J band
6.6 for H band
7.2 for K_s band

Because a star needs only one band brighter than m=5 to be included in this "very bright parent" list, DB_MAPCOR checks the "parent's" magnitude in each band before searching for diffraction spikes. If the "parent's" magnitude is fainter than 10.0 in J, 9.5 in H, or 9.0 in K_s (improbable but certainly possible), DB_MAPCOR does not search for diffraction spikes in that band.

Confusion

The size of the confusion radius in arcsec is:

r = r_0 { 10^[ b ( m_0 - mag_parent ) ] }

where the r_0, b, and m_0 parameters, at present, do not depend on the band; the parameters are the same for all three bands:

r_0	b	m_0
24.0	0.2	6.2

The maximum radius allowed is 2100 arcsec (35.0 arcmin).

Example values, in arcmin, are:

mag	r arcmin
-3.54	35.49
-1.5	13.87
0.0	6.95
2.5	2.20
5.0	0.695

These large confusion radii also get rid of all filter and dichroic glints and ghosts. If more than one Read2-Read1 source is found within the confusion radius, all of the Read2-Read1 sources are identified as artifacts, but a single Read2-Read1 source and most Read1 sources found within the confusion radius are considered real sources but are flagged as confused. However, these "parent" sources that are saturated in R1 can also create false R1 extractions surrounding the core, out to a radius of about 10 arcsec. Therefore, all R1 srcs within 10" of these very bright parent stars have also been marked as artifacts.

Last updated: 2000 July 26, by T. Evans