MAPCOR Processing Algorithms and Input Parameters

MAPCOR works inside the 2MASS processing pipeline, and works on one band of one scan at a time; thus it only has data from the point sources found in that band of that scan.  The possible "parent" sources, sources which cause artifacts, are the stars in the input point source list brighter than or equal to certain magnitudes:

• 10.5 for J band
• 10.0 for H band
• 9.5 for K_s band

The positional parameters below are all given in units of arcsec (which is the approximate size of the Atlas image pixels).  MAPCOR works in band-scan (x,y) coordinates, where (x,y) is the position of the parent source in arcsec relative to the band's zero-point of the scan.  The x and y coordinates preserved in the database are in the universal(U)-scan coordinate system, which is in arcsec and independent of band.  The y-axis of the U-scan system is parallel to the telescope scan direction, pointing in a northward direction.  Converting from each of the three band-scan coordinates to U-scan coordinates involves a 5-parameter linear transformation including x and y scale factors, x and y translations and a rotation angle.  All 15 of these parameters change slightly with scan, but the rotation angles remain less than 0.5 degrees regardless of scan.   In addition, the relationship between the U-scan system and J2000 RA-Dec axes changes with scan, but the x-axis of the U-scan system never gets more than 3-4 arcmin out of the J2000 equatorial plane.  Thus, away from the equatorial poles, to an approximation, x or cross-scan is in the negative RA direction and y or in-scan is in the Dec direction.    (For more information on 2MASS coordinate systems and transformations, please see the POSMAN Subsystem Design Specification.)  Note, therefore, that any calculation of MAPCOR artifact areas using the U-scan coordinates found in the database and the parameters herein is only a close approximation to the area calculated by MAPCOR itself.

Diffraction Spikes

Only sources with J<=10.0 mag, H<=9.5, or K_s<=9.0 mag can create diffraction spikes that are visible on the image, because as parents get fainter, the diffraction spikes disappear faster than other artifacts.  Therefore, possible parents fainter than those magnitudes skip the diffraction step in the artifact processing.  We know that spikes from very bright stars fan out as they extend farther from the parent source, but the diffraction spike width can be approximated as a constant for stars fainter than m ~ 5.  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 ) ] }

The half-width of the diffraction spikes is diff_width, also an input parameter.  The half-width is the same for all of the spikes except the northern telescope's western spike, which is wider and more diffuse than the others.

The input parameter values, in arcsec or mag, for the north and south telescopes are:
 

Persistence

There can be many persistence artifacts following very bright stars, but not all of them are extracted as point sources.   The predicted position of the ith persistence source, where (x,y) is the position of the parent source, is approximately:

x(i) = x + i(xoffs)
y(i) = y + i(yoffs)

For north-going scans:
xoffs ~ 0.5
yoffs ~ 83

For south-going scans:
xoffs ~ -0.5
yoffs ~ -83
 

mag(i) = mag_parent + pers_dm(i),

(pers_dm(i) is not well-determined for values of i above 7 because the parents of those later persistence sources are very bright.)

Mapcor determines the persistence probability for each source within 6 arcsec of predicted persistence positions.   The probability that a certain source is a persistence artifact of a certain parent source is determined from a chi-squared quantity, computed as:

chi² = [(x_s - x_pred)² / sig_x² ] + [(y_s - y_pred)² / sig_y² ] + [(m_s - m_pred)² / sig_m² ]

How many persistence sources could be found, or in other words how large i can get, depends on the parent source's magnitude.  The persistence search continues for values of i up to the point at which the predicted persistence mag minus its sigma, 1.0 mag, is fainter than the approximate source detection limits of J=17.7 mag, H=17.2 mag, or K_s=16.7 mag in the north or J=17.7 mag, H=17.0 mag, or K_s=16.6 mag in the south.

Filter and Dichroic Glints

The northern telescope J, H, and K_s bands and the southern telescope H and K_s bands each have two filter/dichroic glints per parent source; the southern telescope J band has three.  The brightest glints are normally closest to the parent source, and for very bright parents, all glints tend to be lost within the confusion region in the core of the parent source.  The predicted position of glint j, where j = 1, 2, or 3, is:

x(j) = x + dx(j)
y(j) = y + dy(j)

All sources within the uncertainty radius  g_rad of the correct position, with a magnitude within ± 3.0 mag of mag(j), the predicted mag of the jth glint:

mag(j) = mag_parent + glint_dm(j),

Stripes

Only sources brighter than the approximate source detection limits in each band minus 11 mag can create NICMOS3 horizontal stripes that are visible on the images, so possible parents fainter than those magnitudes skip this step in the artifact processing.  The three stripes per parent are located at the parent's y position and at y ± 256 arcsec, and extend horizontally across the entire coadd image.  They are 8 arcsec wide (full width, north to south), and often have small ghosts at the locations (x ± 0, 256"), (y ±  0, 256").   Anything found within the stripe area is flagged as contaminated by the stripe.

Bright Star Confusion

The size of the confusion radius in arcsec is:

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

Note that the main differences between the northern and southern parameters are found in the K_s band; the radius was made extra-large in that band to include the "hat" artifact that sometimes occurs for bright southern K_s band sources.  It is probably caused by non-optimal placement of the telescope stop.  (The different m_0 values account for the different instrumental zero points between the two telescopes.)  There is a minimum confusion radius of 2.0", and a maximum radius of 160" in the southern K_s band and 60" in the southern J and H bands and all northern bands.  If the calculated length is less than the minimum, it is set to the minimum value; if it is greater than the maximum, it is set to the maximum value.

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 any Read1 sources found within the confusion radius are considered real sources but are flagged as confused.
 

Last updated: 2000 July 24, by  T. Evans