2MASS All-sky Point Source Catalog Statistics Service

Instructions/Help

Service Summary

The service computes the approximate statistics of 2MASS PSC source distributions in a specified circular region of the sky. The basic output includes binned source counts and color histograms, integrated and mean surface brightnesses and areal coverage. The results delivered by this service are statistically correct, but may differ slightly from explicit queries run on the PSC. The key advantage of using this tool is its fast execution speed, at the expense of a degradation in spatial resolution.

Input

Coordinates or Object Name

Object name resolution is provided by NED and SIMBAD. Coordinates can be specified in a variety of formats. Examples are given on the top statistics page, and full details can be found here.

Radius

Enter the radius of the area you wish to search in decimal degrees. A default of 1 degree will be used if nothing is entered. A maximum allowable search radius is 90 degrees.

Results

Search Area

Catalog:

2MASS Point Source Catalog

Object:

Astronomical object name or coordinate used as sky location to search around.

Location:

Right ascension and declination in J2000 decimal degrees.

Spatial Bins:

The number of spatial bins in the grid that the search circle touched, and therefore the number that was used in the statistical summary (see below for explanation of spatial bins).

Actual Search Area:

Total sky area covered by these spatial bins in square degrees. This is the area that corresponds to the resulting binned output.

User Requested Search Area:

Area in square degrees covered by the user-defined circle.

Radius:

Circular radius searched around location, in degrees.

Source Counts

The total number of 2MASS sources in the summed bin area (see above) per J, H and K_s bands. Only 2MASS sources that have valid detections are used (e.g., upper limits are excluded). The differential source counts are written to an ascii table and plotted as counts (number per deg^2 per mag) versus mag. The J-band counts are shown in blue, H-band in green, and K_s-band in red.

Integrated surface brightness

The integrated surface brightness (mag per deg^2) obtained by summing the flux of all the sources, per wavelength band, within the summed bin area.

Brightness and Color Histograms, and Plots

Differential source counts by magnitude for each wavelength band, and by color for the three near-infrared colors J-H, H-K_s, and J-K_s.

Tables and Postscript Files

The differential source and color counts are written to an ascii table and plotted in a postscript file. In addition, links are provided to the 2MASS All-Sky Quicklook Image Service to see an inventory of the 2MASS images within the defined region, and to the RADAR inventory service that provides a full inventory of the IRSA archives for the defined region.

What the Statistics Tool Does

The 2MASS PSC statistics service utilizes a statistics database table (SDT) that was generated from the PSC itself. In the current implementation, the SDT is built upon the concept of hierarchical triangular mesh (HTM) based spatial bins, where the entire sky, for 2MASS, has been divided into 131072 bins with areas ranging from ~0.000075 to ~0.00015 steradians, or ~0.25 to ~0.5 square degrees.

Each point source (~500-million in the All Sky data release) has been assigned to a spatial bin (SB). For each SB, integrated source counts and brightnesses are collected and computed for all 3 bands. In addition, differential source counts are accumulated in 3 magnitude histograms and 3 color histograms. The range is from 2.0 to 19.0 mag with a bin width of 0.5 mag for the 3 brightness histograms, and from -2.0 to 5.0 mag with a bin width of 0.2 mag for the 3 color histograms.

The first step in the execution of this processor is to determine the list of SBs which overlap with the user-input target and radius values. It then sends a database query to the SDT, collects and processes the returned statistics, and finally outputs the results in various tabular and graphical forms.

The key advantage of using this tool is its fast execution speed, at the expense of a degradation in spatial resolution. Because the SDT contains only 131072 rows, queries are returned much faster than if run on the ~500 million row PSC.

Program Interface

This software can be access via HTTP GET/POST program interface where the request is a set of "parameter=value" pairs. Please click for more details.

Current Limitations on the Use of this Tool

Area definition is restricted to circular ("cone") geometry.

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This page last updated: July 12, 2004