2MASS Six Degree Mosaics
|
1. Overview
The Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006, AJ 131, 1163) was a survey of the entire sky in three infrared wavebands: J (1.235 microns), H (1.662 microns), and KS (2.159 microns). The 2MASS All-Sky Data Release products include 4,121,439 Atlas Images (512 x 1024 pixels, 1 arcsec/pixel) in the three survey bands.
For users wishing to visualize regions of the sky that are significantly larger than the Atlas Images, or that fall close to the edge of an Atlas Image, the NASA/IPAC Infrared Science Archive (IRSA) has created 6 x 6 degree 2MASS mosaics. The primary set consists of 1,734 mosaics with a 1 arcsecond pixel scale (the same pixel scale as the Atlas Images), resulting in individual files that are 1.9 Gb. Also available are 52 Mb mosaics that have been binned to a 6 arcsecond pixel scale.
One of the primary sources of unreliability of 2MASS detections is confusion with artifacts produced by bright stars. These artifacts include filter and dichroic glints and Persistence (latent) images. The positions of spurious detections of image artifacts were identified during 2MASS data processing, and tables of these spurious detections are provided with the 2MASS Atlas Images (as described in Section IV.7 of the 2MASS Explanatory Supplement). These tables have been used to construct analogous tables for the Six Degree Mosaics.
Users interested in photometry derived from 2MASS images should always defer to the 2MASS catalogs and databases, which were derived from the Atlas images as described in the 2MASS Explanatory Supplement. The high-resolution (1 arcsec pixel scale) six degree mosaics are useful for examining regions of the sky that fall close to the edge of an Atlas Image. The lower resolution (6 arcsecond pixel scale) six degree mosaics are useful for examining large scale emission.
2. Data Access
The 2MASS six degree mosaics are available through the IRSA Image Server, which provides program friendly methods for querying astronomical image metadata tables, including support for the IVOA Simple Image Access protocol. Images and image cutouts are available for direct download and are laid out in browsable directories. Visit the IRSA Image Server:
http://irsa.ipac.caltech.edu/ibeA step-by-step guide for using the command line tools wget and curl for downloading images using the query URLs described at the above link is provided here:
http://irsa.ipac.caltech.edu/docs/program_interface/api_images.htmlWhen constructing the search query URL in Step 2, follow the directions for "IRSA Image Server". For example,
http://irsa.ipac.caltech.edu/ibe/sia/twomass/mosaic/sixdeg?POS=10.6847929,41.2690650
will return a VO Table with the metadata for the six degree mosaics covering M31.
The original 2MASS Atlas Images are also available through the IRSA Image Server. Users interested in GUI access to the Atlas Images can use the 2MASS Image Service.
3. Mosaicking
The Six Degree Mosaics described here were made with Montage, a toolkit for assembling FITS images into custom mosaics while preserving the spatial and calibration fidelity of the input images.
In particular, the mosaics were created using the mExec script, which is designed to create mosaics for 2MASS, SDSS, and DSS data. The "levels-only" option was used to find and apply an additive offset correction to each image before combining.
4. Artifacts
These are lists of RA and Dec (degree, J2000) positions for two kinds of bright-star artifacts: dichroic/filter reflections (glints) and latent images (persistence).
The airglow component of the background in the Atlas images can vary in all of the 2MASS bands, but variations are most prevalent in the H band. These variations cannot always be completely removed by the simple background matching employed, resulting in irregular, sometimes blocky bright/dark areas (see Figure 2).
|
| Figure 2. H band six degree mosaic showing airglow variations in the original 2MASS Atlas images. The field is near the south equatorial pole, centered at RA = 12.86 degrees and Dec = -84.5 degrees and extending 2.2 degrees along the horizontal axis. North is up and East is to the left. |
5. Astrometry
The astrometric fidelity of the six degree mosaics is good: 90% of isolated sources have centroids in the six degree mosaics that are within 0.5 arcsec of that measured in the Atlas images. This corresponds to 1/2 pixel or 1/6 of typical 2MASS seeing of 3 arcsec. Figure 1 shows that there are no large offsets along RA or Dec. The mean total offsets in arcsec are 0.14 (J), 0.18 (H), and 0.22 (KS). This modest degradation of positional accuracy is expected as the six degree mosaics re-sample the Atlas images with relatively large pixels.
|
|
|
6. Photometry
Users interested in photometry should consult the 2MASS catalogs and Databases released as part of the All Sky Survey:
http://irsa.ipac.caltech.edu/applications/Gator
Users that require custom aperture photometry should investigating using the Atlas Images released as part of the All Sky Survey. These are available through a GUI interface or program interface:2MASS Image Server (GUI interface)
http://irsa.ipac.caltech.edu/applications/2MASS/IM/
IRSA Image Server (program interface)
http://irsa.ipac.caltech.edu/ibe
Users whose needs cannot be accommodated by the Atlas Images can do photometry on the 2MASS Six Degree Mosaics with a one arsecond pixel scale. We do not recommend doing photometry on the six arcsecond mosaics.
There will be some smearing of compact-source profiles on the six degree mosaics, since the mosaics re-sample the Atlas images with relatively large pixels. The effect on photometry will be less in larger apertures. With a large enough aperture, one can recover the aperture photometry one would get on the Atlas image. Figure 2 shows the photometry differences for a small (3 arcsec radius) aperture and large (5 arcsec radius) aperture. The smearing causes the small offset in the smaller aperture. Going to a large aperture will increase the scatter at low fluxes as there is more background noise in the aperture.
|
|
|
The units of the six degree mosaics are data numbers. To determine the magnitude of an object, first compute the sum of the pixel values within the desired aperture and subtract the background. Convert the resulting value, S, to magnitudes using the following formula:
magnitude = MAGZP - 2.5log10(S)
Here, MAGZP is the magnitude zero point. The values are 20.9044 (J), 20.4871 (H), and 19.9757 (Ks).
Important Caveat: As with the original 2MASS Atlas Images, the pixel values in the six degree mosaics are not absolutely calibrated. These images are good for relative photometry (with respect to the local background) only.
7. Sample Header
Below is an example header from the file mosaic_6deg_j01382_6asec.fits.
SIMPLE = T / file does conform to FITS standard BITPIX = -32 / number of bits per data pixel NAXIS = 2 / number of data axes NAXIS1 = 3600 / length of data axis 1 NAXIS2 = 3600 / length of data axis 2 EXTEND = T / FITS dataset may contain extensions COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy COMMENT and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H CTYPE1 = 'RA---TAN' CTYPE2 = 'DEC--TAN' CRVAL1 = 18.620690 CRVAL2 = 40.000000 CRPIX1 = 1800.500 / CRPIX2 = 1800.500 / CDELT1 = -0.00166666675033 / CDELT2 = 0.00166666675033 / CROTA2 = 0.000000000 EQUINOX = 2000.0 MAGZP = 20.9044 FILTER = 'j' ORIGIN = 'IPAC/IRSA' DATASET = '2MASS' DATEOBS1= '19981016T05:17:34.54' / obs. date/time of earliest frame (UTC) DATEOBS2= '20001017T09:30:48.85' / obs. date/time of latest frame (UTC) HISTORY Montage v3.3 12/14/2010 http://montage.ipac.caltech.edu COMMENT The mosaic was created using the 2MASS All-Sky Atlas Images and COMMENT algorithms that conserve flux and preserve the astrometric information HISTORY HREBIN: Oct 8 09:39:35 2014 Original Image Size Was 21600 by 21600 HISTORY HREBIN: Oct 8 09:39:35 2014 Box Averaging END
8. Resources
2MASS All-Sky Data Release documentation
2MASS Image Service (All-Sky Images)
Last update: March 23, 2015