Appendix 1. Introduction to the 2MASS Extended Mission and Ancillary Data Products

4. Cautionary Notes

d. Full Survey, 6x and Calibration Atlas Images

  1. Additional Information Available
  2. Backgrounds
  3. Saturation
  4. Multiple Images
  5. Artifacts

i. Please see Cautionary Notes for the 2MASS All-Sky Release Image Atlas (I.6.d)

This section contains brief summaries of the key limitations common to the Full Survey, 6x and Calibration Image Atlases. It is not a complete collection of cautionary notes for each of the image sets. Please review the more complete descriptions for each product at:

ii. Backgrounds

There may be offsets in the mean background levels between adjacent 2MASS Atlas Images.

All 2MASS Atlas Image data reduction preserved the observed background sky levels measured relative to camera dark frames with the shutter closed. The mean background level in the images is normally largest in the Ks band, although it can be larger in the H-band on occasion due to atmospheric OH airglow emission.

The only background compensation that is made during Atlas Image construction is to adjust the individual frame backgrounds by a constant to produce seamless combined images within a scan. Because the atmospheric OH airglow (especially in the H-band) often contains structure on scales at or below the 2MASS frame size, the resulting Atlas Images sometimes show large background spatial variations. An example of the low frequency structure due to airglow can be seen in the mosaic constructed from the 6x Atlas Images of the IC1396 field shown in Figure 1.

Figure 1 - 6x J+H+Ks image mosaic showing the field containing IC1396 (the Elephant Trunk nebula). The green-hued background structure is caused by time-variable atmospheric airglow emission that is strongest in the H-band.

iii. Saturation

Defer to the All-Sky PSC and XSC and the Extended Mission point and extended soure tables for photometry of bright point and extended sources that may be saturated on the Atlas Images.

No correction for saturation is made during Atlas Image construction. The survey, 6x and Calibration Atlas Images are assembled from the respective 1.3 s (7.8s for 6x) READ2-READ1 exposures. Saturation occurs in the READ2-READ1 frames at approximately J<9, H<8.5 and Ks<8.0 for the survey and calibration data, and J<11, H<10.5 and Ks<10.0 for 6x data. The Atlas Images do not benefit from the dynamic range expansion provided by the 51 ms READ1 exposures. Saturation is handled correctly during source photometry in the survey, 6x and calibration scan data reduction (e.g. IV.4.a.i). For objects that saturate the READ2-READ1 exposures, source brightness is measured from the 51 ms READ1 readouts.

iv. Multiple Images at a Specific Location

There may be multiple, independent images taken at different times that cover a location of interest to you in the Full Survey, 6x and Calibration Scan Image Atlases.

Selected regions of the sky were observed more than once during survey, 6x and Calibration observations. The various 2MASS Extended Mission Image Atlases may contain more than one image for a point on the sky. The depth-of-coverage is most extreme in the equatorial poles for the Survey Atlas, and in any of the 2MASS Calibration Fields where there may be hundreds to thousands of images available.

Because of the image redundancy, when examining the image of a source selected from the from the All-Sky Catalogs, or the various Extended Mission point or extended source tables, it is important to select the Atlas Image from the same scan in which the source was detected. The IRSA/2MASS Image Services allow specification of a particular date, observatory (hemisphere) and scan number when requesting images from the Survey Atlas. If these are not specified, you may not receive the image from the correct scan.

You may obtain all Atlas Images that cover a region of the sky using the IRSA/2MASS Image Inventory Service.

v. Artifacts

The 2MASS Full Survey, 6x and Calibration Atlas Images may contain artifacts produced by the optical and electronic effects of bright stars, and by transient phenomena such as meteor trails, aircraft, and insects crawling on the camera dewar windows.

With the exception of single-frame transients such as cosmic rays, hot pixels and meteor trails, image artifacts were not actively removed during standard 2MASS Atlas Image construction. Therefore, 2MASS images may contain both compact and diffuse artifacts that do not correspond directly to objects on the sky.

The nature of extended and diffuse artifacts is usually apparent to the user when viewing the Atlas Images. However, compact image artifacts, such as latent images and dichroic glints produced by bright stars may be mistaken for point and small extended sources on the Atlas Images. These compact artifacts are marked as overlays provided by the IRSA 2MASS Image Services, as illustrated in Figure 2. These overlays are produced using the spurious source detections triggered by the artifacts that are found in the Survey PSRT or 6x and Calibration PSWDBs, and that are identified as artifacts via the contamination and confusion flag (cc_flg).

A compendium of the various artifacts present in the 2MASS Atlas Images is given in the Gallery of Atlas Image Anomalies.

Figure 2 - IRSA/2MASS Interactive Image Service result page showing a Ks image containing the star HD100. The red and green crosses marking the location of latent image and dichroic glints, artifacts that can be mistaken for real sources, are generated from the flagged artifacts in the survey point source Reject Table.

[Last Updated: 2006 December 28; by R. Cutri]

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