Calibration Fields
UNDER CONSTRUCTION
Photometric calibration for 2MASS was performed using observations of calibration fields made at regular intervals during each night of survey operations. Measurements of standard stars in the fields were used to derive the photometric zero point offsets as a function of time during each night. Atmospheric extinction coefficients were derived from 2MASS observations made over long periods. The survey's calibration strategy is described in section III.2.d of the on-line 2MASS Explanatory Supplement.
2MASS calibration fields, or tiles, are 1° long in declination and approximately 8.5' wide in right ascension. There are 35 regular calibration fields distributed at approximately two hour intervals in right ascension near declinations of approximately -30°, 0° and +30°. An additional five "special" calibration fields were defined in and around the Large and Small Magellanic Clouds to support the deep observation campaign towards the end of survey operations. Figure 1 contains a map the sky showing the location of the regular (filled squares) and special (open squares) calibration fields.
One field was observed each hour during the night (before October 11, 1997 UT, two fields were observed each two hours). Each field observation consisted of six consecutive scans made in alternating declination direction. Each scan in the set of six was offset from the preceding scan by ~5´´ in RA to avoid systematic pixel effects. Scans of the calibration fields were made using the standard survey freeze-frame scanning strategy, accumulating 7.8 sec of exposure per scan.
Over the course of the survey, the survey calibration fields were scanned between approximately 600 and 3700 times in nominally photometric conditions. Equatorial fields were observed from both observatories, so were observed more frequently than those near ±30° which were observed from only one. The special calibration fields were observed 150 to 470 times.
All of the calibration scan data were processed using the standard 2MASS Production Processing System (2MAPPS) used to process survey data. 2MAPPS generated calibrated Atlas Images and Working Databases containing brightness and positional data for extracted point and extended sources analogous to those produced from the survey observations.
Registering and combining the large number of Atlas Images of each calibration field produces the most sensitive measurements enabled by 2MASS. The combined images will have cumulative exposure times between ~4700 and ~29,000 sec, yielding sensitivities 3.5 to 4.5 mag deeper than the primary survey.
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| Figure 1 - Equatorial sky map showing location of the 40 2MASS Calibration Fields |
Table 1 contains a listing of the central equatorial (J2000) and galactic coordinates of each primary survey calibration field, and the number times each was under photometric conditions (Nscn). The Tile number is the identifier given to each field and is based on the primary calibration star(s) in each field.
Table 1 - Calibration Field Centers and Number of Times Scanned
Nscn Tile ra(J2000) dec(J2000) glon glat ----------------------------------------------------- 2977 90004 28.66074 +0.71693 154.11677 -58.27527 1582 90009 246.80780 -24.68901 352.96954 +16.58465 3515 90013 89.28447 +0.01890 206.63167 -12.04885 2025 90021 6.10619 -1.97294 107.36651 -64.02523 3692 90067 132.81203 +11.84773 215.63879 +31.90474 789 90091 145.75214 +59.06160 154.88828 +44.63064 562 90121 97.37444 -59.65713 268.95105 -25.88052 2593 90161 105.22232 +48.48935 168.28907 +21.45529 1703 90182 279.89627 +49.09363 78.07393 +22.00907 2086 90191 66.58918 +3.62342 190.90892 -29.77810 1687 90217 180.44070 -50.05148 294.81518 +12.03580 2076 90234 307.83812 -49.64775 349.60727 -36.21631 1962 90247 53.01617 +37.34386 155.04346 -15.31752 2776 90266 183.61047 +35.59855 163.59937 +78.43114 1973 90272 224.64303 +37.14173 61.47581 +61.44777 1780 90273 224.21932 -44.81900 325.15505 +12.57220 977 90279 267.09736 -45.42783 346.06515 -8.92593 1141 90290 352.64369 +38.31591 105.94466 -21.87369 2621 90294 8.31622 -39.40154 318.91629 -77.15726 3430 90301 51.72678 -39.84268 244.63466 -55.50638 3501 90312 126.40319 -39.09847 257.57412 -0.66895 1192 90330 247.89420 +30.14552 50.25044 +42.07085 1839 90533 55.26392 +6.93647 179.51313 -36.67609 671 90547 282.82780 -4.27488 29.11141 -1.91959 3396 90565 246.68168 +5.87185 20.52082 +34.69775 1876 90808 285.48438 -4.48794 30.12525 -4.37662 1570 90813 310.27504 -5.06339 41.33281 -26.64299 2452 90860 185.41757 -0.12034 287.00945 +61.82763 949 90867 220.24529 -0.45767 351.08190 +51.86698 2186 90868 225.11368 -0.65787 356.36663 +48.36291 2619 90893 349.54575 +0.54857 80.12434 -54.38199 2158 92026 128.12790 -1.57084 226.54812 +21.54656 2802 92202 331.40247 -11.07477 47.13268 -47.91669 2585 92397 170.45775 -13.22047 271.78770 +44.16553 1439 92409 330.11998 +20.84962 77.72803 -26.65088
Table 2 contains a listing of the approximate centers of the special calibration fields and the total number of photometric scans acquired during the Magellanic Cloud campaign.
Table 2 - Magellanic Cloud Calibration Fields Centers and Number of Times Scanned
Nscn Tile ra(J2000) dec(J2000) glon glat ------------------------------------------------------------ 468 90299 11.25260 -70.58353 303.70812 -46.53497 SMC2 378 90400 74.90247 -65.73341 276.28631 -35.95631 LMC1 156 90401 78.62001 -71.00065 282.10956 -33.37346 LMC2 432 90402 93.56589 -69.66665 279.95935 -28.52288 LMC3 108 90298 356.63061 -74.50079 308.68978 -41.89381 SMC1
Each 2MASS calibration field contains between 10 and 100 photometric standard stars that were used to derive the photometric calibration for the survey. Magnitudes with the highest degree of internal consistency were computed for each star using the global chi-squared minimization technique described by (Nikolaev et al. 2000, AJ, 120, 3340) applied to all measurements of the stars accumulated over the full survey.
The aperture photometry extracted for sources in the stacked cal
field images will be close to the true 2MASS photometric scale because
of the updated photometric zero point computed for the images (magzp).
The source photometry will be tied explicitly to the 2MASS photometric
scale by renormalizing using the mean photometric offset
(in magnitudes) between the extracted photometry of the standard
stars in each field with their a priori magnitudes. This
renormalization will compensate for any aperture curve of growth effects
that may not be easy to measure in some of the images due to confusion.
During pipeline processing, the calibration scans were position reconstructed with respect to the USNO-A2 catalog because there were often very few or no Tycho 2 reference stars in them. Consequently, the calibration scan astrometric solutions often have systematic differences from the main 2MASS survey which was tied directly to the Tycho 2 reference frame. To compensate for this, the positional WCS information in the individual calibration scan images were corrected during the scan stacking procedure using measured offsets between sources extracted in each scan and the positions of those sources in the 2MASS All-Sky Point Source catalog. The stacked image position information is derived from the individual input scans, thus normalizing them to the reference frame of the primary Survey.
The primary data product from the stacked calibration scans are the very deep combined images. All images will be in FITS format with headers containing full WCS and photometric calibration information. For each field, the image products include:
The bandmerged source lists extracted from each field. These lists will contain for each extracted source:
Last Updated: 22 December 2005
R. Cutri, E. Kopan, S. Van Dyk - IPAC