GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire)
GLIMPSE is Spitzer Legacy project, that delivers data to the Spitzer Science Center (SSC). The GLIMPSE I (aka GLIMPSEI) and GLIMPSE II (aka GLIMPSEII) Spring '07 deliveries both contain a "Catalog" and an "Archive", all four are served through IRSA's General Catalog Query Tool, Gator. The GLIMPSEI source lists are from the 20070412 "FINAL" (DR5) data delivery; the GLIMPSEII source lists are from the Mar2007 "first" data delivery. GLIMPSEI and GLIMPSEII images are served through IRSA's general search service Atlas.
GLIMPSEI
The GLIMPSEI Catalog and Archive contain sources for the entire GLIMPSEI Survey (l=10-65 deg, 295-350 deg, b=+- 1 deg) plus the Observation Strategy Validation (OSV) region at about l=284 deg. The SSC pipeline version used for the GLIMPSEI v2.0 source lists is mostly S13.2 (corrected for the jailbar effect), with an occasional re-observation processed with SSC pipeline version S14 or later.
GLIMPSEII
The GLIMPSEII Catalog and Archive contain sources for the entire GLIMPSEII Survey region. The nominal GLIMPSEII Survey region is l= -10 deg to 10 deg (details in table below). It excludes the Galactic center area from about l=+- 1 deg, b=+-0.75 deg, which was observed by a GO program (Program ID=3677; search Spitzer Data at IRSA). The next release (v2.0 GLIMPSEII processing) will include the Galactic center data.
| longitude (deg) | latitude (deg) |
| 5<l<10 and 350<l<355 | -1<b<1 |
| 2<l<5 and 355<l<358 | -1.5<b<1.5 |
| 0<l<2 and 358<l<360 | -2<b<2 |
The SSC pipeline version used for the GLIMPSEII data in this data release is S12.4.0 from data taken September 2005.
Catalog vs Archive (More Reliable vs More Complete)
The highly reliable GLIMPSE I & II Catalogs consists of point sources that are detected at least twice in one band with a S/N > 5, and at least once in an adjacent band. There are also faint and bright flux limits on the Catalog entries. The more complete Archives have less stringent criteria, namely two detections in any bands, those detections having a S/N > 5. The IRAC data were bandmerged with the 2MASS All-Sky Point Source Catalog. See the GLIMPSEI v2.0 and GLIMPSEII v1.1 documents from the Data Products Document Repository for more details. There are about 31.2 million sources in the GLIMPSEI Catalog and about 49.2 million in the GLIMPSEI Archive. There are about 15.6 million sources in the GLIMPSEII Catalog and about 20.7 million sources in the GLIMPSEII Archive.
Catalog and Archive Fields and FlagsEach entry in the GLIMPSE I & II Catalogs and Archives have the following information:
designation -> SSTGLMC GLLL.llll±BB.bbbb, SSTGLMA GLLL.llll±BB.bbbb
2MASS PSC names -> 2MASS designation, 2MASS counter
position -> l, b, dl, db, ra, dec, dra, ddec
flux -> magi, di_m, fi, dfi, rms_fi (IRAC); mag_t, dt_m, ft, df_t (2MASS)
diagnostic -> sky_i, sn_i, dens_i, # detections mi out of ni possible (IRAC); sn_t (2MASS)
flags -> Close Source Flag (csf), Source Quality Flag (sqf_i), Flux Method Flag (mfi) (IRAC);
Source Quality Flag (sqf_t) (2MASS)
where i is the IRAC wavelength number (IRAC bands 1 - 4) (3.6 µm, 4.5 µm, 5.6 µm and 8.0 µm) and t is the 2MASS wavelength bands (J, H, Ks).
Details of the fields are as follows. Note that IRSA's General and Basic Catalog Search Tools (Gator and BabyGator) calculate colors, using magnitude measurements in the GLIMPSE source lists. For more information on these Gator-derived columns, click here.
The 'designation' entry can have the following set of information:
Archive=> SSTGLMA GLLL.llll+-BB.bbbb Catalog=> SSTGLMC GLLL.llll+-BB.bbbb
This is the object designation or name as specified by the IAU recommendations on source nomenclature. It is derived from the coordinates of the source, where G denotes Galactic coordinates, LLL.llll is the Galactic longitude in degrees, and ±BB.bbbb is the Galactic latitude in degrees. The coordinates are preceded by the acronym SSTGLMC (GLIMPSE Catalog) or SSTGLMA (GLIMPSE Archive).
Designation is:
SST = Spitzer Space Telescope GLM = GLIMPSEI, GLIMPSEII, and GLIMPSE-3D projects A = more complete Archive C = highly reliable Catalog G = Galactic coordinates LLL.llll = Galactic longitude (degrees) of the source +-BB.bbbb = Galactic latitude (degrees) of the source
The 2MASS designation is the source designation for objects in the 2MASS All-Sky Release Point Source Catalog. It is a sexagesimal, equatorial position-based source name of the form hhmmssss±ddmmsss, where hhmmssss is the right ascension (J2000) coordinate of the source in hours, minutes and seconds, and ±ddmmsss is the declination (degrees, minutes, seconds). The 2MASS counter is a unique identification number for the 2MASS PSC source.
There are 8 'position' related fields:
l, b, dl (error), db (error), ra, dec, dra (error), ddec (error)
The position is given in both Galactic (l,b) and equatorial (RA, Declination) J2000 coordinates, along with estimated uncertainties. The pointing accuracy is 1 arcsec (Werner et al. 2004). The SSC pipeline does pointing refinement of the images based on comparison with the 2MASS Point Source Catalog, whose absolute accuracy is typically <0.2 arcsec (Cutri et al. 2005). After applying the SSC geometric distortion corrections and updating to the 2MASS positions, the GLIMPSE point source accuracy is typically 0.3 arcsec absolute accuracy, limited by undersampling of the point-spread function. The position uncertainties are calculated by the bandmerger based on the uncertainties of individual detections, propagated through the calculation of the weighted mean position.
The following are 'flux' related fields:
magi & mag_t, di_m & dt_m (error), fi & f_t, dfi & df_t (error), rms_fi
For each IRAC band i = 3.6, 4.5, 5.6, and 8.0 µm and, when available 2MASS band t = J, H, and Ks, the fluxes are expressed in magnitudes (magi, mag_t) and in mJy (fi, f_t). Each IRAC flux is the error-weighted average of all independent detections of a source. The 2MASS magnitudes and uncertainties are from the 2MASS All-Sky Release Point Source Catalog. They are the j_m, j_msigcom, h_m, h_msigcom, and k_m, k_msigcom columns from the 2MASS PSC. The zeropoints for converting from flux to magnitude for the S13.2 and later SSC processing versions are from Reach et al. (2005) for the IRAC bands and Cohen et al. (2003) for 2MASS and given in Table 1 below.
Table 1. Zeropoints for Flux to Magnitude Conversion Band J H K [3.6] [4.5] [5.8] [8.0] GLIMPSEI Zeropoints (Jy) 1594 1024 666.7 280.9 179.7 115.0 64.13 GLIMPSEII Zeropoints (Jy) 1594 1024 666.7 277.5 179.5 116.5 63.13
The IRAC flux/magnitude uncertainties (dim; dfi) are computed during the photometry stage and take into account photon noise, readnoise, goodness of flat fielding, and PSF fitting (Stetson 1987). Magnitude uncertainties are typically <0.2 mag for the Catalog and < 0.3 mag for the Archive. The uncertainties are smaller in bands 1 and 2 than bands 3 and 4 due to lower backgrounds in bands 1 & 2 and the lower sensitivity of the band 3 detector.
The rms deviation (rms_fi) of the individual detections from the final flux of each source is provided.
The 'flux diagnostics' related fields are:
sky_i, sn_i, sn_t, dens_i, # detections mi out of ni possible
The associated flux diagnostics are a local background level (sky_i) (i = 3.6, 4.5, 5.6, and 8.0 µm) in MJy/sr, a Signal/Noise (sn_i), a local source density (dens_i) (number of sources per square arcmin), and number of times (mi) a source was detected out of a calculated possible number (ni). The local background, an output of DAOPHOT, is provided because high backgrounds were shown to affect the reliability of IRAS sources, and for IRAC as well (especially bands 3 and 4). However, the effects may not be easily characterizable in the quoted error. The Signal/Noise is the flux (f_i) divided by the flux error (dfi). The Signal/Noise for the 2MASS fluxes (sn_t) have been taken from the 2MASS PSC (the j_snr, h_snr and k_snr columns). The local source density is measured as follows: The individual IRAC frame is divided into a 3×3 grid, each of the nine cells being 1.71 arcmin x 1.71 arcmin. A source density is calculated for each cell (number of sources per arcmin^2), and is assigned to each source in that cell. The local source density can be used to assess the confusion in a given region, along with the internal reliability. mi and ni can be used to estimate reliability. ni is calculated based on the areal coverage of each observed frame; due to overlaps some areas are observed more than twice per band.
There are three types of flags: the Close Source Flag, the Source Quality Flag and the Flux Calculation Method Flag. The Close Source Flag is set if there are Archive sources that are within 3 arcsec of the source. The Source Quality Flag provides a measure of the quality of the point source extraction and bandmerging. The Flux Calculation Method Flag describes how the final Catalog/Archive flux was determined.
- The Close Source Flag (csf) is set when a source in the Archive is within 3.0 arcsec of the source. It was found that the magnitudes of a source with nearby sources closer than about 2 arcsec are not reliably extracted and bandmerged. A source that has Archive sources within 2.0 arcsec of the source are culled from the Catalog. A source that has Archive sources within 0.5 arcsec of the source are culled from the Archive. The flag is defined as follows:
0 no sources in the Archive within 3" of the source 1 sources in the Archive between 2.5" and 3" of the source 2 sources in the Archive between 2.0" and 2.5" of the source 3 sources in the Archive between 1.5" and 2.0" of the source 4 sources in the Archive between 1.0" and 1.5" of the source 5 sources in the Archive between 0.5" and 1.0" of the source 6 sources in the Archive within 0.5" of the source
-
The Source Quality Flag (sqf) is generated from SSC-provided masks and the GLIMPSE pipeline, during point source extraction on individual IRAC frames and bandmerging. Each source quality flag is a binary number allowing combinations of flags (bits) in the same number. Flags are set if an artifact (e.g., a hot or dead pixel) occurs near the core of a source - i.e. within 3 pixels. A non-zero SQF will in most cases decrease the reliability of the source. Some of the bits, such as the DAOPHOT tweaks, will not compromise the source s reliability, but has likely increased the uncertainty assigned to the source flux. If just one IRAC detection has the condition requiring a bit to be set in the SQF, then the bit is set even if the other detections did not have this condition. Sources with hot or dead pixels within 3 pixels of source center (bit 8), those in wings of saturated stars (bit 20), and those within 3 pixels of the frame edge (bit 30) are culled from the Catalog.
Table 2 shows the SQF sequence for the GLIMPSEI v2.0 data release. The stray light flag has been implemented for this data release. We have determined that false sources from such regions do not make it into the Catalog due to our 2+1 source selection criterion. In addition, our photometry algorithm was modified substantially to find sources in high background regions that gives it the ability to find sources in stray light and banded regions as well, increasing the photometric uncertainties accordingly.
The value of the SQF is sum{2^{(bit-1)}}. For example, a source with bits 1 and 4 set will have SQF = 2^0 + 2^3 = 9. If the SQF is 0, the source has no detected problems. More information about these flags and a bit value key can be found in Appendix B of the delivery document.
Source Quality Flag (SQF) Bits SQF bit Flag Description Source 1 poor pixels in dark current SSC 2 flat field applied using questionable value SSC 3 latent image SSC 3 persistence (p) 2MASS 4 photometric confusion (c) 2MASS 7 muxbleed correction applied GLIMPSE *8 hot, dead or otherwise unacceptable pixel SSC,GLIMPSE 9 muxbleed correction applied is > 3-sigma above bkg GLIMPSE 9 electronic stripe (s) 2MASS 10 allstar tweak positive GLIMPSE 11 allstar tweak negative GLIMPSE 13 confusion in in-band merge GLIMPSE 14 confusion in cross-band merge (IRAC) GLIMPSE 14 confusion in cross-band merge (2MASS) GLIMPSE 15 column pulldown correction GLIMPSE 16 banding correction GLIMPSE 17 stray light GLIMPSE 19 data predicted to saturate GLIMPSE *20 saturated wing area GLIMPSE *20 diffraction spike (d) 2MASS 21 pre-lumping in in-band merge GLIMPSE 22 post-lumping in cross-band merge GLIMPSE *30 edge of frame (within 3 pixels of edge) GLIMPSEFor 2MASS data, the following flags from the 2MASS All-Sky PSC are mapped into bits 3,4,20 and 9 in the source quality flag see 2MASS Documentation for more information about these 2MASS flags. Users should consult the 2MASS All-Sky Point Source Catalog for the complete information about the source, including all of their quality flags.
2MASS Flag => SQF ---------- --- "p" persistence latent (bit 3) "c" photom confusion corr sat star wing (bit 4) "d" diffraction spike uncorrectable sat star (bit 20) "s" electronic stripe muxbleed (bit 9) bit values: bt 1 => 1; 2 => 2; 3 => 4; 4 => 8; 5 => 16; 6 => 32; 7 => 64; 8 => 128 bt 9 => 256; 10 => 512; 11 => 1024; 12 => 2048; 13 => 4096; 14 => 8192; 15 => 16384; bt 16 => 32768; 17 => 65536; 18 => 131072; 19 => 262144; 20 => 524288; bt 21 => 1048576; 22 => 2097152; 30 => 536870912 * Note that sources with flags 8, 20, and 30 have already been removed from Catalog or the magnitude nulled. -
Flux calculation Method Flag (mfi) indicates by bit whether a given frametime was present, and whether that frametime was used in the final flux. Table 3 below defines the values for this flag for v2.0 GLIMPSEI: value= 2^(present bit - 1) + 2^(used bit - 1).
Table 3.
Method Flag (MF) for v2.0 GLIMPSE I: exp bit bit 0.6 1 2 2 3 4 12 5 6 30 7 8 100 9 10
This flag is more useful for High Dynamic Range (HDR) mode data which has two frametimes, one of which is not necessarily used (i.e. the 12 second frametime data is not used for the very bright sources). For the 2 second frametime of the GLIMPSE surveys, the method flag always equals 12 (2^2 + 2^3).
For v1.0 GLIMPSEII, this is a flag for each band indicating how the final averaged flux was determined by weighting individual flux measurements. An mf=2 could indicate a variable source. Table 4 below defines the values for this flag for v1.0 GLIMPSEII:
Table 4.
Method Flag (MF) for v1.0 GLIMPSE II: mf bit 0 all fluxes in agreement within errors 1 some discrepant fluxes discarded 2 all fluxes were discrepant; use this source with caution
REFERENCES
Cohen, M., Wheaton, W.A., and Megeath, S.T. 2003, AJ, 126, 1090. Cutri, R. et al. 2005, http://pegasus.phast.umass.edu/ipac_wget/releases/allsky/doc/sec2_2.html#pscastrprop Reach, W. et al. 2005, PASP, 117, 978. Stetson, P. 1987, PASP, 99, 191. Werner et al. 2004, ApJS, 154, 1.