II. User's Guide to the 2MASS All-Sky Data Release

4. Atlas Images

a. Atlas Image Header Description

The 2MASS Atlas and Quicklook Images are in FITS format.

The 2MASS Atlas and Quicklook Images are in the same format but the Quicklook Images are lossy-compressed. With the exception of the "bitpix" keyword, headers are identical.

An actual 2MASS Atlas Image header may be found here.

Click on the underlined Field Names for more detail on those parameters.

Field Name Format Units null Description

Primary Fits Information

bitpix %20d --- no -32 => 32 bit IEEE floating point data (Atlas Images)
16 => 16 bit integer data (lossy compressed Quicklook Images)

naxis %20d --- no (always= 2) 2 dimensional image

naxis1 %20d --- no Number of columns in image

naxis2 %20d --- no Number of lines in image

Observatory Data Identifiers

ordate %20s --- no Observation reference date in the YYMMDD. This is the UT day value at the start of operations for the night on which the source was observed. Because the UT day incremented during the night at CTIO, the actual UT observation date for a source can differ from this value by one day.

daynum %20d day no Sequential survey day number, where day 1 is defined to be 1 March 1997 UT. Observations were started at Mt. Hokpins on daynum=109 (7 June 1997 UT) and at CTIO on daynum=384 (19 March 1998 UT).

fn_prfx %20s --- no Observatory Filename Prefix

type %20s --- no Scan type: dar (dark), flt (flat), sci (survey), cal (calibration), or tst (test)

scanno %20d --- no Nightly scan number

scandir %20s --- no Scan Direction: n (north), s (south)

strip_id %20d --- no 2MASS Tile number.

positnid %20s --- no Observatory Scan Position Identifier (Hddhhmm -> H = hemisphere, dd = dec at tile center, hhmm = hours and minutes at tile center)

origin %20s --- no 2MASS Survey Camera

Projection and Location Information

ctype1 %20s --- no Orthographic Projection

ctype2 %20s --- no Orthographic Projection

crpix1 %20f pix no Column Coordinate at Image Center (crval1/crval2)

crpix2 %20d pix no Line Coordinate at Image Center (crval1/crval2)

crval1 %20f deg no Right Ascension at Image Center (J2000)

crval2 %20f deg no Declination at Image Center (J2000)

crota2 %20f deg no Twist about Image Center, +AXIS2 WofN (J2000)

cdelt1 %20f deg no Axis1 Pixel Size (=1 arcsec)

cdelt2 %20f deg no Axis2 Pixel Size (=1 arcsec)

Calibration and Auxillary Image Information

usxref %20f asec no 2MASS U-scan X Coordinate at Image Center

usyref %20f asec no 2MASS U-scan Y Coordinate at Image Center

skyval %20f dn no Modal Sky Estimate for Image (image dn)

skysig %20f dn no Point Source Filtered Noise Estimate for Image (image dn)

magzp %20f mag no Calibrated Zero Point for Image (magnitudes)

calid %20s --- no Calibration Identifiers for Image

seesh %20f --- no Seeing Shape Parameter for Image (image dn)

dxfs %20f asec no Cross-scan Frame Step for Image

dyfs %20f asec no In-scan Frame Step for Image

pixnam %20s --- no PIXPHOT Processing Namelist Configuration Identifier

QT0227 %20f --- no Image Histogram 2.27 Percentile (Image DN)

QT0668 %20f --- no Image Histogram 6.68 Percentile (Image DN)

QT1586 %20f --- no Image Histogram 15.86 Percentile (Image DN)

QT3085 %20f --- no Image Histogram 30.85 Percentile (Image DN)

QT5000 %20f --- no Image Histogram 50.00 Percentile (Image DN)

QT6914 %20f --- no Image Histogram 69.14 Percentile (Image DN)

QT8413 %20f --- no Image Histogram 84.13 Percentile (Image DN)

QT9331 %20f --- no Image Histogram 93.31 Percentile (Image DN)

QT9772 %20f --- no Image Histogram 97.72 Percentile (Image DN)

Auxillary Observatory Information

base %20d ms no Pixel Dwell Time (milliseconds)

wavef_p %20s --- no Secondary Waveform Data File

timer0 %20d ms no Read1 Time (milliseconds)

timer1 %20d ms no Read2-Read1 Time (milliseconds)

timer2 %20d ms no Secondary Settling Time (milliseconds)

icsversn %20s --- no ICS Script Version

schedver %20s --- no Scan Scheduler Version

scriptfn %20s --- no Script Template Filename

scrptver %20s --- no Script Template Version

ut_off1 %20f sec no UT Offset - SOS to Frame 1

xs_off1 %20f asec no X-scan Offset - SOS to Frame 1

is_off1 %20f asec no In-scan Offset - SOS to Frame 1

xs_off2 %20f asec no X-scan Offset - EOS to End Frame

is_off2 %20f asec no In-scan Offset - EOS to End Frame

focus %20d --- no Telescope Focus Setting

numfrms %20d --- no Number of Frames in Scan

skiprdos %20d --- no Number of Frames Skipped at Start of Scan

telname %20s --- no Telescope Location - Hopkins or CTIO

filter %20s --- no Wavelength Band - j, h, or k (J, H, or Ks)

ut_date %20s --- no UT Date of Scan (YYMMDD)

ut %20s --- no UT Time of Scan (HH:MM:SS.SSS)

amass_fc %20s --- no Approximate Airmass at Image Center

ut_date %20s --- no UT Date of Scan (YYMMDD)

Auxillary Observatory Information at Start Of Scan

date_sos %20s --- no UT Date at Start of Scan (YYMMDD)

equinox %20s --- no Equinox at Start of Scan

am_sos %20f --- no Airmass at Start of Scan

zd_sos %20f deg no Zenith Distance at Start of Scan

ha_sos %20s --- no Hour Angle at Start of Scan (HH:MM:SS.SS)

ut_sos %20s --- no UT at Start of Scan (HH:MM:SS.SS)

st_sos %20s --- no Sidereal Time at Start of Scan (HH:MM:SS.SS)

ra_sos %20f --- no Scan Reference RA J2000 (HH:MM:SS.S)

dec_sos %20f --- no Scan Reference Declination J2000 (DD:MM:SS.S)

wspd_sos %20d --- no Wind Speed at Start of Scan (not implemented)

wdir_sos %20d --- no Wind Direction at Start of Scan (not implemented)

see_sos %20d --- no Seeing at Start of Scan (not implemented)

bar_sos %20d in Hg no Barometric Pressure at Start of Scan

airt_sos %20f deg C no Air Temperature at Start of Scan

telt_sos %20f deg C no Telescope Temperature at Start of Scan

Auxillary Observatory Information at End Of Scan

date_eos %20s --- no UT Date at End of Scan (YYMMDD)

am_eos %20f --- no Airmass at End of Scan

zd_eos %20f deg no Zenith Distance at End of Scan

ha_eos %20s --- no Hour Angle at End of Scan (HH:MM:SS.SS)

ut_eos %20s --- no UT at End of Scan (HH:MM:SS.SS)

st_eos %20s --- no Sidereal Time at End of Scan (HH:MM:SS.SS)

ra_eos %20f --- no Scan Reference RA J2000 (HH:MM:SS.S)

dec_eos %20f --- no Scan Reference Declination J2000 (DD:MM:SS.S)

wspd_eos %20d --- no Wind Speed at End of Scan (not implemented)

wdir_eos %20d --- no Wind Direction at End of Scan (not implemented)

see_eos %20d --- no Seeing at End of Scan (not implemented)

bar_eos %20d in Hg no Barometric Pressure at End of Scan

airt_eos %20f deg C no Air Temperature at End of Scan

telt_eos %20f deg C no Telescope Temperature at End of Scan

aryt_eos %20f deg C no Array Temperature at End of Scan

Field Name	Format	Units	null	Description
Primary Fits Information
bitpix	%20d	---	no	-32 => 32 bit IEEE floating point data (Atlas Images) 16 => 16 bit integer data (lossy compressed Quicklook Images)
naxis	%20d	---	no	(always= 2) 2 dimensional image
naxis1	%20d	---	no	Number of columns in image
naxis2	%20d	---	no	Number of lines in image
Observatory Data Identifiers
ordate	%20s	---	no	Observation reference date in the YYMMDD. This is the UT day value at the start of operations for the night on which the source was observed. Because the UT day incremented during the night at CTIO, the actual UT observation date for a source can differ from this value by one day.
daynum	%20d	day	no	Sequential survey day number, where day 1 is defined to be 1 March 1997 UT. Observations were started at Mt. Hokpins on daynum=109 (7 June 1997 UT) and at CTIO on daynum=384 (19 March 1998 UT).
fn_prfx	%20s	---	no	Observatory Filename Prefix
type	%20s	---	no	Scan type: dar (dark), flt (flat), sci (survey), cal (calibration), or tst (test)
scanno	%20d	---	no	Nightly scan number
scandir	%20s	---	no	Scan Direction: n (north), s (south)
strip_id	%20d	---	no	2MASS Tile number.
positnid	%20s	---	no	Observatory Scan Position Identifier (Hddhhmm -> H = hemisphere, dd = dec at tile center, hhmm = hours and minutes at tile center)
origin	%20s	---	no	2MASS Survey Camera
Projection and Location Information
ctype1	%20s	---	no	Orthographic Projection
ctype2	%20s	---	no	Orthographic Projection
crpix1	%20f	pix	no	Column Coordinate at Image Center (crval1/crval2)
crpix2	%20d	pix	no	Line Coordinate at Image Center (crval1/crval2)
crval1	%20f	deg	no	Right Ascension at Image Center (J2000)
crval2	%20f	deg	no	Declination at Image Center (J2000)
crota2	%20f	deg	no	Twist about Image Center, +AXIS2 WofN (J2000)
cdelt1	%20f	deg	no	Axis1 Pixel Size (=1 arcsec)
cdelt2	%20f	deg	no	Axis2 Pixel Size (=1 arcsec)
Calibration and Auxillary Image Information
usxref	%20f	asec	no	2MASS U-scan X Coordinate at Image Center
usyref	%20f	asec	no	2MASS U-scan Y Coordinate at Image Center
skyval	%20f	dn	no	Modal Sky Estimate for Image (image dn)
skysig	%20f	dn	no	Point Source Filtered Noise Estimate for Image (image dn)
magzp	%20f	mag	no	Calibrated Zero Point for Image (magnitudes)
calid	%20s	---	no	Calibration Identifiers for Image
seesh	%20f	---	no	Seeing Shape Parameter for Image (image dn)
dxfs	%20f	asec	no	Cross-scan Frame Step for Image
dyfs	%20f	asec	no	In-scan Frame Step for Image
pixnam	%20s	---	no	PIXPHOT Processing Namelist Configuration Identifier
QT0227	%20f	---	no	Image Histogram 2.27 Percentile (Image DN)
QT0668	%20f	---	no	Image Histogram 6.68 Percentile (Image DN)
QT1586	%20f	---	no	Image Histogram 15.86 Percentile (Image DN)
QT3085	%20f	---	no	Image Histogram 30.85 Percentile (Image DN)
QT5000	%20f	---	no	Image Histogram 50.00 Percentile (Image DN)
QT6914	%20f	---	no	Image Histogram 69.14 Percentile (Image DN)
QT8413	%20f	---	no	Image Histogram 84.13 Percentile (Image DN)
QT9331	%20f	---	no	Image Histogram 93.31 Percentile (Image DN)
QT9772	%20f	---	no	Image Histogram 97.72 Percentile (Image DN)
Auxillary Observatory Information
base	%20d	ms	no	Pixel Dwell Time (milliseconds)
wavef_p	%20s	---	no	Secondary Waveform Data File
timer0	%20d	ms	no	Read1 Time (milliseconds)
timer1	%20d	ms	no	Read2-Read1 Time (milliseconds)
timer2	%20d	ms	no	Secondary Settling Time (milliseconds)
icsversn	%20s	---	no	ICS Script Version
schedver	%20s	---	no	Scan Scheduler Version
scriptfn	%20s	---	no	Script Template Filename
scrptver	%20s	---	no	Script Template Version
ut_off1	%20f	sec	no	UT Offset - SOS to Frame 1
xs_off1	%20f	asec	no	X-scan Offset - SOS to Frame 1
is_off1	%20f	asec	no	In-scan Offset - SOS to Frame 1
xs_off2	%20f	asec	no	X-scan Offset - EOS to End Frame
is_off2	%20f	asec	no	In-scan Offset - EOS to End Frame
focus	%20d	---	no	Telescope Focus Setting
numfrms	%20d	---	no	Number of Frames in Scan
skiprdos	%20d	---	no	Number of Frames Skipped at Start of Scan
telname	%20s	---	no	Telescope Location - Hopkins or CTIO
filter	%20s	---	no	Wavelength Band - j, h, or k (J, H, or Ks)
ut_date	%20s	---	no	UT Date of Scan (YYMMDD)
ut	%20s	---	no	UT Time of Scan (HH:MM:SS.SSS)
amass_fc	%20s	---	no	Approximate Airmass at Image Center
ut_date	%20s	---	no	UT Date of Scan (YYMMDD)
Auxillary Observatory Information at Start Of Scan
date_sos	%20s	---	no	UT Date at Start of Scan (YYMMDD)
equinox	%20s	---	no	Equinox at Start of Scan
am_sos	%20f	---	no	Airmass at Start of Scan
zd_sos	%20f	deg	no	Zenith Distance at Start of Scan
ha_sos	%20s	---	no	Hour Angle at Start of Scan (HH:MM:SS.SS)
ut_sos	%20s	---	no	UT at Start of Scan (HH:MM:SS.SS)
st_sos	%20s	---	no	Sidereal Time at Start of Scan (HH:MM:SS.SS)
ra_sos	%20f	---	no	Scan Reference RA J2000 (HH:MM:SS.S)
dec_sos	%20f	---	no	Scan Reference Declination J2000 (DD:MM:SS.S)
wspd_sos	%20d	---	no	Wind Speed at Start of Scan (not implemented)
wdir_sos	%20d	---	no	Wind Direction at Start of Scan (not implemented)
see_sos	%20d	---	no	Seeing at Start of Scan (not implemented)
bar_sos	%20d	in Hg	no	Barometric Pressure at Start of Scan
airt_sos	%20f	deg C	no	Air Temperature at Start of Scan
telt_sos	%20f	deg C	no	Telescope Temperature at Start of Scan
Auxillary Observatory Information at End Of Scan
date_eos	%20s	---	no	UT Date at End of Scan (YYMMDD)
am_eos	%20f	---	no	Airmass at End of Scan
zd_eos	%20f	deg	no	Zenith Distance at End of Scan
ha_eos	%20s	---	no	Hour Angle at End of Scan (HH:MM:SS.SS)
ut_eos	%20s	---	no	UT at End of Scan (HH:MM:SS.SS)
st_eos	%20s	---	no	Sidereal Time at End of Scan (HH:MM:SS.SS)
ra_eos	%20f	---	no	Scan Reference RA J2000 (HH:MM:SS.S)
dec_eos	%20f	---	no	Scan Reference Declination J2000 (DD:MM:SS.S)
wspd_eos	%20d	---	no	Wind Speed at End of Scan (not implemented)
wdir_eos	%20d	---	no	Wind Direction at End of Scan (not implemented)
see_eos	%20d	---	no	Seeing at End of Scan (not implemented)
bar_eos	%20d	in Hg	no	Barometric Pressure at End of Scan
airt_eos	%20f	deg C	no	Air Temperature at End of Scan
telt_eos	%20f	deg C	no	Telescope Temperature at End of Scan
aryt_eos	%20f	deg C	no	Array Temperature at End of Scan

[Last Updated: 2003 March 14, by Gene Kopan & R. Cutri]

Return to Section II.4.