| Keyword | Description |
|---|---|
| RUN | Unique exposure number. Corresponds to the run number found in the observing logs of the Isaac Newton Telescope. |
| DATE‑OBS | Start time of the exposure in UTC, given in the ISO 8601 format "YYYY-MM-DDTHH:MM:SS". |
| EXPTIME | Exposure time in seconds. This may deviate slightly from the value found in the raw data, because the INT/WFC is known to occasionally record incorrect exposure times in the headers. This has been corrected, as best we can, in the images hosted on the IGAPS website. |
| PHOTZP | Photometric zeropoint, computed such that it absorbs the required corrections for atmospheric extinction, gain variations, exposure time, and the IGAPS re-calibration shift (if computed). Hence, the number counts stored in the images can be converted straight into Vega-based magnitudes using: mag(Vega) = -2.5*log(pixel value) + PHOTZP. |
| FLUXCAL | Identifies the status of PHOTZP. If set to 'IGAPS-UNIFORM' it means that the image has been globally re-calibrated to an accuracy of 0.03 mag rms (see Monguio et al (2020)). If set to 'CASU-PIPELINE' it indicates that the image has only been calibrated using the nightly average zeropoint obtained by observing standard stars, which may or may not be accurate depending on the zeropoint stability during the night. |
| MAGZPT | Original nightly zeropoint for default extinction measured at unit airmass. Obtained by taking the average zeropoint of all standard star fields observed within the same night. Unlike PHOTZP, this value does NOT include the required corrections for atmospheric extinction at the altitude of the observed pointing, gain variations, the exposure time or for the global re-calibration. We do not recommend its use. |
This is an example of a full header:
XTENSION= 'BINTABLE' / binary table extension BITPIX = 8 / 8-bit bytes NAXIS = 2 / 2-dimensional binary table NAXIS1 = 8 / width of table in bytes NAXIS2 = 4096 / number of rows in table PCOUNT = 5621622 / size of special data area GCOUNT = 1 / one data group (required keyword) TFIELDS = 1 / number of fields in each row TTYPE1 = 'COMPRESSED_DATA' / label for field 1 TFORM1 = '1PB(1477)' / data format of field: variable length array EXTNAME = 'COMPRESSED_IMAGE' / name of this binary table extension ZIMAGE = T / extension contains compressed image ZBITPIX = 16 / data type of original image ZNAXIS = 2 / dimension of original image ZNAXIS1 = 2048 / length of original image axis RUN = 484995 / Run number OBSERVAT= 'LAPALMA ' / Name of observatory (IRAF style) OBJECT = 'intphas_3260 i' / Title of observation LATITUDE= 28.761939 / Telescope latitude (degrees), +28:45:43.0 LONGITUD= -17.877587 / Telescope longitude (degrees), -17:52:39.3 HEIGHT = 2348 / [m] Height above sea level. SLATEL = 'LPO2.5 ' / Telescope name known to SLALIB TELESCOP= 'INT ' / 2.5m Isaac Newton Telescope MJD-OBS = 53709.1836527 / Modified Julian Date at start of observation JD = 2453709.6836527 / Julian Date at start of observation PLATESCA= 6.856013 / [d/m] Platescale ( 24.68arcsec/mm) TELFOCUS= 0.043653 / Telescope focus (metres) AIRMASS = 1.119865 / Effective mean airmass TEMPTUBE= 7.793302 / Truss Temperature (degrees Celsius) INSTRUME= 'WFC ' / INT wide-field camera is in use. WFFPOS = 4 / Position-number of deployed filter WFFBAND = 'i ' / Waveband of filter WFFID = '215 ' / Unique identifier of filter SECPPIX = 0.333 / Arcseconds per pixel DETECTOR= 'WFC ' / Formal name of camera CCDSPEED= 'FAST ' / Readout speed CCDXBIN = 1 / Binning factor in x axis CCDYBIN = 1 / Binning factor in y axis CCDSUM = '1 1 ' / Binning factors (IRAF style) CCDTEMP = 154.957 / [K] Cryostat temperature NWINDOWS= 0 / Number of readout windows DATE-OBS= '2005-12-05T04:24:30.1' / Start time of the exposure [UTC] ZNAXIS2 = 4096 / length of original image axis ZTILE1 = 2048 / size of tiles to be compressed ZTILE2 = 1 / size of tiles to be compressed ZCMPTYPE= 'RICE_1 ' / compression algorithm ZNAME1 = 'BLOCKSIZE' / compression block size ZVAL1 = 32 / pixels per block BZERO = 32768.00000000 / Pixel-data have 32K offset. BSCALE = 1.00000000 / Pixel-data are not scaled. INHERIT = T / Extension inherits primary HDU. EXTVER = 4 / Extension version number IMAGEID = 4 / Image identification DASCHAN = 4 / Number of readout channel WINNO = 0 / Number of readout window CHIPNAME= 'A5382-1-7 ' / Name of detector chip. CCDNAME = 'A5382-1-7 ' / Name of detector chip. CCDCHIP = 'A5382-1-7 ' / Name of detector chip. CCDTYPE = 'EEV42-80 ' / Type of detector chip. CCDXPIXE= 0.00001350 / [m] Size of pixels in x. CCDYPIXE= 0.00001350 / [m] Size of pixels in y. AMPNAME = 'LH ' / Name of output amplifier. GAIN = 2.90000000 / Nominal Photo-electrons per ADU. READNOIS= 5.80000000 / Nominal Readout noise in electrons. SATURATE= 56008.0 / Highest value that is unsaturated BIASSEC = '[11:53,3:4098]' / Bias pixels. TRIMSEC = '[54:2101,3:4098]' / Illuminated pixels. RTDATSEC= '[-52:2101,1:4200] ' / Location in d-space for RTD. HISTORY HISTORY Updated 2020-03-05 HISTORY ------------------ HISTORY This frame contains pipeline-reduced IGAPS data that was originally HISTORY processed by the Cambridge Astronomical Survey Unit (CASU), but the HISTORY headers have been updated following Monguio et al. in 2020 HISTORY to add a re-calibrated zeropoint and to tweak the WCS keywords. HISTORY RADESYS = 'ICRS ' / WCS calibrated against Gaia-DR2 EQUINOX = 2000.0 CTYPE1 = 'RA---ZPN' / Algorithm type for axis 1 CTYPE2 = 'DEC--ZPN' / Algorithm type for axis 2 CRUNIT1 = 'deg ' / Unit of right ascension coordinates CRUNIT2 = 'deg ' / Unit of declination coordinates PV2_1 = 1.0 / Coefficient for r term PV2_2 = 0.0 / Coefficient for r**2 term PV2_3 = 212.409021 / Coefficient for r**3 term PV2_5 = 0.0 / Coefficient for r**5 term CRVAL1 = 93.72109 / [deg] Right ascension at the reference pixel CRVAL2 = 21.1490626 / [deg] Declination at the reference pixel CRPIX1 = 1801.30718 / [pixel] Reference pixel along axis 1 CRPIX2 = 2967.45576 / [pixel] Reference pixel along axis 2 CD1_1 = -1.2741E-06 / Transformation matrix element CD1_2 = -9.244E-05 / Transformation matrix element CD2_1 = -9.2439E-05 / Transformation matrix element CD2_2 = 1.2691E-06 / Transformation matrix element STDCRMS = 0.0305099337399365 / Astrometric fit error (arcsec) HISTORY trimmed to range : 54 2101 3 4098 HISTORY linearity LUT applied: /home/mike/pipeline/nonlincoeffs/int_non HISTORY bias-corrected with : bias.fit HISTORY flatfielded with : i_flat.fit HISTORY gain-corrected by : 1.145 DEFRING0= 'Defringe image i_fringe_aug2003.fits scale = 0.0111' MOONDIST= 152.1999969482422 / Distance to the moon in degrees MOONALT = -85.69999694824219 / Altitude of the moon above the horizon MOONPHAS= 16.29999923706055 / Phase of the moon SKYLEVEL= 57.02 / Sky level SKYNOISE= 5.610000133514404 / Sky noise PERCORR = 0. / Sky calibration correction (mags) MAGZPT = 23.77 / Uncorrected nightly ZP (per second) MAGZRR = 0.05 / Photometric ZP error (mags) EXTINCT = 0.09 / Extinction coefficient (mags) PHOTZP = 26.3149 / mag(Vega) = -2.5*log(pixel value) + PHOTZP PHOTZPER= 0.03 / Default 1-sigma PHOTZP uncertainty in IGAPS PHOTSYS = 'Vega ' / Photometric system FLUXCAL = 'ABSOLUTE' / Certifies the validity of PHOTZP SEEING = 0.897102 / Average FWHM (arcsec) ELLIPTIC= 0.153999999165535 / Average ellipticity EXPTIME = 10.0 / [sec] Exposure time adopted CONFMAP = 'iphas_dec2005/i_conf.fit' CHECKSUM= '43Af416e41Ae415e' / HDU checksum updated 2020-03-05T12:21:38 DATASUM = '908599464' / data unit checksum updated 2020-03-05T12:21:38 COMMENT ------------------ COMMENT _____ _____ _____ _____ COMMENT |_ _|/ ____| /\ | __ \ / ____| COMMENT | | | | ___ / \ | |__) | (___ COMMENT | | | ||__ | / /\ \ | ___/ \___ \ COMMENT _| |_| |__| |/ ____ \| | ____) | COMMENT |_____|\_____|_/ \_\_| |_____/ COMMENT COMMENT Data origin COMMENT ----------- COMMENT This image is part of the INT Galactic Plane Survey COMMENT (IGAPS). COMMENT COMMENT Photometric calibration info COMMENT ---------------------------- COMMENT The pixel values (number counts) in this image can be converted into COMMENT Vega-based magnitudes using the PHOTZP keyword as follows: COMMENT COMMENT mag(Vega) = -2.5*log10(pixel value) + PHOTZP. COMMENT COMMENT The PHOTZP value has been computed such that it absorbs the required COMMENT factors for atmospheric extinction, gain variations, exposure time, COMMENT and the re-calibration shift. COMMENT As these images still include moonlight and other sources of COMMENT non-astronomical background, they can only support flux measurements COMMENT that include a suitably-chosen local background subtraction. COMMENT COMMENT Uncompressing the images COMMENT ------------------------ COMMENT These data are compressed using Rice lossless compression algorithm. COMMENT This is a standard compression algorithm. Details can be found here: COMMENT http://heasarc.gsfc.nasa.gov/fitsio/fpack/ COMMENT Compressed images can be opened using ds9. COMMENT COMMENT Acknowledgement instructions COMMENT ---------------------------- COMMENT If you use these data, please cite Monguio et al. (2020) and COMMENT Greimel et al. (2020). END
The astrometric solution is recorded in the FITS header of an image by means of a series of World Coordinate System (WCS) keywords. IGAPS uses the Zenithal PolyNomial (ZPN) projection, which is required to model the radial distortions in the focal plane (see Calabretta & Greisen, 2002).
An example WCS for an IGAPS image (same as above) is excerpted below. Note that each image contains its own solution. Its quality can be judged from the value assigned to the keyword, STDCRMS. For more detail, see section 4 in Monguio et al (2020).
HISTORY This frame contains pipeline-reduced IGAPS data that was originally HISTORY processed by the Cambridge Astronomical Survey Unit (CASU), but the HISTORY headers have been updated following Monguio et al. in 2020 HISTORY to add a re-calibrated zeropoint and to tweak the WCS keywords. HISTORY RADESYS = 'ICRS ' / WCS calibrated against Gaia-DR2 EQUINOX = 2000.0 CTYPE1 = 'RA---ZPN' / Algorithm type for axis 1 CTYPE2 = 'DEC--ZPN' / Algorithm type for axis 2 CRUNIT1 = 'deg ' / Unit of right ascension coordinates CRUNIT2 = 'deg ' / Unit of declination coordinates PV2_1 = 1.0 / Coefficient for r term PV2_2 = 0.0 / Coefficient for r**2 term PV2_3 = 212.409021 / Coefficient for r**3 term PV2_5 = 0.0 / Coefficient for r**5 term CRVAL1 = 93.72109 / [deg] Right ascension at the reference pixel CRVAL2 = 21.1490626 / [deg] Declination at the reference pixel CRPIX1 = 1801.30718 / [pixel] Reference pixel along axis 1 CRPIX2 = 2967.45576 / [pixel] Reference pixel along axis 2 CD1_1 = -1.2741E-06 / Transformation matrix element CD1_2 = -9.244E-05 / Transformation matrix element CD2_1 = -9.2439E-05 / Transformation matrix element CD2_2 = 1.2691E-06 / Transformation matrix element STDCRMS = 0.0305099337399365 / Astrometric fit error (arcsec)