This server provides access to the entire collection of pipeline-reduced images that have been obtained as part of the IPHAS and UVEX surveys of the northern Galactic Plane. The data set consists of 527 760 CCD frames obtained between 2003 and 2018. The g, r, i and Hα frames that have undergone uniform calibration as part of the construction of the IGAPS point source catalogue number 349 175. The volume occupied by the compressed images is 3.0 TB. This page also provides the associated metadata and documents the use of the images.
Note that the raw telescope data and the calibration frames are not available here, they may be found in the Isaac Newton Group Archive instead. The raw and processed data are also available through the CASU dqc interface.
Use the form below to retrieve a list of IPHAS CCD frames at a given position.
CCD frames may also be downloaded directly from our server. Each frame is uniquely identified by the combination of its run number, which identifies the exposure, and the CCD number, which identifies the detector within the INT/WFC camera used by the survey (i.e. CCD number 1, 2, 3 or 4). The run and CCD numbers for a desired position or epoch may be found in the metadata table provided below. They may also be found in the "detectionID" columns of the source catalogue, which are in the form "#RUN-#CCD-#SOURCE".
When the numbers of a desired frame are known, the URL of the image is composed as follows:
http://www.igapsimages.org/data/images/r{first 3 digits of #RUN}/r{#RUN}-{#CCD}.fits.fz
For example, the image for a source with rDetectionID equal to "570144-1-5487", i.e. detected in run 570144 by CCD 1, is located at:
http://www.igapsimages.org/data/images/r570/r570144-1.fits.fz
Note that the images are compressed using the Rice algorithm. Although this compression scheme is supported by professional software such as DS9 and Aladin, it is not supported by every FITS viewer yet. If you struggle to open an image, try to decompress it using funpack first.
A table detailing the run numbers, ccd numbers, sky coordinates, observing dates, conditions and quality information is available for download in FITS format:
The columns in the metadata table are as follows:
| Column | Unit | Description |
|---|---|---|
| run | Number of the exposure, corresponding to the run number found in the observing logs of the Isaac Newton Telescope (INT). | |
| ccd | Number of the CCD of the INT Wide Field Camera (1, 2, 3, or 4). | |
| url | URL of the FITS file containing the image, e.g. "http://www.igapsimages.org/data/images/r534/r534002-2.fits.fz". | |
| ra | degrees | Right Ascension (J2000) in the centre of the frame, i.e. at pixel position (x,y)=(1024,2048). |
| dec | degrees | Declination (J2000) in the centre of the frame, i.e. at pixel position (x,y)=(1024,2048). |
| band | Waveband of the filter used. One of "r", "i", or "halpha". | |
| utstart | Universal Time (UTC) at the start of the exposure, given as a ISO 8601 timestamp (e.g. "2003-08-08T22:07:10.4"). | |
| fieldid | Internal IGAPS identifier for the observation composed of the field number and the observing month, e.g. "0001o_aug2003". | |
| 2020 | Boolean column to indicate if the image was used in the construction of the IGAPS catalogue (Monguio et al 2020). | |
| calibrated | Boolean column, true if the exposure passed all the quality control criteria and was selected for inclusion in the IGAPS Catalogue Release. When this column is set to false, it is likely that the frame suffers from severe quality problems (e.g. clouds). | |
| qcgrade | IGAPS quality score. One of "A++", "A+", "A", "B", "C" or "D". D-graded data is generally unsuited for quantitative scientific use due to e.g. clouds or electronic noise in the image. | |
| qcproblems | Brief description of the issues encountered during quality control. | |
| exptime | seconds | Exposure time adopted. |
| seeing | arcsec | Average Full Width at Half Maximum (FWHM) of point sources in the CCD frame. |
| ellipticity | Average ellipticity of point sources in the CCD frame, defined as e = (1 - b/a) with b the semi-minor and a the semi-major axis. | |
| skylevel | counts/pixel | Median sky brightness. An automatic 2D background-following algorithm is used to track and "remove" slowly varying background features such as image gradients etc. |
| skynoise | counts/pixel | Pixel noise at sky level, estimated using a robust MAD estimator for noise scaled to equivalent Gaussian rms value, ie. = MAD x 1.48, after removing large scale sky background variations. MAD = Median of the Absolute Deviations about the median. |
| airmass | The airmass (sec z) of the observation. | |
| moon_phase | The phase of the moon expressed as a fraction, with 1.0 as full moon and 0.0 as new moon. | |
| moon_separation | degrees | The angle between the moon and telescope pointing. |
| moon_altitude | degrees | The height of the moon above the horizon at time of image acquisition, expressed as an angle. A negative value indicates the moon was below the horizon. |
| depth | mag | 5-sigma limiting magnitude in the Vega system. |
| photzp | mag | Photometric zeropoint, computed such that it absorbs the required corrections for atmospheric extinction, gain variations, exposure time, and the IGAPS re-calibration shift (included only if the frame was part of the IGAPS release). See section on flux calibration below. |
| stdps | mag. | Only given for 2020 calibrated g,r,i data. This magnitude is the standard deviation around the photometric offset measured between the pipeline-calibrated IGAPS point source data for the CCD and the equivalent data from PanSTARRS. Where (in a very small number of cases) this is given as 9999.0, it could not be determined because of e.g. a very bright star flooding the CCD. |
| confmap | Filename of the confidence map, relative to their location on the IPHAS server: http://www.igapsimages.org/data/images/confmaps | |
| ra_min | degrees | Right Ascension (J2000) in the western-most corner of the CCD frame. |
| ra_max | degrees | Right Ascension (J2000) in the eastern-most corner of the CCD frame. |
| dec_min | degrees | Declination (J2000) in the southern-most corner of the CCD frame. |
| dec_max | degrees | Declination (J2000) in the northern-most corner of the CCD frame. |
The metadata table explained above provides essential quality information such as the seeing, ellipticity, skylevel, and skynoise. The quality of each image is summarised in the qcgrade column, which classifies images as "A", "B", "C", or "D":
Although the use of D-graded images is generally discouraged, we note that the quality grades were assigned on a field-by-field basis. That is, each trio of exposures of a field were given a single quality grade based on their combined properties. Hence, the D-graded category does contain some single-band exposures which are perfectly fine, but were graded poorly because of a problem in a different filter.
Some fields in the survey footprint were observed more than once to improve data quality. The best-available data for each field are flagged by the calibrated column, which marks the exposures that were used to compile the point-source catalogue. These are also the only exposures for which the photometric zeropoints are calibrated in a global sense to an accuracy of 0.03 mag (see Monguio et al 2020).
| 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 | Certifies the validity of PHOTZP. If set to 'ABSOLUTE' it means that the image has been globally re-calibrated to an accuracy of 0.03 mag (rms). If set to 'UNCALIBRATED' 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. We recommend against using PHOTZP in the latter case. |
| 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. |
A typical header looks as follows:
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 MAXBIAS = 65535.00000000 / Maximum expected bias level 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
Each image contains a photometric zeropoint in the FITS header (keyword PHOTZP) which allows the pixel values to be converted into Vega-based magnitudes as follows:
mag(Vega) = -2.5*log10(pixel value) + PHOTZP
The PHOTZP value has been computed such that it absorbs the required corrections for atmospheric extinction, gain variations, exposure time, and the DR2 re-calibration shift (if available).
To estimate absolute narrow-band Hα fluxes from the image data, we note that the integrated in-band energy flux for Vega in the WFC narrow-band Hα filter is 1.51 x 10−7 erg cm−2 s−1 at the top of the Earth’s atmosphere, which is the flux obtained by folding the CALSPEC SED with the filter transmission curve only (the correction for atmosphere and detector quantum efficiency, otherwise scales down the narrow-band flux by 0.707). We note that the Hα magnitude for Vega is now set to a value of 0.045 (see table 2 in Monguio et al 2020), implying that the in-band flux corresponding to zero magnitude is:
F(Hα=zero) = 1.57 x 10−7 erg cm−2 s−1
An astrometric solution for the IGAPS images has been determined by comparing the positions of our stars in all bands against those found in the DR2 release of the Gaia mission. This is a change from IPHAS DR2 where the astrometry was referred to the Two-Micron All Sky Survey (2MASS). All positions are specified in the International Celestial Reference System (ICRS). The Root Mean Square (RMS) residual of our astrometric fit against Gaia DR2 is in most cases appreciably better than 0.1 arcsec (see header keyword STDCRMS).
The astrometric solution is recorded in the FITS headers 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 extracted from an IGAPS image (same as above) is given below. Note that each image contains its own solution. 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)