Accessing IPHAS images

This server provides access to the entire collection of pipeline-reduced images that have been obtained as part of the IPHAS survey of the Northern Galactic Plane. The data set consists of 268 180 CCD frames obtained between 2003 and 2012. The data volume is 1.6 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.

Search by coordinates
Search by object name

Direct access

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.iphas.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.iphas.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.

Note: this data set includes all the images obtained during our observing runs, including those that were affected by poor conditions (e.g. clouds, bad seeing, electronic noise). You should verify the quality of the images using the metadata provided below. We recommend that you avoid images for which the qcgrade equals "D".

A table detailing the run numbers, ccd numbers, sky coordinates, observing dates, and quality information is available in FITS, SQLite and ASCII format:

Columns

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.iphas.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 IPHAS identifier for the observation composed of the field number and the observing month, e.g. "0001o_aug2003".
in_dr2 Boolean column, true if the exposure passed all the quality control criteria and was selected for inclusion in IPHAS Data Release 2 (DR2). When this column is set to false, then it is likely that the frame suffers from severe quality problems (e.g. clouds).
qcgrade IPHAS DR2 quality score. One of "A++", "A+", "A", "B", "C" or "D". D-graded data is generally unsuited for 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 in DR2.
seeing arcsec Average Full Width at Half Maximum (FWHM) of point sources in the CCD frame.
elliptic 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.
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 DR2 re-calibration shift (included only if the frame was part of DR2). Hence, the number counts stored in the frame can be converted into Vega-based magnitudes using: mag(Vega) = -2.5*log(pixel value) + PHOTZP.
confmap Filename of the confidence map, relative to their location on the IPHAS server: http://www.iphas.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.

Quality grades

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":

  • "A"-graded images were found to meet all our quality criteria;
  • "B"- and "C"-graded images do not meet the criteria in a strict sense, but may nevertheless be adequate for a range of scientific applications. For example, this category includes images that were obtained under sub-standard seeing (2 to 2.5 arcsec), but are fine otherwise.
  • "D"-graded images were excluded from DR2 due to serious quality concerns.

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 r/i/H-alpha 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 partner exposure.

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 in_dr2 column, which marks the exposures that were used to compile the DR2 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 Barentsen et al 2014).

Header keywords

Useful keywords in the image headers include:
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, to our best ability, in the images hosted on the IPHAS website.
PHOTZP Photometric zeropoint, computed such that it absorbs the required corrections for atmospheric extinction, gain variations, exposure time, and the DR2 re-calibration shift (included only if the frame was part of DR2). 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) as part of DR2. 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 the DR2 global re-calibration. We do not recommend its use.

A typical header looks as follows:

XTENSION= 'IMAGE   '           / binary table extension                         
BITPIX  =                   16 / data type of original image                    
NAXIS   =                    2 / dimension of original image                    
NAXIS1  =                 2048 / length of original image axis                  
NAXIS2  =                 4096 / length of original image axis                  
PCOUNT  =                    0 / size of special data area                      
GCOUNT  =                    1 / one data group (required keyword)              
RUN     =               570144 / Run number                                     
OBSERVAT= 'LAPALMA '           / Name of observatory (IRAF style)               
OBSERVER= 'Drew    '           / Name of principal investigator                 
OBJECT  = 'intphas_6686 Ha'    / Title of observation                           
LATITUDE=            28.761915 / Telescope latitude  (degrees), +28:45:42.9     
LONGITUD=           -17.877588 / 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 =        54277.1902816 / Modified Julian Date at start of observation   
JD      =      2454277.6902816 / Julian Date at start of observation            
PLATESCA=             6.856013 / [d/m] Platescale ( 24.68arcsec/mm)             
TELFOCUS=             0.043476 / Telescope focus (metres)                       
AIRMASS =             1.142659 / Effective mean airmass                         
TEMPTUBE=            11.330218 / Truss Temperature (degrees Celsius)            
INSTRUME= 'WFC     '           / INT wide-field camera is in use.               
WFFPOS  =                    2 / Position-number of deployed filter             
WFFBAND = 'Halpha  '           / Waveband of filter                             
WFFID   = '197     '           / 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 =              150.167 / [K] Cryostat temperature                       
NWINDOWS=                    0 / Number of readout windows                      
DATE-OBS= '2007-06-26T04:34:02.5' / Start time of the exposure [UTC]            
BZERO   =       32768.00000000 / Pixel-data have 32K offset.                    
BSCALE  =           1.00000000 / Pixel-data are not scaled.                     
INHERIT =                    T / Extension inherits primary HDU.                
EXTVER  =                    1 / Extension version number                       
IMAGEID =                    1 / Image identification                           
DASCHAN =                    1 / Number of readout channel                      
WINNO   =                    0 / Number of readout window                       
CHIPNAME= 'A5506-4           ' / Name of detector chip.                         
CCDNAME = 'A5506-4           ' / Name of detector chip.                         
CCDCHIP = 'A5506-4           ' / 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    =           2.80000000 / Nominal Photo-electrons per ADU.               
READNOIS=           6.40000000 / Nominal Readout noise in electrons.            
SATURATE=       65535.00000000 / Highest value that is unsaturated              
MAXBIAS =       65535.00000000 / Maximum expected bias level                    
BIASSEC = '[11:50,3:4098]'     / Bias pixels.                                   
TRIMSEC = '[51:2098,3:4098]'   / Illuminated pixels.                            
RTDATSEC= '[2062:4215,13:4212]             ' / Location in d-space for RTD.     
RADESYS = 'ICRS    '           / WCS calibrated against 2MASS                   
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   =                220.0 / Coefficient for r**3 term                      
CRVAL1  =            324.17822 / [deg] Right ascension at the reference pixel   
CRVAL2  =            57.634918 / [deg] Declination at the reference pixel       
CRPIX1  =           -388.29001 / [pixel] Reference pixel along axis 1           
CRPIX2  =               3050.0 / [pixel] Reference pixel along axis 2           
CD1_1   =       -1.2477742E-06 / Transformation matrix element                  
CD1_2   =       -9.2437171E-05 / Transformation matrix element                  
CD2_1   =       -9.2437804E-05 / Transformation matrix element                  
CD2_2   =        1.2915958E-06 / Transformation matrix element                  
PEDESTAL=                  0.0 / Artificial pedestal added to data              
HISTORY  trimmed to range     :     51  2098     3  4098                        
HISTORY  linearity LUT applied: /home/mike/pipeline/nonlincoeffs/int_non        
HISTORY  bias-corrected with  : bias.fits                                       
HISTORY  flatfielded with     : ../Haflat.fits                                  
HISTORY  gain-corrected by    :      1.000                                      
HISTORY                                                                         
HISTORY Updated 2014-05-02                                                      
HISTORY ------------------                                                      
HISTORY This frame contains pipeline-reduced IPHAS data that was originally     
HISTORY processed by the Cambridge Astronomical Survey Unit (CASU), but the     
HISTORY headers have been updated by Geert Barentsen (Hertfordshire) in 2014    
HISTORY to add a re-calibrated zeropoint and to tweak the WCS keywords.         
NUMBRMS =                  623 / Number of standards used                       
STDCRMS =                0.075 / Astrometric fit error (arcsec)                 
PERCORR =               -0.005 / Sky calibration correction (mags)              
MAGZPT  =                21.61 / Uncorrected nightly ZP (per second)            
MAGZRR  =                 0.03 / Photometric ZP error (mags)                    
EXTINCT =                 0.05 / Extinction coefficient (mags)                  
NUMZPT  =                   -1                                                  
NIGHTZPT=                  30.                                                  
NIGHTZRR=                  -1.                                                  
SEEING  =                2.633 / Average FWHM (pixels)                          
NIGHTNUM=                   -1                                                  
EXPTIME =               120.04 / [sec] Exposure time adopted in DR2             
PHOTZP  =              26.6162 / mag(Vega) = -2.5*log(pixel value) + PHOTZP     
PHOTZPER=                 0.03 / Default 1-sigma PHOTZP uncertainty in IPHAS DR2
PHOTSYS = 'Vega    '           / Photometric system                             
FLUXCAL = 'ABSOLUTE'           / Certifies the validity of PHOTZP               
CONFMAP = 'iphas_jun2007/Ha_conf.fits'                                          
CHECKSUM= 'aNjcdLgZaLgaaLgY'   / HDU checksum updated 2014-05-02T12:27:04       
DATASUM = '362546080'          / data unit checksum updated 2014-05-02T12:27:04 
COMMENT  _____ _____  _    _           _____                                    
COMMENT |_   _|  __ \| |  | |   /\    / ____|                                   
COMMENT   | | | |__) | |__| |  /  \  | (___                                     
COMMENT   | | |  ___/|  __  | / /\ \  \___ \                                    
COMMENT  _| |_| |    | |  | |/ ____ \ ____) |                                   
COMMENT |_____|_|    |_|  |_/_/    \_\_____/                                    
COMMENT                                                                         
COMMENT Data origin                                                             
COMMENT -----------                                                             
COMMENT This image is part of the INT Photometric H-Alpha Survey                
COMMENT of the Northern Galactic Plane (IPHAS). For more information,           
COMMENT visit http://www.iphas.org.                                             
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 corrections for atmospheric extinction, gain variations, exposure time, 
COMMENT and the DR2 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 Acknowledgement instructions                                            
COMMENT ----------------------------                                            
COMMENT If you use this data, please cite Drew et al. (2005) and                
COMMENT Barentsen et al. (2014), and include the acknowledgement text           
COMMENT that is available from www.iphas.org.                                   
END

Flux calibration

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).

Note: only the images which were used to compile the DR2 source catalogue (i.e. for which column in_dr2 equals "true" in the metadata) have photometric zeropoints that are globally calibrated to an accuracy of 0.03 mag (rms).

To estimate absolute narrow-band Hα fluxes from the image data, we note that the integrated in-band energy flux for Vega in the IPHAS Hα filter is 1.52 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 set by convention to 0.03 (Fukugita et al. 1996), which implies that the in-band flux corresponding to zero magnitude is:

F(Hα=zero) = 1.56 x 10^−7 erg cm^−2 s^−1
Note: as these images still include moonlight and other sources of non-astronomical background, they can only support flux measurements that include a suitably-chosen local background subtraction.

Astrometry

An astrometric solution for the IPHAS images has been determined by comparing the positions of our stars against those found in the Two-Micron All Sky Survey (2MASS), which itself is calibrated in the International Celestial Reference System (ICRS). The Root Mean Square (RMS) residual of our astrometric fit against 2MASS is typically better than 0.1 arcsec (see header keyword STDCRMS). We warn that the residuals of individual stars near CCD corners can occasionally exceed 0.5 arcsec however, even when the RMS is below 0.1 arcsec.

The astrometric solution is recorded in the FITS headers by means of a series of World Coordinate System (WCS) keywords. IPHAS uses the Zenithal PolyNomial (ZPN) projection, which is required to model the radial distortions in the focal plane (see Calabretta & Greisen, 2002). As part of DR2 we have reprocessed the WCS keywords to ensure that they are in line with the current standard.

An example WCS extracted from an IPHAS image is given below. Note that each image contains its own solution.

CTYPE1  = 'RA---ZPN'           / Algorithm type for axis 1                      
CTYPE2  = 'DEC--ZPN'           / Algorithm type for axis 2                      
CRPIX1  =           -388.29001 / [pixel] Reference pixel along axis 1           
CRPIX2  =               3050.0 / [pixel] Reference pixel along axis 2  
CRVAL1  =            324.17822 / [deg] Right ascension at the reference pixel   
CRVAL2  =            57.634918 / [deg] Declination at the reference pixel       
CRUNIT1 = 'deg     '           / Unit of right ascension coordinates            
CRUNIT2 = 'deg     '           / Unit of declination coordinates                
CD1_1   =       -1.2477742E-06 / Transformation matrix element                  
CD1_2   =       -9.2437171E-05 / Transformation matrix element                  
CD2_1   =       -9.2437804E-05 / Transformation matrix element                  
CD2_2   =        1.2915958E-06 / Transformation matrix element  
PV2_1   =                  1.0 / Coefficient for r term                         
PV2_2   =                  0.0 / Coefficient for r**2 term                      
PV2_3   =                220.0 / Coefficient for r**3 term                      
RADESYS = 'ICRS    '           / WCS calibrated against 2MASS                   
EQUINOX =               2000.0                                                  
NUMBRMS =                  623 / Number of standards used                       
STDCRMS =                0.075 / Astrometric fit error (arcsec)   

You can use the keywords to transform pixel coordinates into equatorial coordinates using a range of libraries. The example below demonstrates the conversion of pixel coordinates (x,y)=(10,20) into RA and Dec using the AstroPy library:

import astropy
myheader = astropy.io.fits.open('http://www.iphas.org/data/images/r570/r570144-1.fits.fz')[1].header
mywcs = astropy.wcs.WCS(myheader)
ra, dec = mywcs.all_pix2world([[10, 20]], 1)[0]
Note: we do not recommend using the astrometric solution for applications which require a positional accuracy better than 0.5 arcsec. You should employ relative astrometry in such cases.

If you encounter a problem in using IPHAS DR2, please tell us about it by filling in the report form (shared with VPHAS+ presently). The form asks for your name and email contact so that we might contact you if either we can suggest a fix or we need to better understand your problem.