Accessing IGAPS images

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 527 736 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 349 159. The volume occupied by the compressed images is 3.0 TB. This page also provides some information on the flux calibration and astrometry, and presents the metadata summarising image properties.

The raw telescope data and calibration frames are not available here: if wanted, they may be found in the Isaac Newton Group Archive.

Use the form below to retrieve IGAPS CCD frames that either capture a given sky position or tile a square box of up to 1 degree on a side.

To recover the IPHAS filters, choose the Hα, rI, and i filter options. For UVEX only, select URGO, g and rU.

Frame Search

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 available for download below. They may also be found in the "detectionID" columns of the source catalogue, which are in the form "#RUN-#CCD-#SOURCE".

When the details of a desired frame are known, the URL accessing it 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 delivered Rice-compressed. This compression scheme can be read directly by professional software such as DS9, Aladin and Fv. If you need to decompress an image, try using HEASARC's funpack. See NASA's round up on fits viewers for more possibilities.

Note: this repository includes all the images obtained during our observing runs, including those 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 avoiding images for which the qcgrade equals "D".

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 IGAPS re-calibration shift (when available).

Note: only the images identified in the header as FLUXCAL='IGAPS-UNIFORM' have photometric zeropoints that were globally calibrated (Monguio et al 2020). All other images still carry the zeropoints assigned by the CASU pipeline on the basis of standards obtained within the same observing run.

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

As these images include moonlight and/or other sources of non-astronomical background, efforts to measure fluxes directly from them must involve a suitably-chosen local background subtraction.

Image astrometry

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 RMS residual of our astrometric fit against Gaia DR2 is in most cases appreciably better than 0.1 arcsec (see header keyword STDCRMS).

Users of the URGO band data should bear in mind that the astrometry achieved in this band is less precise, with residuals relative to Gaia DR2 cross-matches (median in CCD4: 79 mas) almost twice those seen in the longer wavelength bands. Differences in its optical properties, as a liquid Cu2SO4 filter, also lead to higher-order distortion within the camera footprint (flagged by non-zero PV2_5 in the header).

Header information

For more information on the header content, and a complete example of a header, go to this page.

Common artefacts in the images

For more information on common artefacts, please see this page (in prep).

A regular issue with Wide Field Camera data is that the CCD mosaic suffers from a small amount of vignetting that mainly impacts the furthest south-east corner of CCD3. Over the 15 years of data acquisition this corner area grew somewhat. Users of the images should bear this in mind when faced with choices between different CCD frames covering a region of interest.

Quality grades

The metadata table made available below 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. The very best are awarded "A++".
  • "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 inferior seeing (2 to 2.5 arcsec), but are fine otherwise.
  • "D"-graded images were not used in building the catalogue due to more significant quality concerns.

Although the use of D-graded images is generally discouraged, it is appropriate to point out that quality grades refer to complete exposure sets (not to single exposures). Briefly, each IPHAS or UVEX trio of exposures per sky pointing has been assigned a grade based on an overview of the trio's properties. Hence, D-graded exposure sets will sometimes contain one or more usable single-band exposures. Users, having reason to consider using them, are advised to carry out their own inspections of such data to confirm their fitness for purpose.

Many fields in the survey footprint were observed more than once to improve data quality. Data flagged as uni_cal, underwent uniform photometric calibration in the process of preparing for IGAPS point-source catalogue. For these exposures, the photometric zeropoints in the headers have been reset according to results of this re-calibration (see Monguio et al 2020).

A table detailing the run numbers, ccd numbers, sky coordinates, observing dates, conditions and quality information for the entire dataset is available to download in FITS format:

The columns provided in the igaps-images.fits file 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 appraised in the construction of the IGAPS catalogue (Monguio et al 2020).
uni_cal Boolean column, set to true if the exposure was uniformly calibrated in preparation for the IGAPS Catalogue Release. This flag is set to false for all URGO frames.
qcgrade IGAPS quality score. One of "A++", "A+", "A", "B", "C", or "D". D-graded data is deprecated for quantitative scientific use (but see comments above about grades).
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 percentage, with 100.0 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 the remarks on flux calibration above.
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 their PanSTARRS cross-matches. Where (in a very small number of cases) stdps is set to 9999.0, it signals no useful measurement because e.g. a very bright star floods 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.