The IPHAS DR2 Source Catalogue

Key properties

The INT/WFC Photometric H-Alpha Survey of the Northern Galactic Plane (IPHAS) is a 1860 deg² imaging survey of the Northern Milky Way at red visible wavelengths. It covers Galactic latitudes |b| < 5° and longitudes l = 30-215° in the broad-band r, i and narrow-band Hα filters using the Wide Field Camera (WFC) on the 2.5-m Isaac Newton Telescope (INT) in La Palma.

IPHAS Data Release 2 (DR2) is the first quality-controlled and globally calibrated source catalogue derived from the survey, providing single-epoch photometry for 219 million unique sources across 92% of the footprint. The observations were carried out at a median seeing of 1.1 arcsec (sampled at 0.33 arcsec/pixel) and to a mean 5-sigma depth of 21.2 (r), 20.0 (i) and 20.3 (H-alpha). The photometric calibration is in the Vega magnitude system and carries an external precision of 0.03 mag (root-mean-square error).

For each source, the catalogue provides magnitudes measured using a series of circular apertures, along with coordinates, morphology information, and quality warning flags. This page details how the data may be accessed, provides a table detailing the column definitions, explains a set of recommended quality criteria, and finally provides a FAQ which addresses common caveats.

Spatial coverage

IPHAS DR2 includes all the quality-approved data which the survey obtained between 2003 and 2012, covering 92% of the final footprint. The remaining spots of missing data are currently being observed and will likely be part of IPHAS DR3.

You can download the footprint, shown in the figure above, as a high-resolution PDF file, or as a Multi-Order Coverage map (MOC) which can be visualised using the Aladin software.

Online queries

The easiest way to retrieve data from the catalogue is to use the Vizier data portal, which hosts a full copy of the catalogue and allows excerpts from the catalogue to be downloaded in various formats, including html, ascii and fits. To use this service, please head to the Vizier IPHAS2 page at:

http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=IPHAS2

You can also query the Vizier service in an automated fashion using their conesearch API. For example, to download and save all the sources located within a 0.1° radius around (RA, Dec) = (324°, 58°), you can use the following Python script:

from astropy.vo.client.conesearch import conesearch
search = conesearch(center=(324.0, 58.0),
                    radius=0.1,
                    verb=3,
                    catalog_db="http://vizier.u-strasbg.fr/viz-bin/votable/-A?-source=IPHAS2&-out.all&")
search.to_table().write('iphas-data.fits', format='fits')

Note that the script above requires the AstroPy package to be installed.

Downloading a full copy (48 GB)

The catalogue is also made available for download as a series of binary FITS tables, each covering a 5x5 deg² area of the survey. You may download all these tiles to a local directory using the following wget command:

wget -c -r -np -nH --cut-dirs=3 --accept=fits.gz http://www.iphas.org/data/dr2/full/

Be aware however that the full size of the catalogue is 48 GB. To obtain the smaller "light" version (8 GB), which contains only a very limited subset of columns, use the following command instead:

wget -c -r -np -nH --cut-dirs=3 --accept=fits.gz http://www.iphas.org/data/dr2/light/

You may also download individual 5x5 deg² tiles using the links provided in the table below:

The following table summarises the meaning of each column. Note that the catalogue is described in more detail in Barentsen et al. (2014).

An important feature introduced by this release is the availability of user-friendly quality warning flags. These are necessary because the catalogue includes any source detected at a signal-to-noise ratio of just 5 or better in any band. Many applications will require a combination of quality criteria to be applied to exclude low-significance, saturated, or confused photometry.

The choice of quality criteria tensions completeness against reliability, and hence depends on the requirements of a project. To aid users, the data release paper recommends two sets of quality criteria, named "a10" and "a10point", which should satisfy most projects.

As a minimum, the "a10" criteria select objects which have been detected at the minimum level of 10-sigma in all bands, without being saturated. Additional constraints are provided by the "a10point" criteria, which require objects to be point sources free of blending, unaffected by nearby bright stars, as well as being unsaturated >10-sigma detections in all bands.

Definition of the "a10" class

All of the following criteria must be satisfied for an object to be included in the "all-band 10-sigma detection" class, which is flagged by the a10 column in the catalogue:

Definition of the "a10point" class

All of the following criteria must be satisfied for an object to be included in the "unconfused, point-like, all-band 10-sigma detection" class, which is flagged by the a10point column in the catalogue:

Like any other photometric survey, there are caveats associated with using IPHAS DR2 data. We recommend reading the frequently asked questions below to get a view on the most common caveats and questions:

• Does IPHAS DR2 provide variability information?
• Why are some stars classified as galaxies or noise?
• How do I obtain photometry for extended sources?
• How do I access the image data for a source?
• How accurate is the astrometry?
• What software was used to produce the catalogue?
• How do I reference IPHAS and this catalogue in my papers?

Does IPHAS DR2 provide variability information?

IPHAS revisited roughly 30% of the survey footprint at a second or third epoch, often years apart, to improve the data quality in fields affected by poor seeing or depth. The survey hence provides some variability information, but in DR2 we have decided to focus on providing single-epoch photometry based on the highest-quality observations alone. Multi-epoch data for a source can nevertheless be obtained by querying the IPHAS image database, or by mining the full set of field-by-field catalogues . The variability information has not been compiled into a user-friendly catalogue at present.

Why are some stars classified as galaxies or noise?

The morphological classification values (e.g. rClass, iClass, haClass) are determined by comparing the point spread function (PSF) of a source against the mean shape of the PSF measured across the CCD. It is common for stars to be incorrectly flagged as extended objects because the PSF deviates from the mean for a different reason. For example, this may affect stars which are (i) very faint, (ii) have a nearby neighbour, (iii) are located in a nebulous region, or (iv) fall near the edge of the focal plane. The band-merged morphological information (e.g. mergedClass, pStar) tends to be more robust against these effects. For fine-grained control, we recommend using the stellarness-of-profile statistics included in the catalogue (e.g. mergedClassStat, rClassStat) or inspecting the image data by eye.

How do I obtain photometry for extended sources?

We do not recommend using the photometry in the DR2 catalogue to study extended objects. The catalogue photometry was obtained using reasonably small apertures which have been corrected for the flux lost outside of the aperture by assuming that the source is a point source. To enable the analysis of diffuse sources, our website provides access to the pipeline-processed imaging data. These have been updated to include the DR2 re-calibrated zeropoints (header keyword PHOTZP). The information required to convert the image data into fluxes is provided at www.iphas.org/images.

How do I access the image data for a source?

Images may be downloaded directly from our server. In brief, each image is uniquely identified by the combination of its run number and CCD number, which are encoded within the detection identifiers (catalogue columns rDetectionID, iDetectionID and haDetectionID). For example, the r-band image for a source with rDetectionID equal to "570144-1-9999" (#run-#ccd-#source) is located at:

http://www.iphas.org/data/images/r570/r570144-1.fits.fz

For more documentation on image access, please visit www.iphas.org/images.

How accurate is the astrometry?

The astrometric solution for IPHAS has been determined by comparing the positions of stars against those found in the Two-Micron All Sky Survey (2MASS). The Root Mean Square (RMS) residual of our astrometric fit against 2MASS is typically better than 0.1 arcsec. We warn however that the residuals of individual stars near CCD corners can occasionally exceed 0.5 arcsec, even when the RMS is below 0.1 arcsec. We hence do not recommend using IPHAS astrometry for applications which require a positional accuracy better than 0.5 arcsec: such applications should employ relative astrometry techniques instead. More information about the astrometric solution can be found in the dedicated section at www.iphas.org/images.

What software was used to produce the catalogue?

The images were pipeline-processed by the Cambridge Astronomy Survey Unit using the CASUTOOLS software package. Next, the single-band detection tables produced in Cambridge were homogeneously re-calibrated and band-merged by the IPHAS team using a custom-built Python package, which is available on GitHub in the spirit of reproducibility.

How do I reference IPHAS and this catalogue in my papers?

Publications arising from IPHAS data should cite and acknowledge the survey and the data release. The primary reference for defining the survey is Drew et al 2005 [bibtex], and the reference for this data release is Barentsen et al 2014 [bibtex].

Here is text to be used in acknowledgment sections of papers:

This paper makes use of data obtained as part of the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS, www.iphas.org) carried out at the Isaac Newton Telescope (INT). The INT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. All IPHAS data are processed by the Cambridge Astronomical Survey Unit, at the Institute of Astronomy in Cambridge. The bandmerged DR2 catalogue was assembled at the Centre for Astrophysics Research, University of Hertfordshire, supported by STFC grant ST/J001333/1.

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.