The IPHAS and VPHAS+ catalogues

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The IPHAS and VPHAS+ catalogues  A talk by Geert Barentsen (@GeertHub) for the RAS Milky Way Workshop in London on 30 March 2015

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The IPHAS and VPHAS+ catalogues  A talk by Geert Barentsen (@GeertHub) for the RAS Milky Way Workshop in London on 30 March 2015

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Image: Nick Risinger

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Summary  - We recently published IPHAS DR2, providing riHα photometry for 219 million unique sources in the North. - The ﬁrst VPHAS+ multi-band catalogue is expected soon, providing ugriHα in the South.

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Isaac Newton Telescope (INT); 100-inch Schmidt-Cassegrain, Short for “the INT Photometric Hα Survey  of the Northern Galactic Plane” Wide Field Camera;  4 CCDs, 32 megapixel,  0.33 arcsec/pixel.  Survey operated entirely in competitive “open access time”.

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IPHAS DR2 - First quality-controlled, globally-calibrated release. - Median seeing 1.1 arcsec. - 219 million sources (50% have SNR > 5 in all bands). - Median 5σ-limit: 21.2 (r), 20.3 (Hα), 20.0 (i). - Lists the best detection of each unique source. ! ! Barentsen et al. (2014)

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 r - i 0.0 0.2 0.4 0.6 0.8 1.0 1.2 r - H↵ A0V K0V M0V M3V M4III E(B-V)=1 E(B-V)=2 E(B-V)=3 (`, b) = (180 , 3 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 r - i 0.0 0.2 0.4 0.6 0.8 1.0 1.2 r - H↵ (`, b) = (45 , 2 ) Galactic  anti-centre: First  quadrant: The r-i, r-Hα diagram Reddening (using the “a10point” quality ﬂag)

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0.0 0.1 0.2 0.3 0.4 r 0.0 0.1 0.2 0.3 0.4 i 12 13 14 15 16 17 18 19 20 21 22 Magnitude 0.0 0.1 0.2 0.3 0.4 H↵ Poissonian magnitude uncertainties ! Barentsen et al. (2014)

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0.0 0.1 0.2 0.3 0.4 |r r2 | (b) a10point sources 0.0 0.1 0.2 0.3 0.4 |i i2 | 12 13 14 15 16 17 18 19 20 21 22 Magnitude 0.0 0.1 0.2 0.3 0.4 |H↵ H↵2 | Photometric repeatability: σ=0.03 mag (r<18) ! Barentsen et al. (2014)

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Raw images Quality control Calibrated images Single-band  source lists Multi-band  catalogue debias, linearise, ﬂat-correct, gain-correct, defringe, nightly zeropoint (Mike Irwin / CASU) source detection,  photometry (Mike Irwin / CASU) cross-match,  ﬂag duplicates IPHAS DR2 Recalibration

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IPHAS DR2 can be queried using Vizier

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Also available via VO Conesearch  (e.g. using TopCat or AstroPy)

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Also available in FITS format (48 GB) http://www.iphas.org/dr2

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But wait,  there’s more!

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New and improved website: www.iphas.org

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http://www.iphas.org/images All the calibrated images and metadata are now available on the survey website (inc. WCS ﬁxes)

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The DR2 set contains 268,180 CCD frames (1.6TB)

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Multi-epoch information is made available  by means of field-by-field catalogues. http://www.iphas.org/data/dr2/fields

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Derived products have also been made available, e.g. Stuart Sale’s 3D extinction map: http://www.iphas.org/extinction

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3Three lessons learnt while  building IPHAS DR2.

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Lesson #1  Quality control is a big effort

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Isaac Newton Telescope, La Palma Clouds

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A single INT/WFC exposure:  (4 CCDs; ~10x20 arcmin each)

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Transparent clouds

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Transparent clouds

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Automated methods were used to aid quality control,  e.g. photometric repeatability in overlap regions: Good Bad

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Readout noise

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Lunar straylight

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Condensation

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All images were examined by eye: the quality of the data included in DR2 is excellent!

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Lesson #2  1% photometric calibration is hard

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 r - i 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 r - H↵ (b) After re-calibration 0.0 0.5 1.0 1.5 2.0 2.5 3.0 r - i 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 r - H↵ (a) Before re-calibration The initial calibration error exceeded  0.1 mag across 9% of the footprint. using nightly standard star observations using a retrospective calibration

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=> ﬁeld edges invariably suffer from inaccurate ﬂat and aperture corrections (~1% systematics) The utility of overlap regions to carry out retrospective calibration was found to be limited

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Ideally, surveys ought to use “random” footprints,  e.g. simulations by (Holmes et al. 2012): Retrospective calibration improved  by 1 dex!

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Similarly, SDSS “ubercal” was enabled by orthogonality in the coverage

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Solution: 52% of the IPHAS ﬁelds were cross-calibrated against APASS; overlaps were then used to correct the others. AAVSO Photometric All-Sky Survey (APASS)  uses 20 cm f/3.6 Newtonians (8 deg2 FOV) => www.aavso.org/apass

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Lesson #3  Catalogues are imperfect

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The classic caveat: source confusion

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5 arcmin

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4 arcsec Example: crowded VPHAS+ image (i-band)

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Other caveats include e.g. cosmic rays on top of stars:

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Raw images Calibrated images Single-band  source lists Multi-band  catalogue cross-match,  ﬂag duplicates IPHAS DR2

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Raw images Calibrated images Single-band  source lists Multi-band  catalogue cross-match,  flag duplicates VPHAS+ Master source list DAOPHOT PSF photometry flag poor fits

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An AstroPy-based catalogue generation tool for VPHAS+ is in an advanced stage at ! https://github.com/barentsen/surveytools

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Coordinated by Greenﬁeld (STSci), Robitaille (Heidelberg), Tollerud (Yale). AstroPy brings distributed and transparent peer review to core astronomical software.

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Summary - IPHAS DR2: brand new release, ﬁrst one to include thorough quality control and re-calibration. World public & ready for science! - VPHAS+: multi-band catalogue in preparation, single-band source lists already available in the ESO archive (credit: CASU). Visit www.iphas.org and www.vphas.eu for more info.

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Thank you. @GeertHub www.geert.io  github.com/barentsen 