The IPHAS and VPHAS+ catalogues

Talk presented at the "Milky Way Astrophysics from Wide-Field Surveys" workshop at the Royal Astronomical Society in London on 30 March 2015.

Workshop website: http://astro.kent.ac.uk/~df/gp/index.html

1. The IPHAS and VPHAS+ catalogues
 A talk by Geert Barentsen (@GeertHub) for the RAS Milky Way Workshop in London on 30 March 2015
2. The IPHAS and VPHAS+ catalogues
 A talk by Geert Barentsen (@GeertHub) for the RAS Milky Way Workshop in London on 30 March 2015
3. Image: Nick Risinger
4. 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.
6. 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”.
7. 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)
8. 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)
9. 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)
10. 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)
11. 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
12. IPHAS DR2 can be queried using Vizier
13. Also available via VO Conesearch
 (e.g. using TopCat or AstroPy)
14. Also available in FITS format (48 GB) http://www.iphas.org/dr2
15. But wait,
 there’s more!
16. New and improved website: www.iphas.org
17. http://www.iphas.org/images All the calibrated images and metadata are now available on the survey website (inc. WCS fixes)
18. The DR2 set contains 268,180 CCD frames (1.6TB)
19. Multi-epoch information is made available
 by means of field-by-field catalogues. http://www.iphas.org/data/dr2/fields
20. Derived products have also been made available, e.g. Stuart Sale’s 3D extinction map: http://www.iphas.org/extinction
21. 3Three lessons learnt while
 building IPHAS DR2.
22. Lesson #1
 Quality control is a big effort
23. Isaac Newton Telescope, La Palma Clouds
24. A single INT/WFC exposure:
 (4 CCDs; ~10x20 arcmin each)
25. Transparent clouds
26. Transparent clouds
27. Automated methods were used to aid quality control,
 e.g. photometric repeatability in overlap regions: Good Bad
28. Readout noise
29. Lunar straylight
30. Condensation
31. All images were examined by eye: the quality of the data included in DR2 is excellent!
32. Lesson #2
 1% photometric calibration is hard
33. 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
34. => 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
35. Ideally, surveys ought to use “random” footprints,
 e.g. simulations by (Holmes et al. 2012): Retrospective calibration improved
 by 1 dex!
36. Similarly, SDSS “ubercal” was enabled by orthogonality in the coverage
37. 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
38. Lesson #3
 Catalogues are imperfect
40. The classic caveat: source confusion
41. 5 arcmin
42. 4 arcsec Example: crowded VPHAS+ image (i-band)
43. Other caveats include e.g. cosmic rays on top of stars:
44. Raw images Calibrated images Single-band
 source lists Multi-band
 catalogue cross-match,
 flag duplicates IPHAS DR2
45. 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
46. An AstroPy-based catalogue generation tool for VPHAS+ is in an advanced stage at ! https://github.com/barentsen/surveytools
47. Coordinated by Greenfield (STSci), Robitaille (Heidelberg), Tollerud (Yale). AstroPy brings distributed and transparent peer review to core astronomical software.
48. 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.
49. Thank you. @GeertHub www.geert.io
 github.com/barentsen