Data

To characterize the foreground population of the Orion A molecular cloud we will make use of existing surveys together with raw data from Canada-France-Hawaii Telescope (CFHT), Calar Alto Observatory (CAHA 1.23m), and the Spitzer satellite, which were processed and analyzed for the purpose of this investigation.

Catalogs

We retrieved the astrometry and photometry for all sources within a box of 5\(\times\)15 centered around RA=85.7 and Dec=-4(J2000) in the Sloan Digital Sky Survey III, the Wide-field Infrared Survey Explorer \citep[WISE ][]{2012yCat.2311....0C}, the Third XMM-Newton serendipitous source \citep{2009A&A...493..339W} and the 2MASS catalogs \citep{2006AJ....131.1163S}. Table \ref{table_obs} gives an overview of the properties of these catalogs.

CFHT/Megacam

A mosaic of 2\(\times\)2 pointings covering 2\(\times\)2 centered on the ONC was observed with CFHT/Megacam \citep{2003SPIE.4841...72B} with the Sloan \(ugr\) filters on 2005 February 14 (P.I. Cuillandre). Figure \ref{fig:coverage} gives an overview of the area covered by these observations. The conditions were photometric, as described in the Skyprobe database \citep{2004ASSL..300..287C}. Seeing was variable, oscillating between 1–2 as measured in the images. A total of five exposures of 150 s (\(u\)-band), 40 s (\(g\)-band), and 40 s (\(r\)-band) each were obtained at each of the four positions. The observations were made in dither mode, with a jitter width of a few arcminutes at each position. This allows filling the CCD-to-CCD and position gaps and correcting for deviant pixels and cosmic ray events. The images were processed using the recommended Elixir reduction package \citep{2004PASP..116..449M}. Aperture photometry was then extracted using SExtractor \citep{1996A&AS..117..393B} and the photometric zero-points in the SDSS system were derived by cross-matching with the SDSS catalog. The CFHT/Megacam observations complement the SDSS data in one critical aspect: they provide data for regions around bright stars and nebulae, in particular the Orion nebula region that is missing in the SDSS data.

Calar Alto/1.23m CCD camera

Selected pointings of the ONC (see Table \ref{table_obs_caha} and Fig. \ref{fig:coverage}) were observed on 2011 December 15 with the Calar Alto CCD camera mounted on the 1.23m telescope (hereafter CAHA123). The CCD camera is a 2k\(\times\)2k optical imager with a 17 field-of-view. The Sloan filters available at Calar Alto vignet the field and reduce it to a circular 11 diameter field-of-view. These observations are meant to complement the CFHT and SDSS observations below their saturation limits (at \(ugr\approx\)12 mag), and in the vicinity of bright saturated stars. Short exposures of 0.1 and 5.0 s were obtained in the Sloan \(gr\) filters, and of 0.1 and 10 s in the Sloan \(u\) filter. The telescope was slightly defocused to avoid saturation of the brightest stars. Three standard fields were observed during the course of the night to derive accurate zero-points. Each pointing was observed with a small dithering of a couple of arcminutes to correct for deviant pixels and cosmic ray events. The images were pre-processed (bias subtraction and flat-field correction) using standard procedure with the Eclipse reduction package \citep{1997Msngr..87...19D}. The astrometric registration and stacking were then performed using the AstrOmatic software suite \citep[][]{2010jena.confE.227B}. Aperture photometry was finally extracted using SExtractor and the photometric zero-points in the SDSS system were derived by cross-matching with the SDSS and Megacam catalogs. The night was clear but not photometric. We observed a dispersion in the zero-point measurements through the night of 0.06 mag in \(u\) and \(g\), and 0.16 mag in \(r\), which we added quadratically to the photometric measurement uncertainties.

Spitzer IRAC

The ONC has been extensively observed with IRAC on-board the Spitzer observatory in the course of programs 30641, 43, and 50070. We retrieved the corresponding IRAC BCD images and associated ancillary products from the public archive and processed them following standard procedures with the recommended MOPEX software \citep{2005PASP..117.1113M}. The observations were all made in the high dynamics range mode, providing short (0.6 s) and long (12 s) exposure. We processed the two sets independently so as to cover the widest dynamic range. The procedure within MOPEX includes overlap correction, resampling, interpolation (to have an output pixel scale of 06) and outlier rejection. The individual frames were then median-combined with Swarp \citep{SWARP} using the rms maps produced by MOPEX as weight maps. Aperture photometry of all the sources brighter than the 3-\(\sigma\) noise of the local background was extracted using SExtractor. We verified that the corresponding photometry agrees well with the IRAC photometry from \citet{2009A&A...504..461F} within the uncertainties.

Spitzer MIPS1

The ONC was observed with the Spitzer Space Telescope and its MIPS instruments in the course of programs 202, 30641, 30765, 3315, 40503, 47, 50070, and 58. We retrieved all corresponding MIPS1 (24 \(\mu\)m) BCD images from the public archive and processed them with the recommended MOPEX software. The procedure includes self-calibration (flat-fielding), overlap correction, outlier rejection, and weighted coaddition into the final mosaic. Aperture photometry of all sources brighter than the 3-\(\sigma\) noise of the local background was extracted using SExtractor. We also verified that the corresponding photometry agrees with the MIPS photometry from \citet{2009A&A...504..461F} within the uncertainties.