DR1 Detailed Description

Content of the Archive

In this data release we provide the following data:

  • Catalogues. Photometric information of objects detected in the images has been stored in several tables in a database. Two main catalogues are provided:
    • Dual-mode catalogue. This is the main catalogue and has been constructed using SExtractor's dual mode with the r band image used as detection image. Before running SExtractor, the PSF of the detection and analysis images were homogenized. In this catalogue, each entry corresponds to a detection in r band and contains the photometry in all the bands.
    • Single-mode catalogue. This catalogue has been constructed running SExtractor independently for detection and photometry in each photometric band. In this case, each entry of the catalogue corresponds to the a measurement in a given band.
  • Images. We offer access to the coadded images and weight images for each field and filter. These images are the result of combining several individual exposures and are not PSF homogenized. Although single frames are not currently offered, you can access to basic information (eg. observing date, exposure time,...) of the individual images that have been combined in each case.
  • Masks. We also offer masks for each pointing which identify areas with known problems that can produce spurious detections or can affect the photometry of detected objects (eg., satellite trails, edges of images, halos of bright stars,...).

Survey Strategy

Observations of J-PLUS are carried out following a sky tiling in which each tile has the size of the field of view of T80Cam with an overlapping of adjacent pointings to ensure continuity of coverage.

The priority is to observe all the bands of one pointing before passing to the next pointing. The order in which the different filters are observed has been chosen to minimize overheads during the change of filters.

The survey strategy of J-PLUS has suffered an important modification during 2016. Initially, observations were carried out with fixed exposure times irrespective of the sky conditions. During 2016 there was a transition to a strategy in which exposure times were adapted as a function of the the sky brightness to achieve a given depth. This can be detected in the variability of the photometric depth of the images taken before the new observing strategy was fully implemented.

Image Reduction and Coadded Image Construction

Reduction of single exposures is done following the standard procedures:

  • Over- and pre-scan subtraction. Due to temporal variations detected in the bias images, the bias subtraction in each single frame is done using over- and pre-scan areas of each amplifier.
  • Flat field correction. Master flat fields are constructed using flat fields observations in periods of at least one month, ensuring the stability within that period.
  • Fringing correction. Fringing is observed mainly in z band and much weak on J0861 band. For the latter, no correction is attempted. For the former, master fringing images are constructed using scientific images.
  • Astrometric calibration. Astrometric calibration is computed for each single frame using Astromatric's Scamp software and Gaia DR1 star catalogue as reference.
  • PSF analysis. PSF analysis is performed using Astromatic's PSFEx software.

Single exposures of the same filter and pointing are combined to generate deeper images or coadds. Coadds are constructed using the Astromatic's software Swarp using weight images of each single exposure. Before the combination, single frames are re-astrometrized altogether, however, preliminary tests using Gaia DR1 point to the fact that this could be not necessary using Gaia DR1 as external catalogue for astrometrization.

Object Detection and Photometry

The detection of the objects is done on the coadds using Astromatic's software SExtractor.

Two types of catalogues are generated, using SExtractor's single and dual modes:

  • Dual-mode catalogues. SExtractor's dual-mode allows to use one image as reference image (r band image in the case of J-PLUS) for object detection and definition of the apertures for the photometry while performing the actual photometry in other image. This has the advantage of performing the photometry for each object in all the bands in the same position and in the same apertures. In addition, to reduce the effect of differences in the PSF among images in different bands of the same pointing, before running SExtractor in dual-mode, the detection and the analysis images are homogenized to have the same PSF.
    J-PLUS tables: jplus.MagABDualObj; jplus.FLambdaDualObj; jplus.FNuDualObj
  • Single-mode catalogues. In single-mode, the detection and photometry is carried out totally independent among images of the different bands of the same pointing. This allows to recover objects that are not detected in the reference band but they are in other bands (eg., faint objects with emission lines in one narrow band or extreme blue or red objects).
    J-PLUS tables: jplus.MagABSingleObj; jplus.FLambdaSingleObj; jplus.FNuSingleObj

Being aware that different scientific objectives need different ways of performing the photometry, the results of different photometric procedures are offered in our catalogues:

  • Common SExtractor photometric outputs:
    • Circular aperture photometry (SExtractor's MAG_APER). Photometry in simple circular apertures of different sizes is computed.
    • Isophotal photometry (SExtractor's MAG_ISO).
    • Kron photometry (SExtractor's MAG_AUTO).
    • Petrosian photometry (SExtractor's MAG_PETRO).
  • Custom photometric outputs. We offer photometry following three particular procedures:
    • Photometry after degrading the images of all the bands to the worst PSF.
    • Photometry after convolving all the images with a Gaussian kernel of σ=1.5".
    • Photometry applying a PSF correction as that applied in Molino et al., 2014.

Flagging and masking

For each object and each band we provide with two flags parameters to warn about known issues that can affect the quality of the photometry:

  • FLAGS. The FLAGS parameter is inherited from SExtractor's FLAGS parameter. Table 1 shows the values of the different flags. The final value of the FLAGS parameter for a given detection is the sum of the values of all the flags that affect such detection. In addition to SExtractor's usual values (from 1 to 128), we have added two additional values:
    • 1024. Since adjacent pointings are partially overlapping, in the overlapping areas objects can have more than one measurement. In the reference catalogue, only the best detection is stored, however, we provide also tables with the additional measurements and these are identified with this flag.
    • 2048. This flag is used to identified objects which are known to be variable from the cross-match with external catalogues of variable objects.
  • MASK_FLAGS. This parameter contains information of the different masks that can affect a given object. Table 2 summarizes the possible values. Like the FLAGS parameter, the actual value of a given object is the sum of all the values of that table affecting such object.

Table 1. List of possible values of the FLAGS parameter
Value Description
1 The object has neighbors, bright and close enough to significantly bias the photometry, or bad pixels (more than 10% of the integrated area affected).
2 The object was originally blended with another one.
4 At least one pixel of the object is saturated (or very close to).
8 The object is truncated (to close to an image boundary).
16 Object's aperture data are incomplete or corrupted.
32 Object's isophotal data are incomplete or corrupted.
64 A memory overflow occurred during deblending.
128 A memory overflow occurred during extraction.
1024 The object is duplicated. There is a better detection of the object in other image.
2048 The object is a known variable.
Table 2. List of possible values of the MASK_FLAGS parameter
Bit Value Name Description
0 not mask Not inside a mask
0 1 window Object is outside the window frame
1 2 bright star Object is bright star or near one
2 4 artifact Object masked due to nearby artifact

Photometric Calibration

The photometric calibration of J-PLUS faces two main challenges, namely the variety of observational conditions in which, throughout the project, J-PLUS images are being taken, and the use of a unique set of purposely defined filters. However, the difficulty of these tasks is to some extent alleviated by the large amount of external data made available by projects like SDSS, PanSTARRS and Gaia.

In this context, with the ultimate goal of being able to calibrate J-PLUS at the widest variety of observing conditions, we have chosen to apply a battery of calibration procedures rather than relying on a single calibration technique. Here we briefly outline the calibration procedures being currently applied on current J-PLUS data:

  • SDSS spectroscopy. In those areas in which there are SDSS spectra available we estimate zero points using synthetic photometry of spectra of stars with median(SNR)>10. This allows to calibrate most of the J-PLUS photometric bands. Only uJAVA and zSDSS are always outside the spectra wavelength range and J0378 can be calibrated only when BOSS spectra are available.
  • SDSS photometry. The significant (~80%) overlap between J-PLUS and SDSS footprints allows to calibrate the J-PLUS broad band observations against the corresponding ones in SDSS, after applying the needed color term corrections. This calibration technique is used to calibrate the u and z bands, uncovered by SDSS spectra.
  • Pan-STARRS photometry. The full Northern Sky coverage of Pan-STARRS helps us to carried out the photometric calibration in the photometric bands in common (g, r, i).
  • Spectrophotometric standard stars. The observation of spectrophotometric standard stars (hereafter SSSs) is used to calibrate those images in which other procedures are not available, most notably the calibration of the J0378 filter when no BOSS spectra are available and all non-standard bands when no SDSS spectra at all are available.
  • Stellar locus. The stellar locus is the region on color-color diagrams in which stars are found. In general, this region is narrow and its shape is independent of the photometric calibration. Computing the offset between the location of the stellar locus in instrumental colors with respect to a calibrated stellar locus allows a photometric calibration of colors. Using several color-color diagrams and calibrating independently one given band (eg., calibrating r band with SDSS or PanSTARRS) allows a full consistent calibration of all the bands of the same pointing. After having gathered a large amount of pointings of J-PLUS calibrated with other procedures, it has been possible to compute a reference empirical stellar locus which has been used to calibrate all the pointings in a consistent way. Once a reference stellar locus is constructed, this procedure can be applied to any pointing and ties the calibration of the different bands in a consistent way- For these reason, this has been chosen as the reference calibration procedure of this data release.

Value Added Catalogues

In this data release we provide several value-added catalogues:

  • Bayesian star-galaxy separation. Complemented SExtractor's CLASS_STAR, we provide an improved star-galaxy classification using the Bayesian approach described in López-Sanjuan et al. (2018).

    J-PLUS table jplus.StarGalClass.

  • Photometric redshifts. Photometric redshifts have been computed for all the galaxies (CLASS_STAR<0.2) with rSDSS<21.5 using three codes: Le Phare, BPZ and TPZ. In addition to basic outputs from each code that are store in the main database, we provide also the probability density functions (PDFs).

    J-PLUS tables: jplus.PhotoZLephare, jplus.PhotoZBpz, and jplus.PhotoZTpz.

  • Cross-matches with large sky surveys. Using CDS X-match service, we offer cross-matches of our dual-mode catalogue with the following large sky surveys:
    • GALEX AIS (jplus.xmatch_galex_ais). Cross-match using 4".
    • SDSS DR12 (jplus.xmatch_sdss_dr12). Cross-match using 1.5".
    • PanSTARRS DR1 (jplus.xmatch_panstarrs_dr1). Cross-match using 1.5".
    • Gaia DR2 (jplus.xmatch_gaia_dr2). Cross-match using 1.5".
    • AllWISE (jplus.xmatch_allwise). Cross-match using 4".
  • Stellar parameters (Coming soon!). Within few weeks we will provide an additional value-added catalogue with stellar effective temperatures for stars with high quality photometry. These parameters will be computed with different techniques by different groups within the J-PLUS collaboration.