Transportation of calibration from J-PLUS to J-PAS

Let's resume with the main objective of J-PLUS, ie. the photometric calibration of J-PAS. At this point, after calibrating J-PLUS fields we need a reliable way to transport the photometric calibration of J-PLUS to J-PAS.

For this purpose, we are going to use a particular set of stars, those with high signal to noise (SNR>50) and without close companions. For these stars we will be able to obtain accurate photometry in all the bands of J-PLUS, obtaining a rough spectral energy distribution (SED) for each of these stars. After having performed several simulations, it has come out that for these stars that we will be able, through a fit with a library of stellar spectra, to recover the photometry in J-PAS from the photometry in J-PLUS (always in the same photometric system, ie. that in which J-PLUS is based upon). This particular set of stars we call them Calibration Transport Stars (CTSs).

Nevertheless, we need to ensure that we are going to have enough CTSs in each of the fields of one CCD of JPCam (the camera mounted at JST to carry out J-PAS). The field of view of a single CCD of JPCam is roughly 35'x35'. Using data from SDSS, we have found out that we need to reach at least mAB=18 to be sure that we will have at least several dozens of CTSs in the worst case. With this information and the exposure time calculator constructed for J-PLUS, we have established the J-PLUS exposure strategy.

J-PAS will start its observations, for each single exposure (or even for combined exposures) in a given J-PAS band, first of all, we will identify the CTSs in the field. Then, thanks to the spectral fitting we will have for these stars the calibrated magnitudes in the J-PAS filter and comparing the observed instrumental magnitudes with the standard magnitudes we will be able to compute a photometric zero point (which includes atmospheric extinction) for that exposure. This photometric zero point (ZP) will be applied to the instrumental photometry of the rest of the objects of the same exposure, calibrating the full J-PAS exposure.

Eventually, and already beyond the duties of J-PLUS, the observational strategy of J-PAS, with four exposures per band with high overlapping, will allow us to perform an "übercalibration" to reach a high degree of internal homogeneity of the photometry in each single band. Moreover, the spectral sampling of J-PAS filter system, similar to a spectra of very low resolution (R~50), combined with the observation of many spectrophotometric standard stars in the area of J-PAS, will enable us to go a step further from the "übercalibration" and perform a “spectro-übercalibration” achieving a high degree of consistency in the photometry not only band by band but in the whole spectral range covered by J-PAS filter system (~3500Å – 11000Å).