The photometric calibration is the process that converts the raw data that it is stored in the images observed at the telescope to the actual flux of energy that arrives from a celestial object before entering in the atmosphere.

This is usually done inserting observations of particular stars called standard stars between the observations of the fields of interest. The standard stars have the property that their fluxes outside the atmosphere have been well established. Therefore, comparing how the standard stars are observed with their actual flux it is possible to derive the conversion between the “observed” and the “real” flux of any object observed in the same conditions as the standard stars.

However, observing standard stars reduces the amount of time that can be dedicated to the observation of the fields of interest, increasing the overheads of the observations. To avoid such overheads when observing the fields of J-PAS with the JST250 telescope, an alternative path has been devised and this is based on the used of the JAST80 telescope and the J-PLUS survey.

J-PLUS will observe the same area of the sky as J-PAS but with a different set of filters. The field of view of T80Cam at JAST80 is huge (~1.5° x 1.5°) and using less number of filters means that J-PLUS is going to be done much faster than J-PAS (and it is going to start before). Then, it is possible to observed the sky with J-PLUS and calibrate photometrically many stars and then used these stars to calibrate the J-PAS exposures.

But, first of all, we need to calibrate the J-PLUS observations. The photometric calibration (once the images have been properly reduced) can be split in two components. The first one is the effect of the atmosphere, mainly its extinction. The second one is the effect of all the optical elements that interact with the light before arriving to the CCD and then also the quantum efficiency of the CCD to convert photons in electrons. This two components differ also in an important way: while the effect of the atmosphere can vary quite fast, even within one night; the effects of the optical and electronic elements are much more stable. This means that to characterize properly the extinction one needs to sample the extinction in small time intervals, meanwhile the characterization of the optical and electronic system can be done in a less frequent way. And for this reason, it has been decided to measure both effects in an independent way.

First, the extinction is going to be measured with a dedicated smaller telescope (40cm aperture) provided with a special set of 10 filters. The set of filters has been chosen to sample in a proper way the change of the extinction with the wavelength, ie. the extinction curve of the atmosphere. The extinction monitor will observed several fields at different airmasses along the night with a cadence of one field in the ten filters each five minutes. This will allow not only to measure the extinction coefficients of the whole night but also to determine temporal variations within the night.

Having computed the extinction curve with the extinction monitor, we will be able to compute the extinction coefficients in the J-PLUS filter system (convolving the extinction curve with the response of J-PLUS filters and CCD) and, therefore, we will know the actual extinction along the line of any observation given the airmass of the observation.

With the extinction determined, the next step is the computation of the effects of the instruments on the light. This will be done with observation of spectrophotometric standard stars (SSSs) during each night. To have enough statistics, at least three observations of SSS will be performed each night at low airmasses.

Since J-PLUS has several filters that are not standard we are in the need of observing spectrophotometric standard stars. This way, through synthetic photometry, convolving the known flux calibrated spectra of the SSSs with the known response of our instruments we will be able to compute the standard flux which with we will compare the observed flux of the SSSs, obtaining the photometric zero points to be applied to the rest of the objects observed the same night as the SSSs.

To avoid missing some are of the sky due to pointing uncertainties, it is planned some overlapping among adjacent pointings. This will help to cross-check the photometric calibration through the comparison of objects observed in different pointings. Once J-PLUS is finished, this overlapping will be used to improve the internal photometric calibration through the so called "übercalibration" (Padmanabhan et al., 2007).