= Photometric Calibration information in postage stamp headers =
This note describes the what information about single exposure photometric calibration will be available 
in the headers of postage stamp images.

PS1 exposures are calibrated photometrically in multiple stages of the pipeline.

 * During the '''camera''' stage psastro compares the point sources detected at the '''chip''' stage with a reference catalogue and computes the astrometric solution and measures the photometric zero point.
 * The '''ubercal''' process [XXX: insert reference] refines the zero points. These values are then inserted into the DVO catalog.
 * During '''DVO''' calibration, exposures that were not ubercaled are calibrated by relphot [XXX: insert reference or text describing the process].  New zero points are measured for the non-ubercal exposures, ubercal exposures keep the zero points measured there.


It has been requested that the IPP postage stamp server include calibration information in the headers of the postage stamps. This project is in the process of being implemented. After DVO calibration the resulting information will be extracted and saved in the IPP meta data database (the mysql database named gpc1). The postage stamp server will include the data in the postage stamp FITS headers. 

The following table describes the associated keywords that provide the information.


{{{
keyword     type    Description 

ZPCALIB     F32     calibrated zero point extracted from DVO. Also saved in relExp.zpt_obs
ZPCALERR    F32     error in calibrated zero point. Zero for ubercal exposures. Also saved in relExp.zpt_obs
                    
DVOFLAGS    U32     photometry calibration flags extracted from dvo image flags. Includes flag indicating if ubercal
                    exposure.
UCALDIST    U32     Distance to ubercaled image XXX: 0 for ubercaled exposures. XXX: Describe more completely.
UCALFILE    STR     base name of ubercal file (in lieu of ubercal version)
DVODB       STR     base name of DVO database used for calibration (where information was extracted)

CALDATE     STR     timestamp relExp was set to calibrated
PS1REL      STR     ippRelease name (for example 3PI.PV1) for processing/calibration
                    XXX: drop '1' ? change PS1 to IPP ?
}}}

All keywords are absent if exposure's relExp for given release is not "calibrated"

Related keywords
{{{
ZPT_OBS     psastro zero point and error
ZPT_ERR
PSREFCAT    base name of reference catalog used by psastro.
}}}

Issues
 * While there is a single zero point for an exposure the data is extracted from the DVO images table. There are a small number of exposures (15) where the zero point (MCAL) is not the same for all chips. This problem will be ignored and the first value will be chosen.