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 table relExp 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.
ZPCALERR    F32     error in calibrated zero point. Zero for ubercal exposures. 
                    
DVOFLAGS    U32     photometry calibration flags extracted from dvo image flags. Includes flag indicating if the exposure is an ubercal
                    exposure.
UCALDIST    U32     Distance to ubercaled image XXX: 0 for ubercaled exposures. XXX: Describe more completely.
UCALFILE    STR     base name of ubercal file (serves as ubercal version)
DVODB       STR     base name of DVO database used for calibration (Source of calibration information)

CALDATE     STR     timestamp relExp was last updated (presumably when it's state was changed to calibrated)
PSREL       STR     ippRelease name for processing/calibration (for example 3PI.PV1) 

All keywords will be absent if image does not have an associated row in the relExp table or if the relExp state is not equal to "calibrated"

Related keywords currently in use and will not be changed.

ZPT_OBS     zero point and error measured by psastro
ZPT_ERR
PSREFCAT    base name of reference catalog used by psastro.

Examples

o5352g0420o (ubercal exposure because DVOFLAGS & 512 == 512)

ZPCALIB  = 24.65917590
ZPCALERR = 0.0
ZPT_OBS  = 24.66297
ZPT_ERR  = 0.021101
DVOFLAGS = 512
UCALDIST = 0
DVODB    = 'LAP.ThreePi.20120706.v2'
UCALFILE = 'ucalqw_noref.fits'
CALDATE  = '2013-08-01T00:00:00'
PSREL    = '3PI.PV1'
PSREFCAT = 'PS1.REF.20120524'


o5352g0549o (not an ubercal exposure because DVOFLAGS & 512 == 0)

ZPCALIB  = 24.65723240
ZPCALERR = 0.00005263
ZPT_OBS  = 24.64897
ZPT_ERR  = 0.01906586
DVOFLAGS = 0
UCALDIST = 1
DVODB    = 'LAP.ThreePi.20120706.v2'
UCALFILE = 'ucalqw_noref.fits'
CALDATE  = '2013-08-01T00:00:00'
PSREL    = '3PI.PV1'
PSREFCAT = 'PS1.REF.20120524'

Currently DVOFLAGS is the value of FLAGS in the DVO Images table. The bits are

typedef enum {
  ID_IMAGE_NEW            = 0x00000000,  /* no calibrations yet attempted */
  ID_IMAGE_PHOTOM_NOCAL   = 0x00000001,  /* user-set value used within relphot: ignore */
  ID_IMAGE_PHOTOM_POOR    = 0x00000002,  /* relphot says image is bad (dMcal > limit) */
  ID_IMAGE_PHOTOM_SKIP    = 0x00000004,  /* user-set value: assert that this image has bad photometry */
  ID_IMAGE_PHOTOM_FEW     = 0x00000008,  /* currently too few measurements for photometry */
  ID_IMAGE_ASTROM_NOCAL   = 0x00000010,  /* user-set value used within relastro: ignore */
  ID_IMAGE_ASTROM_POOR    = 0x00000020,  /* relastro says image is bad (dR,dD > limit) */
  ID_IMAGE_ASTROM_FAIL    = 0x00000040,  /* relastro fit diverged, fit not applied */
  ID_IMAGE_ASTROM_SKIP    = 0x00000080,  /* user-set value: assert that this image has bad astrometry */
  ID_IMAGE_ASTROM_FEW     = 0x00000100,  /* currently too few measurements for astrometry */
  ID_IMAGE_PHOTOM_UBERCAL = 0x00000200,  /* externally-supplied photometry zero point from ubercal analysis */