A Primer on DVO Photcodes and Magnitude Representation

There are two types of DVO table which store magnitude data. The Average table stores magnitudes which represent averages of multiple measurements. The Measure table stores magnitudes which represent single measurements.

Each type of entry is defined by a Photcode. The individual Measure entries have their photcodes stored as part of the structure. The Average entries have photcodes which are determined externally by their sequence. For historical reasons, the Average photcode are divided into the Primary photcode (of which there may only be one) and Secondary (of which there may be several). The average magnitude associated with the Primary photcode is stored with the Average structure while the Secondary photcodes are stored in a separate, associated table called Secfilt. The layout of the Average and Secfilt tables, and the photcodes assigned to them, are defined when the database is created based on the layout of the photcode table. This table is defined with a single Primary and a number of Secondary photcodes. When the database tables are created, primary photcode is automatically associated with the Average entry while the sequence of Secondary photcodes defines the sequence of the entries in the Secfilt table. This is very fragile, but very fast for making the association. Thus, changing the number or sequence of secondary photcodes would cause an existing database table to be mis-interpretted.

The Measure entries are broken into two photcode types as well: internal and external measurements. Internal measurements are those for which the data source is well understood; the instrumental magnitudes are available and the instrumental parameters which are required to determine a calibrated magnitude. External measurements are provided by external sources: published catalogs, reference sources, etc. For these measurements, there is no control over the instrumental parameters which determined the measurement, and in many cases the measurement represents an average of an ensemble.

The photcode table defines relationships between photcodes, representing different photometry systems. Each photcode entry defines the target of the calibration (the equivalence), and it may potentially define the zero-point (Co), the airmass slope (Ko), the appropriate color terms (C1, C2), the reference color term (Color), and the polynomial coefficients of the color equation (starting with the 1st order term).

I need to include operations to handle varying color terms between CCDs. I have measured color terms between CCDs in the range +/- 0.1 mag/mag. This is an appropriate time to clean up some of the code that handles the photcodes, etc. In particular, the photcode table should provide the relationship beween relate between filter systems. For all of the functions below, a REF photcode simply returns the stored magnitude without transformation. PhotInst (measure) returns the instrumental magnitude of the given Measure Minst = measure[0].M - measure[0].dt - iZERO_POINT; PhotCat (measure) returns the 'catalog' magnitude of the given Measure (best guess given no other information like color): Mcat = measure[0].M - iZERO_POINT + Klam*(measure[0].airmass - 1000) + photcodes[0].code[Np].C; PhotSys (measure, average, secfilt) returns the 'system' magnitude of the given Measure (best guess using existing color information, but without application of relative photometry offsets): Msys = Mcat + f(color) color is derived from measure.photcode.c1,c2 (measure.source.c1,c2) PhotRel (measure, average, secfilt) returns the 'relative' magnitude of the given Measure (best guess using existing color information and current relative photometry measurment for this image). This is the same photometry system as the Average (PRI/SEC) measurement equivalent to this measurement photcode: Mrel = Msys + measure[0].Mcal; color is derived from measure.photcode.c1,c2 (measure.source.c1,c2) PhotCal (measure, average, secfilt, allmeasures, code) returns the 'calibrated' magnitude of the given Measure (best guess using existing color information and current relative photometry measurment for this image transformed to the appropriate reference photometry system): Mcal = Mref + f(color) color is derived from code.c1,c2 the provided code must be either the measure.photcode.equiv or an alternate for that code.

consider two filters, B & V, and two CCDs 00 & 01. also, reference data in B_L92, V_L92

    N  photcode     type  Co      Ko       X    C1    C2    Ao     Color  Equivalent
  100  CFH12K.B.00  dep   26.000 -0.15     -    B     V     0.1    -      B
  101  CFH12K.B.01  dep   26.000 -0.15     -    B     V     0.0    -      B

  200  CFH12K.V.00  dep   26.100 -0.15     -    B     V     0.0    -      V
  201  CFH12K.V.01  dep   26.100 -0.15     -    B     V     0.1    -      V

    1  B            pri    0.000  -        -    B_L92 V_L92 0.016  -      B_L92
    2  V            sec    0.000  -        -    B_L92 V_L92 0.008  -      V_L92

  1003 B_L92        ref    -      -        -    -     -     -      -      -
  1004 V_L92        ref    -      -        -    -     -     -      -      -
  
  B = m + Co + Ko*(secz - 1) + Ao*(B - V)
  B_L92 = B + Co + color*A0 + color^2*A1 + color^3*A2