[TRANSFORM] What is a "frame"?

[Ed Shaya]

Dave,

    Do you like this better?

FluxQ
   Argument
       positionQ
   ValueList
       list of flux values

positionQ
   Argument
        Index(NAxis1)
   Argument
        Index(NAxis2)
   DataFormat
        tuple
            int
            int
   ValueList
         0,0 0,1 0,2 ...
         1,0 1,1 1,2 ...
            ...
   Mapping
         Algorithm
            algorithmID
              http://ivoa.net/algorithms/WCS/position/pixelToCoords.c
        Input
            frame1 (collection)
               epoch (date Q)
               equinox (date Q)
               reference system (string Q)
        Input
            centerRADE  (doublet Q)
        Input
            pixelsPerArcsec (float Q)
        OutputUnits
            (hour min sec)
            (degree arcmin)     
    Mapping
         Algorithm
            algorithmID
              http://ivoa.net/algorithms/WCS/position/pixelToCoords.c
         Input
            alternateFrame2
         Input
            centerGlGb
         Input
            pixelsPerArcsec
         OutputUnits
            degree
            degree
            

Ed

David Berry wrote:

>Ed,
>
>  
>
>>Here is what we are talking about in a rough, possible serialized form.
>>I am just guessing that this algorithm would like input in the form of
>>tuples of the index values (which normally would be generated by some other
>>algorithm, but here I just type them in).
>>    
>>
>
>  
>
>>Frame is now a collection
>>of the various quantities (epoch, centerRA, centerDE, pixelsPerArcsec, etc)
>>needed for the transformation.
>>    
>>
>
>>From our experience with this sort of system over the past 8 years, we
>find the system is much more versatile and more easily extended if you
>keep the definitions of the coordinate systems separate to the definitions
>of the Mappings between coordinate systems. So, a Frame describing (say)
>(RA,Dec) would contain epoch, equinox, reference system
>(i.e. FK4/5/ICRS,etc), units, plus other axis annotation, but would *not*
>contain what you call centerRA, centerDE, pixelsPerArcsec, etc, since
>these do not form part of the definition of the coordinate system, but
>instead define the pixel->(ra,dec) *Mapping*. This is also the approach
>taken by Arnold Rots in his STC schema.
>
>
>  
>
>>FluxQ
>>    Argument
>>       positionQ
>>    ValueList
>>       list of flux values
>>[snip]
>>    
>>
>
>My version of this would be:
>
>FluxQ
>    WCS          (defines data positions in various coordiate systems)
>    UNITS        (defines data values in various flux systems)
>    ValueList
>        list of the flux values
>
>
>WCS
>    Frame0   (describes the "list index" coordinate system)
>    Frame1   (describes another coordinate system, e.g. (RA,Dec) )
>    Frame2   (describes another coordinate system, e.g. focal plane position)
>    ...      (more Frames as required)
>    Mapping1 (describes how to transform a position from Frame0 to Frame1)
>    Mapping2 (describes how to transform a position from Frame0 to Frame2)
>    ...      (more Mappings one for each extra Frame)
>
>UNITS
>    Frame0   (describes the raw data values, e.g. uncalibrated ADU)
>    Frame1   (describes another data system, e.g. calibrated flux density)
>    Frame2   (describes another data system, e.g. antenna temperature)
>    ...      (more Frames as required)
>    Mapping1 (describes how to transform a data value from Frame0 to Frame1)
>    Mapping2 (describes how to transform a data value from Frame0 to Frame2)
>    ...      (more Mappings one for each extra Frame)
>
>
>There would be no need to call this a "FluxQ", just a "Q" would do
>since the UNITS component defines what is held by the ValueList.
>
>As another example, there follows a quantity holding a collection of
>star (RA,Dec) positions. In this case, "data value" is an (RA,Dec)
>pair instead of a flux value, and the only sensible coordinate system
>for specifying "data position" is the system which describes the index
>of an (RA,Dec) pair within the ValueList.
>
>Q
>    WCS
>    UNITS
>    ValueList
>        list of the flux values
>
>WCS
>    Frame0   (describes the "list index" coordinate system)
>             (no Mappings needed since there is only 1 Frame)
>UNITS
>    Frame0   (describes the raw data values, e.g. (RA,Dec))
>             (no Mappings needed since there is only 1 Frame)
>
>
>I *think* that our two approaches provide broadly similar facilities
>(except possible for the use of UNITS), just using different structures.
>
>David
>
>
>----------------------------------------------------------------------
>Dr David S. Berry    (dsb at ast.man.ac.uk)
>
>STARLINK project                        Centre for Astrophysics
>(http://www.starlink.ac.uk/)            University of Central Lancashire
>Rutherford Appleton Laboratory          PRESTON
>DIDCOT                                  United Kingdom
>United Kingdom                          PR1 2HE
>OX11 0QX
>
>
>  
>