Native Star Files
Keywords: gcx

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Native Star Files

The file format used by GCX for star catalogs, recipies and observation reports consists of lists of token-value pairs. Each list is enclosed in brackets. The value part of each pair can be a number, a string (enclosed in double quotes) or another list. The order of the pairs within the list is not important.

A file can contain several top-level lists. Each such list is called a ``frame''. A typical recipe frame looks like:

    ( recipe ( object "aucyg" ra "20:18:32.76" dec "34:23:21.3"
        equinox 2000 comments "generated from tycho2" )
      sequence "tycho2"
      stars (

        (name "2680-1551" type std mag 7.48
        ra "20:17:54.500" dec "34:05:25.22" perr 0.0071
        comments "p=T "
        smags "v=7.344/0.020 b=8.654/0.020 vt=7.483/0.011 bt=9.024/0.017"
        flags ( astrom ) )

        (name "2680-1588" type std mag 7.79
        ra "20:16:56.755" dec "34:08:22.86" perr 0.0057
        comments "p=T "
        smags "v=7.795/0.020 b=7.745/0.020 vt=7.790/0.011 bt=7.731/0.015"
        flags ( astrom ) )

        (name "aucyg" type target mag 9.5
        ra "20:18:32.76" dec "34:23:21.3"
        comments "p=V t=M s=M6e-M7e m(p)=9.50/15.30"
        flags ( var ) ) )
      )

An observation report frame can look like:

    ( observation ( filter "b" object "aucyg" ra "20:18:39.31"
        dec "34:23:05.6" equinox 2000 mjd 53236.9839856
        telescope "SCT" aperture 30 exptime 20 sns_temp 238.3
        latitude "44:25:50.0" longitude "-26:06:50.0"
        altitude 75 airmass 1.223 observer "R. Corlan" )
      noise ( read 7 eladu 2 flat 0 )
      ap_par ( r1 5 r2 9 r3 13 sky_method "synthetic_mode" )
        (name "AU_CYG_39" type std mag 12.1
        ra "20:17:47.15" dec "34:32:01.4"
        smags "v=12.062/-0.001 b-v=1.982/-0.002 b=14.044/0.002"
        imags "b=-7.256/0.167"
        residual -0.209 stderr 1.25
        noise (photon 0.13 sky 0.074 read 0.075 scint 0.0021)
        centroid (x 876.89 y 385.24 xerr 0.0085 yerr 0.0065 dx -0.18 dy -0.15)
        flags ( astrom centered ) )

        (name "AU_CYG_10" type std mag 13
        ra "20:17:44.52" dec "34:21:00.0"
        smags "v=12.961/-0.001 b-v=0.765/-0.001 b=13.726/0.001"
        imags "b=-7.991/0.089"
        residual 0.208 stderr 2.33
        noise (photon 0.066 sky 0.038 read 0.038 scint 0.0021)
        centroid (x 569.61 y 331.56 xerr 0.002 yerr 0.0017 dx 0.19 dy -0.17)
        flags ( astrom centered ) )

        (name "aucyg" type target mag 9.5
        ra "20:18:32.76" dec "34:23:21.3"
        comments "p=V t=M s=M6e-M7e m(p)=9.50/15.30"
        smags "b=12.642/0.085"
        imags "b=-8.867/0.042"
        centroid (x 782.90 y 615.40 xerr 0.0079 yerr 0.0064 dx 0.17 dy 0.07)
        noise (photon 0.031 sky 0.017 read 0.017 scint 0.0021)
        flags ( var centered ) ) )
      transform ( band "b" zp 21.509 zperr 0.074 zpme1 1.29 ) )

A typical catalog frame would be:

    ( catalog (comments "Internal catalog output")
        stars (
          ( name "piuma" type catalog ra "08:39:11.70" dec "65:01:15.0"
            flags ( var ) )
          ( name "lpup" type catalog ra "07:13:31.90" dec "-44:38:39.0"
            flags ( var ) )
          ( name "piori" type catalog ra "04:54:15.10" dec "02:26:26.4"
            flags ( var ) ) ) )

All three forms of the star file have in common the top-level token `stars` which is followed by the list of all stars within the frame. Each star is also represented by a list, having some of the following tokens:

name | string | The name of the star;
type | symbol | The type of the star: `std` for standards, `target` for targets, `catalog` far catalog objects and `field` for field stars;
ra | string | Right ascension in HMS format;
dec | string | Declination in DMS format;
perr | number | Position error in arc seconds;
equinox | number | Equinox of coordinates;
mag | number | Generic magnitude of the star, used for display purposes only;
smags | string | A string describing the standard magnitudes of the star and their errors. See Section 7.7 for the format;
imags | string | A string describing the instrumental magnitudes of the star and their errors;
centroid | list | A list containing positional information. `x` and `y` are frame coordinates of the star's centroid. `xerr` amd `yerr` are the estimated position errors calculated by the centroiding code. `dx` and `dy` are the amounts by which the star's centroid is shifted from the catalog position;
noise | list | A list of components of the star's instrumental noise model;
residual | number | The residual of the star after the zeropoint fit;
stderr | number | The standard error (scaled residual) of the star after the zeropoint fit;
flags | list | A list of reduction flags.

Some top-level tokens of the star files are:

stars | list | List of stars in the frame;
recipe | list | A list of recipe parameters. Most important are the target object and field center coordinates;
observation | list | Parameters of the ccd frame from which the data was obtained. See Section 7.5 for more details;
catalog | list | Identifies the frame as a catalog frame. The most important token in this list is `comments`;
sequence | string | The source of the standard magnitudes in the star list;
noise | list | A list of parameters of the CCD noise model used to calculate instrumental errors;
ap_par | list | Parameters used by the aperture photometry routine (aperture sizes and sky estimation method);
transform | list | transformation coefficients, zeropoint and extinction coefficients used to calculate the standard magnitudes of target stars.

The native star format can be converted to a tabular format using the report converter function as described in Appendix E.