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#### 6.1.2 `addpixsample`

Usage:

       addpixsample [-radius <expr-rad>] [-systems <in-sys> <pix-sys>]
                    <expr-lon> <expr-lat> <healpix-file>

Samples pixel data from an all-sky image file in HEALPix format. The `<healpix-file>` argument must be the filename of a table containing HEALPix pixel data. The URL of such a file can be used instead, but local files are likely to be more efficient.

The `<expr-lon>` and `<expr-lat>` arguments give expressions for the longitude and latitude in degrees for each row of the input table; this is usually just the column names. The long/lat must usually be in the same coordinate system as that used for the HEALPix data, so if the one is in galactic coordinates the other must be as well. If this is not the case, use the `-systems` flag to give the input long/lat and healpix data coordinate system names respectively. The available coordinate system names are:

- `icrs`: ICRS (Right Ascension, Declination)
- `fk5`: FK5 J2000.0 (Right Ascension, Declination)
- `fk4`: FK4 B1950.0 (Right Ascension, Declination)
- `galactic`: IAU 1958 Galactic (Longitude, Latitude)
- `supergalactic`: de Vaucouleurs Supergalactic (Longitude, Latitude)
- `ecliptic`: Ecliptic (Longitude, Latitude)

The `<expr-rad>`, if present, is a constant or expression giving the radius in degrees over which pixels will be averaged to obtain the result values. Note that this averaging is somewhat approximate; pixels partly covered by the specified disc are weighted the same as those fully covered. If no radius is specified, the value of the pixel covering the central position will be used.

The `<healpix-file>` file is a table with one row per HEALPix pixel and one or more columns representing pixel data. A new column will be added to the output table corresponding to each of these pixel columns. This type of data is available in FITS tables for a number of all-sky data sets, particularly from the LAMBDA archive; see for instance the page on foreground products (including dust emission, reddening etc) or WMAP 7 year data. If the filename given does not appear to point to a file of the appropriate format, an error will result. Note the LAMBDA files mostly (all?) use galactic coordinates, so coordinate conversion using the `-systems` flag may be appropriate, see above.

Syntax for the `<expr-lon>` , `<expr-lat>` and `<expr-rad>` arguments is described in the manual.

This filter is somewhat experimental, and its usage may be changed or replaced in a future version.

Note: you may prefer to use the `pixsample` command instead.

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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk