# Virtual Observatory

How to get an information about astronomical objects by using of Virtual Observatory.

## Introduction

Virtual observatory (VO) is a network infrastructure developed by astronomers which standardizes access for observational data, object catalogs and bibliographic sources. Any astronomical data (as images, spectra, catalogs, …) are directly accessible from VO without filling of web forms in browsers or hardly understanding data stored in a home-made format. Moreover, the data access should be easy automatized, the data can be preprocessed, different network sources could be merged, etc.

VO is using VO Table for data exchange (for data list, data serialization and as a control data). The VOTable is basically XML with exactly defined structure, keywords, status codes etc. VOTables are mutually convertible to FITS tables. VOTable is primary designed as a machine to a machine communications format.

Have in mind: Munipack support for VO is unfinished and also unmatured. Just only cone search and limited VO table parsing is implemented.

## Cone search

Cone search is a simple search in a catalog around specified point in spherical coordinates in a given solid angle.

The functionality is activated when `cone` is passed as an argument. Equatorial spherical coordinates are specified as `ra,dec` and search radius by `-r` switch. All quantities must be specified in degrees (sexadecimal notation or radians are not supported). The ICRS is used as the coordinate frame.

The simplest form is search specified catalgue passing just only angular coordinates Right Ascension and Declination in degrees separated by colon: the example below shows query for Pleiades on roughly coordinates α = 92.4°, δ = 24.1°. The objects are scanned within cone (radius) of 0.2° around the position.

    $ munipack cone -c Hipparcos -r 0.2 92.4 24.1
    $ xmunipack cone.fits

The switch `-o` can be used to store output in another named file and the switch `-t` chooses format of the output file (see details below).

The sort of output table by a column can be useful when we got a lot of data. The most common example is the astrometry, where it is convenient put bright stars on begin of the table. The sort option requires an exact column designation. The designation is (cataloue) site-specific and there is no way how to get the designations before a part of a table is transferred.

### Data sources

When cone search (or another VO request) is performed, the default source (Simbad) is connected. Another sources can be selected with `--cat` switch for a set of predefined catalogs or `--url` and `--par` switches for any available cone-search capable service.

Available data catalogs can be listed by using `--list` option:

    $ munipack cone --list
    Available catalogues  (use --url to specify another):
     Alias  URL --------------------------
    UCAC5  http://vizier.u-strasbg.fr/viz-bin/votable/-A?-source=I/340&
    ...

Just for example, the astrometric catalog UCAC5 is selected by

    $ munipack --cat=UCAC5 cone 92.4 24.1
    ...

Any cone-search service must support CGI get method, parameters for coordinates and (optionally) some site-specific parameters. Generally, the cone search has the form

    http://some.place.suffix/dir/cone-script.cgi?RA=92.4&DEC=24.1&parX=X

where the first part (up to question mark `?`) points to Internet address of the service along with a service script and the second part specifies parameters for the search. More detailed description offers Cone Search specification document.

To access of an arbitrary cone-search server, simply set `--url` switch to a value, as here:

    $ munipack --url="http://some.place.suffix/dir/cone-script.cgi?" cone 92.4 24.1

Note use of quotes (or apostrophes) to prevent a possible shell expansion and ending URL with question mark `?`. The cone search position and radius are added, to the query, automatically.

Some services supports generic parameters for the search. Usually ones sets limits for number of object search, limiting magnitude, etc. The parameters can be passed as `--par` (multiple parameters are allowed):

    $ munipack --par="max=666" cone 92.4 24.1

## VOTable

VOTable is a basic format for data provided by VO. The VOTable is supposed to be internal format for any data exchange. Therefore any manipulation with data in VOTable is non-trivial (requires XML parser). Munipack offers utility for conversion of VOTables to another formats which ones can find useful.

Available formats for conversion:

- `fits` table
- `csv` (comas separated) for spreadsheet applications
- `txt` for both human or machine processing
- `svg` for drawing of star charts
- `xml` just reformat of its input

A plain text representation of VOTable is generated as

    $ munipack votable -t txt pleiades.xml

The similar way can be used for conversion to FITS and CVS formats.

The convert to SVG is the most useful for drawing of star charts from VOTable (see output).

    $ munipack votable -t svg -pa 92.4 -pd 24.1 -ps 500 -ml 2 -mk R
               -o pleiades.svg pleiades.xml

## Advanced features

The cone search is implemented as a simple HTTP downloader with additional VOTable processing. Alternative to the way is downloading by the hand with an utility like wget or cURL followed by using of the votable to process of their output.

## See Also

- VOTable
- Cone Search