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- NVAS Queries (`astroquery.nvas`)

# NVAS Queries (`astroquery.nvas`)

## Getting started

This module may be used to retrieve the NVAS VLA archive images. All images are returned as a list of `HDUList` objects. Images may be fetched by specifying directly an object name around which to search - in this case the name will be resolved to coordinates by astropy name resolving methods that use online services like SESAME. The search centre may also be entered as a coordinate using any coordinate system from `astropy.coordinates`. ICRS coordinates can also be entered directly as a string that conforms to the format specified by `astropy.coordinates`. Some other parameters you may optionally specify are the `radius` and the frequency band for which the image must be fetched. You can also specify the maximum allowable noise level in mJy via the `max_rms` keyword parameter. By default this is set to 10000 mJy

    >>> from astroquery.nvas import Nvas
    >>> import astropy.units as u
    >>> images = Nvas.get_images("3c 273", radius=2*u.arcsec, band="K", max_rms=500)
    1 images found.
    >>> images
    [[<astropy.io.fits.hdu.image.PrimaryHDU at 0x3376150>]]

The `radius` may be specified in any appropriate unit using a `Quantity` object. Apart from that it may also be entered as a string in a format parsable by `Angle`. The frequency bands are specified using the `band` keyword parameter. This defaults to a value of `all` - i.e all the bands. Here’s a list of the valid values that this parameter can take.

    "all", "L", "C", "X", "U", "K", "Q"

Let’s look at an example that uses coordinates for specifying the search centre.

    >>> from astroquery.nvas import Nvas
    >>> import astropy.coordinates as coord
    >>> import astropy.units as u
    >>> images = Nvas.get_images(coord.SkyCoord(49.489, -0.37,
    ...                          unit=(u.deg, u.deg), frame='galactic'), band="K")
    2 images found.

You may also fetch UVfits files rather than the IMfits files which is the default. To do this simply set the `get_uvfits` to `True`, in any of the query methods. You can also fetch the URLs to the downloadable images rather than the actual images themselves. To do this use the `get_image_list()` which takes in all the same arguments as `get_images()` above except for the `verbose` argument which isn’t relevant in this case.

    >>> from astroquery.nvas import Nvas
    >>> import astropy.coordinates as coord
    >>> import astropy.units as u
    >>> image_urls = Nvas.get_image_list("05h34m31.94s 22d00m52.2s",
    ...                               radius='0d0m0.6s', max_rms=500)
    >>> image_urls
    ['http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.5+220114/1.51I4.12_T75_1986AUG12_1_118.U3.06M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.5+220114/1.51I3.92_T75_1986AUG20_1_373.U2.85M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.5+220114/4.89I1.22_T75_1986AUG12_1_84.8U2.73M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/1.44I1.26_AH0336_1989FEB03_1_197.U8.29M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/1.44I1.32_AH0336_1989FEB03_1_263.U3.84M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/4.91I0.96_AH595_1996OCT14_1_41.3U2.45M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/4.91I0.89_AH595_1996OCT11_1_43.2U2.45M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/4.91I0.99_AH0595_1996OCT16_1_66.4U2.55M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/8.46I2.18_AM503_1996FEB23_1_243.U2.59M.imfits',
     'http://www.vla.nrao.edu/astro/archive/pipeline/position/J053431.9+220052/8.46I1.60_AM503_1996FEB01_1_483.U2.59M.imfits']

## Troubleshooting

If you are repeatedly getting failed queries, or bad/out-of-date results, try clearing your cache:

    >>> from astroquery.nvas import Nvas
    >>> Nvas.clear_cache()

If this function is unavailable, upgrade your version of astroquery. The `clear_cache` function was introduced in version 0.4.7.dev8479.

## Reference/API

### astroquery.nvas Package

Revision History

Refactored using common API as a part of Google Summer of Code 2013.

Originally contributed by:

  Adam Ginsburg (adam.g.ginsburg@gmail.com)

#### Classes

`NvasClass`() |
`Conf`() | Configuration parameters for `astroquery.nvas`.