title: Catalog matching using astropy.coordinates.match_to_catalog_sky
body: <p>I have two catalogs of RAs and decs and am trying to find objects that show up in both catalogs.  I want to do so using astropy.coordinates.match_to_catalog_sky.  Right now, this function picks up all the objects that have matches, but also includes in its return indices that have no match.  Here is my code (it is a version of the code on the astropy documentation website with data about distance removed since I am not using distance data):</p>
  <pre><code>c = SkyCoord(ra=ra1*u.degree, dec=dec1*u.degree)
  catalog = SkyCoord(ra=ra2*u.degree, dec=dec2*u.degree)
  idx, d2d, d3d = c.match_to_catalog_3d(catalog)
  </code></pre>
  <p>catalog[idx] should just be a set of objects in catalog that have matches in c, but for some reason it is a set of those objects as well as some other ones on the edges of the region covered by catalog.</p>
  <p>Is there perhaps a way to lower the threshold for separation between matches?  Or is there another solution to this?</p>