[AstroPy] astropy.vo.client conesearch radius problem?

[Daniel Evans]

I'm not entirely sure what radius you're querying, as your quoted
coordinates vary by much more than 0.2 arcsec, and the nearest object I can
find in USNO-A2.0 is 26 arcsec away. It looks more like you've used 2
degrees?

Running the code with 0.20 degrees as the search radius, the RA coordinate
values do indeed differ by +-0.22 degrees. However, this is entirely
correct, because a change of 0.2 degrees in RA coordinate does not equal an
angular separation of 0.2 degrees between two objects (unless you're on the
equator). The RA coordinate difference must be multiplied by cos(Dec), so
because you're querying at Dec=-27.65, the angular distance along the RA
axis is (0.22 degrees) * cos(-27.65) = 0.20 degrees.

(If anything, it seems the USNO ought to be congratulated for seemingly
being the only catalog providers to remember the cos(Dec) term!)

Regards,
Daniel

On 16 January 2017 at 19:57, Francisco Gallardo lópez <
f.gallardo.lopez at gmail.com> wrote:

> Hi Evert!
>
> Thanks a lot for answering.
>
> I would say it only happens with the USNO catalogs, both 'The PMM
> USNO-A1.0 Catalogue (Monet 1997) 1' and 'The USNO-A2.0 Catalogue (Monet+
> 1998) 1'.
>
> I created a small code to show the thing (please find it attached to this
> email).
>
> It iterates through all the available catalogs and breaks into the
> debugger as soon as the query returns some sources (with the AR,DEC
> hard-coded in the program, only USNO catalogs will return sources). It
> stores the result in the “temp” variable. No big deal.
>
> For instance, with the second catalog (*'The USNO-A2.0 Catalogue (Monet+
> 1998) 1')* and the sky position (ra=159*.31*5075782, dec=-27.6515145412
> <(651)%20514-5412>):
>
>
> <Table masked=True length=49>
>     _r       USNO-A2.0    RAJ2000    DEJ2000   ...   Bmag    Rmag   Epoch
>    deg                      deg        deg     ...   mag     mag      yr
>  float64       str13      float64    float64   ... float32 float32 float64
> ---------- ------------- ---------- ---------- ... ------- ------- --------
>   0.041089 0600-13508332 159.269631 -27.643284 ...    19.3    17.9
> 1981.701
>   0.039111 0600-13508442 159.271653 -27.644431 ...    20.2    17.9 1981.701
>   0.038170 0600-13508467 159.272078 -27.654039 ...    20.6    17.5 1981.701
>   0.039771 0600-13508743 159.277409 -27.629875 ...    19.8    17.9 1981.701
>   0.031614 0600-13508894 159.280334 -27.644278 ...    19.8    17.9 1981.701
>   0.037095 0600-13509081 159.283342 -27.675723 ...    16.1    15.6 1981.701
>        ...           ...        ...        ... ...     ...     ...      ...
>   0.040114 0600-13512624 159.348720 -27.678370 ...    15.9    15.4
> 1981.701
>
> =========================================================
>
>
> I would say it is not paying attention to the requested radius. (Again,
> this does not happen with other catalogs, only with the USNO catalogs)
>
> Thanks a lot!
> BR
> Fran
>
> 2017-01-16 8:46 GMT+01:00 Evert Rol <evert.rol at gmail.com>:
>
>>  Hi Francisco,
>>
>> Could you send a short working code example around that shows the actual
>> problem?
>> Possibly for both the GSC (ok) and the USNO catalogs (not ok).
>>
>> Evert
>>
>>
>>
>> > Hi all!
>> >
>> > I looked in the Internet but I did not find references to this problem,
>> so maybe I'm making something wrong.
>> >
>> > So I'm using the conesearch from astropy.vo.client to query some
>> catalogs (indeed all the catalogs that are returned by
>> conesearch.list_catalogs() ).
>> >
>> > The thing is, I'm using a radius of 0.2 seconds of arc and it seems
>> like when querying some catalogs the results seem to be out of that
>> requested radius. For instance, two sources returned by the query were, one
>> at ra=5.602814 deg and another one @ ra=5.536589 deg.
>> >
>> > That does not happen with the 'Guide Star Catalog v2 1' which seems to
>> work properly, it happens when requesting the The PMM USNO-A1.0 Catalogue
>> (Monet 1997) 1'.
>> >
>> > Am I missing something or making something wrong or is it a known bug?
>> >
>> > Thanks a lot!
>> > BR
>> > Fran Gallardo
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