[AstroPy] Question re objects within radius of given co-ordinates
Jim Singh
jimmyboysingh at gmail.com
Sun Apr 4 08:31:32 EDT 2021
Well this has certainly been an education - thank you so much for taking
the time to explain!
I can see now that even pseudo code like:
If NAME is not blank, *name*=NAME
ElseIf * is not blank, *name*=*
Else *name*=HR Code
would not be sufficient, because there can be multiple * entries (eg *bet
Ori, *19 Ori, *bet Ori A), just as there can be multiple NAMEs (eg NAME
Loadstar, NAME Polaris, NAME North Star), so I will need additional logic
to choose from such occurrences. It'd be no fun if this was too
straightforward :-)
Thanks again
Regards,
Jim
On Sun, Apr 4, 2021 at 4:24 AM Eric Jensen <ejensen1 at swarthmore.edu> wrote:
> Hi Jim,
>
> Glad it’s working for you.
>
> Astronomical naming is a messy topic - not every star has a “common name”
> in the way you’re thinking of, but at the same time, every bright star has
> *many* identifiers (usually associated with various catalogs) that can be
> used to refer to it.
>
> If you want to look up names, querying Simbad will give you the most
> systematic, comprehensive answer, but then you’ll have to work out some
> kind of rubric for how to choose which name you want to use from the many
> provided.
>
> For example, the Bright Star Catalogue objects all have “HR” numbers
> (confusingly, for "Harvard Revised” photometry in the Yale Bright Star
> Catalogue...). If we add ‘HR’ to the list of columns returned in your
> example, you get this:
>
> _RAJ2000 _DEJ2000 Vmag HR Name
> hourangle deg mag
> ---------- ---------- ----- ---- ---------
> 5.139 -8.665000 5.78 1671
> 5.152 -8.754167 4.27 1679 69Lam Eri
> 5.226 -8.147778 6.37 1704
> 5.242 -8.201667 0.12 1713 19Bet Ori
> 5.293 -6.844444 3.60 1735 20Tau Ori
>
>
> So the stars with missing names could be referred to as “HR 1671” and “HR
> 1704”. Those are perfectly good star names.
>
> If you want to go further, here’s a query to fetch all the names from
> Simbad for those stars with blanks in the ’Name’ column from your previous
> query:
>
> from astroquery.simbad import Simbad
> for obj in result[0]:
> if obj['Name'] == '':
> hr_name = 'HR ' + str(obj['HR'])
> print("No name for {}, fetching from Simbad.".format(hr_name))
> s = Simbad.query_objectids(hr_name)
> print("Found {} identifiers:".format(len(s)))
> print(s, "\n")
>
> which yields this:
>
> No name for HR 1671, fetching from Simbad.
> Found 29 identifiers:
> ID
> ----------------------------
> TIC 425213211
> 2MASS J05082019-0839552
> ADS 3722 A
> BD-08 1037
> CCDM J05083-0839A
> CEL 543
> CSI-08 1037 1
> GC 6281
> GCRV 55873
> GEN# +1.00033224A
> ...
> SKY# 8076
> TD1 4085
> UBV 4941
> UBV M 10659
> YZ 98 1353
> uvby98 100033224 A
> WDS J05083-0840A
> TYC 5330-1722-1
> ** STF 649A
> WEB 4664
> Gaia DR2 3182907530429025280
> Length = 29 rows
>
> No name for HR 1704, fetching from Simbad.
> Found 23 identifiers:
> ID
> ----------------------------
> TIC 231305489
> 2MASS J05133327-0808520
> BD-08 1059
> CEL 574
> GC 6392
> GEN# +1.00033948
> GSC 05330-01720
> HD 33948
> HERZ 11718
> HR 1704
> N30 1119
> PPM 187818
> SAO 131887
> SKY# 8229
> SRS 6242
> TD1 4233
> TYC 5330-1720-1
> UBV 5002
> UBV M 10732
> YZ 98 1387
> [HFE83] 362
> uvby98 100033948
> Gaia DR2 3206914301671368960
>
> From there you could decide which name you want to use… If you’re
> looking specifically for the common names, those are formatted as ’NAME X’
> in Simbad (e.g. ‘NAME Polaris’), and Bayer or Flamsteed designations have a
> single ‘*’ at the start, e.g. ‘* alf UMi’ and ‘* 1 UMi’ for Polaris (where
> the ‘UMi’ is the constellation abbreviation, Ursa Minor in this case).
> Again, not every star will have a name like that, but you could check the
> list if you’re trying to prioritize names of that type.
>
> Best,
>
> Eric
>
> P.S. Regarding Peter Dzwig’s point regarding matching across catalogues,
> using coordinates for matching is always the safest way to go, rather than
> names.
>
>
> On Apr 3, 2021, at 7:20 AM, Jim Singh <jimmyboysingh at gmail.com> wrote:
>
> Thanks so much for your response Eric, yes this helps tremendously!
>
> I did some additional filtering and formatting (to enable direct
> comparison of returned results with a star map on the phone), and they
> match up nicely.
> However, as can be seen below, some rows are without names - this is
> expected?
>
> from astroquery.vizier import Vizier
> import astropy.units as u
> v = Vizier(columns=['Name','_RAJ2000', '_DEJ2000',
> 'Vmag'],column_filters={"Vmag":"<8"})
> result = v.query_region("rigel", radius="02d00m00s", catalog="V/50")
> for table_name in result.keys():
> table = result[table_name]
> table['_RAJ2000'] = table['_RAJ2000'].to(u.hourangle)
> table['_RAJ2000'].info.format = '10.3f'
> print(table)
>
> Name _RAJ2000 _DEJ2000 Vmag
> hourangle deg mag
> --------- ---------- ---------- -----
> 5.139 -8.665000 5.78
> 69Lam Eri 5.152 -8.754167 4.27
> 5.226 -8.147778 6.37
> 19Bet Ori 5.242 -8.201667 0.12
> 20Tau Ori 5.293 -6.844444 3.60
>
> Also, presumably another Vizier DB contains a mapping of 'common name'
> (where one exists) to the Bayer name? I tried for example the online form
> for IV/22 "Common Name Cross Index" and although for our first example it
> found 'BET CEN' when given the RA and Dec of Hadar, it did not return
> anything in the 'name' field (where I'd expected 'Hadar' to appear):
>
>
> *RAJ2000"h:m:s"*
> *DEJ2000"d:m:s"*
> *Vmagmag* BFno name
> 14 03 49.408 -60 22 22.79 0.61 BET CEN
>
> Regards,
> Jim
>
> On Sat, Apr 3, 2021 at 5:26 AM Eric Jensen <ejensen1 at swarthmore.edu>
> wrote:
>
>> Hi Jim,
>>
>> If you’re primarily interested in bright stars, and in names of objects,
>> then the VO search probably isn’t the best bet. As you’ve found, it has
>> lots of objects, and no common names for them.
>>
>> I’d suggest querying one of the catalogs from Vizier (
>> https://vizier.u-strasbg.fr/viz-bin/VizieR), for example the Bright Star
>> Catalogue.
>>
>> Here’s a query that will do a 2-degree-radius search around Hadar for
>> stars brighter than V=6:
>>
>> from astroquery.vizier import Vizier
>> result = Vizier.query_region("Hadar", radius="02d00m00s",
>> catalog=‘V/50',
>> column_filters={"Vmag":"<6"})
>> print(result[0])
>>
>> which prints:
>>
>> HR Name HD ADS VarID RAJ2000 DEJ2000 Vmag B-V SpType NoteFlag
>> "h:m:s" "d:m:s" mag mag
>> ---- ------- ------ --- ------- ---------- --------- ----- ----- ------ --------
>> 5267 Bet Cen 122451 Bet Cen 14 03 49.4 -60 22 23 0.61 -0.23 B1III *
>>
>>
>> The reason you have to use ‘print(result[0])’ rather than just
>> ‘print(result)’ is that the query returns a *list* of tables, though in
>> this case the list only has one element. The string ‘V/50’ is the Vizier
>> code for the Yale Bright Star Catalogue. You can find other suitable
>> catalogs to query by searching Vizier and noting down the code, typically a
>> Roman numeral followed by some digits. Each catalog will have different
>> column names, so you may have to adapt your ‘column_filters’ argument above
>> to the names of columns actually present in the catalog, or it won’t have
>> any effect.
>>
>> Hope this helps,
>>
>> Eric
>>
>> On Apr 2, 2021, at 1:23 AM, Jim Singh <jimmyboysingh at gmail.com> wrote:
>>
>> Hi,
>> I am a beginner astropy user. What I'd like to be able to do is obtain
>> the names and RA and Dec of all objects above a certain Magnitude, within a
>> given angular FOV centered on a given named object. If I try this for
>> example...
>>
>> from astropy.coordinates import SkyCoord
>> from astroquery.vo_conesearch import ConeSearch
>> c = SkyCoord.from_name('hadar')
>> print(c)
>> result = ConeSearch.query_region(c, '0.1 deg')
>> result.sort('Mag')
>> print(result)
>>
>> ...I get 2670 rows back. Assuming this is the right basic approach to
>> begin with, is there any way to constrain the search to just those rows
>> with 'Mag'>8 for instance? Only a handful of records would qualify in this
>> case. My typical search radius would be a couple of degrees, so if the
>> query were to return everything like above, it would take forever to
>> execute. Also, how to get the name of each qualifying object? Would it
>> entail a separate lookup on the column 'objID'?
>>
>> Thanks for your help!
>>
>> Regards,
>> Jim
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