[AstroPy] Constructing Angle from a tuple specified as sign string plus numbers for degrees, arcmin, arcsec

Thomas Robitaille thomas.robitaille at gmail.com
Mon Aug 10 17:53:06 EDT 2015

Erik Bray wrote:
> On 08/10/2015 05:26 PM, Eric L. N. Jensen wrote:
>> Hi all,
>> I’m working with a table from CDS (i.e. from an ApJ paper) that specifies the declination coordinate in four separate columns:  a string for the sign of the declination, and three numerical columns for degrees, arcminutes, and arcseconds.  Is there a straightforward way to combine these into a single astropy.coordinates Angle object?  The standard Angle function (constructor?) only accepts a tuple with three entries.
>> I guess what I’m looking for is the inverse method of ‘signed_dms’.
>> If it’s of use, the table I’m working with is here:  http://iopscience.iop.org/0067-0049/184/1/18/fulltext/apjs300673t4_mrt.txt
>> Thanks in advance for your help with this,
> It seems a bit strange that the table format in this case indicates the sign of 
> the declination as a completely separate field, rather than as just part of the 
> numeric string.  But I'm not intimately familiar with the quirks of the CDS 
> table format.  Is that normal?  I wonder if there's a way to get the CDS reader 
> to format those fields as a single column.  Tom Aldcroft would know better.
> You can instantiate an Angle array using a generator, so you could probably just 
> slurp in the data using a generator expression, allowing you to transform the 
> values one a time while creating the Angle rather than transforming the table 
> all at once first (please bear with the LISPiness of this example):
> dec = Angle(((int(row[0] + str(row[1])), row[2], row[3])
>               for row in table['DE-', 'DEd', 'DEm', 'DEs'))
> # (Note: originally I used row[2:] above, but there seems to be an unrelated bug 
> involved in taking slices of rows...I'll report on that.)

In case you want to do this without loops, np.char.add can be used to
concatenate string arrays element-wise. So for instance you could
concatenate the sign column and the degree column of the declination with

np.add(t['DE-'], t['DEd'])

then you would end up with three arrays instead of four.


> Erik
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