Using Python - After a coordinate query in Simbad, how do I extract the image generated by Aladin (opened with the Simbad plot button on the website)?

Question

Basic Goal: Given a set of sky coordinates (ra, dec), this Python function should return a FITS image of the sky centered at those coordinates.

Detailed Goal: The function will perform a coordinate query using astroquery and simbad. Ideally, it will be able to access the database of objects within the given region centered at (ra, dec). Hence, some size parameter will also be input (FoV, or radius). But mainly, when a query is done on the simbad through browser, there is an option to plot the star region centered at the input coordinates, made by AladinLite, and the function should return this image (hopefully as a FITS image).

What I've tried: Everything I've tried to get here has been discovering the use of astroquery and simbad. But I cannot find anything in the documentation on how the image from AladinLite can be obtained through astroquery. Finding a way to get an image straight from Aladin using Python doesn't seem possible either.

Question: Is it possible to get that image generated by AladinLite on the browser version of the simbad coordinate query using Python and astroquery? If so, how can I go about doing it?

I attached a screenshot of the web browser showing the image generated by AladinLite. To be clear - I want the image of the star field, not the image with the circled objects and so on.

Some pseudocode:

def generate_starfield(ra, dec, FoV):

    # define the sky coords
    coords = SkyCoord(ra=ra, dec=dec)

    # do an astroquery.simbad query using above coords
    query = perform_query(coords)

    # extract the image from the query generated by AladinLite
    image = get_aladinlite_image(query, FoV)

    return image

Iguananaut · Accepted Answer · 2020-06-12T14:32:31.363

Caveat: I am not a researcher in Astronomy and while I am an Astropy developer I'm not really a user so there might be some slightly better ways to do this if someone with more experience/understanding of the issue wants to come along.

But AladinLite is not an image catalogue. It's just a web-based UI for image viewing and plotting. However, the AladinLite sample service hosted by U. Strasbourg takes its data from their HiPS Survey services. They provide this data through a hips2fits web-based API. I don't think AladinLite provides an interface to gather all its displayed tiles into a FITS image to download. That's what the hips2fits service is for. Incidentally, it notes on the website:

We will develop an access to hips2fits from astroquery.cds. In the meantime, this notebook should get you started to query hips2fits from Python scripts.

So you have the first part of your workflow right: You would use astroquery to query Simbad. But then you need to query for the image data. There's no Python-specific API for this yet, but the web API is well documented, so it's mostly a matter of constructing the correct URL. Here's an example workflow I came up with using the same coordinates from the screenshot you gave:

>>> from astroquery.simbad import Simbad                                                            
>>> from astropy.coordinates import SkyCoord
>>> coord = SkyCoord('16 14 20.30000000 -19 06 48.1000000', unit=(u.hourangle, u.deg), frame='fk5') 
>>> query_results = Simbad.query_region(coord)                                                      
>>> query_results                                                                                   
<Table length=3>
        MAIN_ID               RA           DEC      ... COO_QUAL COO_WAVELENGTH     COO_BIBCODE    
                           "h:m:s"       "d:m:s"    ...                                            
         object             str13         str13     ...   str1        str1             object      
----------------------- ------------- ------------- ... -------- -------------- -------------------
               [T64]  7 16 14 20.2881 -19 06 48.062 ...        A              O 2018yCat.1345....0G
        IRAS 16114-1858    16 14 22.1     -19 06 14 ...        E              M 1988IRASP.C......0J
2MASS J16142091-1906051 16 14 20.9018 -19 06 05.195 ...        A              O 2018A&A...616A...1G

Now say I want an image centered on the first object in the results (similarly to the AladinLite preview shown on the web UI):

>>> from urllib.parse import urlencode
>>> from astropy.io import fits
>>> object_main_id = query_results[0]['MAIN_ID'].decode('ascii')
>>> object_coords = SkyCoord(ra=query_results['RA'], dec=query_results['DEC'], 
...                          unit=(u.hourangle, u.deg), frame='icrs')
>>> query_params = { 
...     'hips': 'DSS', 
...     'object': object_main_id, 
...     'ra': object_coords[0].ra.value, 
...     'dec': object_coords[0].dec.value, 
...     'fov': (2 * u.arcmin).to(u.deg).value, 
...     'width': 500, 
...     'height': 500 
... }                                                                                               
>>> url = f'http://alasky.u-strasbg.fr/hips-image-services/hips2fits?{urlencode(query_params)}' 
>>> hdul = fits.open(url)                                                                           
Downloading http://alasky.u-strasbg.fr/hips-image-services/hips2fits?hips=DSS&object=%5BT64%5D++7&ra=243.58457533549102&dec=-19.113364937196987&fov=0.03333333333333333&width=500&height=500
|==============================================================| 504k/504k (100.00%)         0s
>>> hdul.info()
>>> hdul.info()                                                                                     
Filename: /path/to/.astropy/cache/download/py3/ef660443b43c65e573ab96af03510e19
No.    Name      Ver    Type      Cards   Dimensions   Format
  0  PRIMARY       1 PrimaryHDU      22   (500, 500)   int16   
>>> hdul[0].header                                                                                  
SIMPLE  =                    T / conforms to FITS standard                      
BITPIX  =                   16 / array data type                                
NAXIS   =                    2 / number of array dimensions                     
NAXIS1  =                  500                                                  
NAXIS2  =                  500                                                  
WCSAXES =                    2 / Number of coordinate axes                      
CRPIX1  =                250.0 / Pixel coordinate of reference point            
CRPIX2  =                250.0 / Pixel coordinate of reference point            
CDELT1  = -6.6666668547014E-05 / [deg] Coordinate increment at reference point  
CDELT2  =  6.6666668547014E-05 / [deg] Coordinate increment at reference point  
CUNIT1  = 'deg'                / Units of coordinate increment and value        
CUNIT2  = 'deg'                / Units of coordinate increment and value        
CTYPE1  = 'RA---TAN'           / Right ascension, gnomonic projection           
CTYPE2  = 'DEC--TAN'           / Declination, gnomonic projection               
CRVAL1  =           243.584534 / [deg] Coordinate value at reference point      
CRVAL2  =         -19.11335065 / [deg] Coordinate value at reference point      
LONPOLE =                180.0 / [deg] Native longitude of celestial pole       
LATPOLE =         -19.11335065 / [deg] Native latitude of celestial pole        
RADESYS = 'ICRS'               / Equatorial coordinate system                   
HISTORY Generated by CDS hips2fits service - See http://alasky.u-strasbg.fr/hips
HISTORY -image-services/hips2fits for details                                   
HISTORY From HiPS CDS/P/DSS2/NIR (DSS2 NIR (XI+IS))

Now for plotting. The various markers and ticks seen in the AladinLite preview are generated by the AladinLite viewer itself, so using a Python-based workflow it's now on you to provide your own plotting. There are many ways to go about this. You could now save the FITS image and use whatever plotting tools you already have available. Though for a pure-Python workflow, the aplpy package is designed specifically for astronomy plots. Here's what I did with this image, just as an example:

>>> import aplpy
>>> gc = aplpy.FITSFigure(hdul)                                                                     
>>> gc.show_grayscale()                                                                             
INFO: Auto-setting vmin to  2.560e+03 [aplpy.core]
INFO: Auto-setting vmax to  1.513e+04 [aplpy.core]
>>> gc.show_markers(object_coords.ra, object_coords.dec, edgecolor='red',
...                 marker='s', s=50**2)         
>>> gc.save('plot.png')

Result:

Actually, showing me hips2fits perfectly solved what I was looking for! I don't even need an object, just an arbitrary set of coords and it gets the image! Thanks! — Zachary, Jun 14 '20 at 05:25
Yep! In this case I was just demonstrating how it was probably working in the exact screenshot you showed, but this is a flexible service you can use probably in whatever way you require for your goals. — Iguananaut, Jun 15 '20 at 10:33

Using Python - After a coordinate query in Simbad, how do I extract the image generated by Aladin (opened with the Simbad plot button on the website)?

1 Answers1