Most commonly used functions will be available from the top pyvo module
import pyvo as vo
We want to create a catalog of available X-ray images of our favorite source (supernova remnant, Cas A).
First we look for archives that have x-ray images:
# find archives with x-ray images
archives = vo.regsearch(servicetype='image', waveband='xray')
archives now contains a list of archives with data
len(archives)
Next, we'll search each archive to find out if they have images of our source. For that, we need to get its position:
pos = vo.object2pos('Cas A')
pos
As we search, we will write our results to a CSV file. We need to make sure that be prepared for failures by catching the DALAccessError. This example was run in October the day after the US goverment shutdown and NASA went off-line; consequently, most of the archive queries failed.
# find images and list them in a CSV file
with open('cas-a.csv', 'w') as csv:
csv.write("Archive short name,Archive title,Image title,RA,Dec,URL\n")
for arch in archives:
print("searching %s..." % arch.shortname)
try:
matches = arch.search(pos=pos, size=0.25)
except vo.DALAccessError as ex:
print("Trouble accesing %s archive (%s)" %
(arch.shortname, str(ex)))
continue
print("...found %d images" % matches.nrecs)
for image in matches:
csv.write(','.join( (arch.shortname, arch.title, image.title,
str(image.ra), str(image.dec),
image.getdataurl()) ))
Iteration as a natural, pythonic way to process results
The interface tries to be self-explanatory
archives[0].title
archives[0].publisher
An archive service is queried via its access URL.
archives[0].accessurl
(slide)
usnob = 'http://www.nofs.navy.mil/cgi-bin/vo_cone.cgi?CAT=USNO-B1&'
srcs = vo.conesearch(usnob, pos=(45.31, 74.0), radius=0.05)
len(srcs)
srcs.fieldnames()
srcs[0]['sRa']
at = srcs.votable.to_table()
b1 = at['B1']
b1
# obtain your list of positions from somewhere
sourcenames = ["ngc4258", "m101", "m51"]
mysources = {}
for src in sourcenames:
mysources[src] = vo.object2pos(src)
# create an output directory for cutouts
import os
os.mkdir("NVSSimages")
We will use a query object to save and re-use query constraints
# setup a query object for NVSS
nvss = "http://skyview.gsfc.nasa.gov/cgi-bin/vo/sia.pl?survey=nvss&"
query = vo.sia.SIAQuery(nvss)
query.size = 0.2 # degrees square
query.format = 'image/fits'
for name, pos in mysources.items():
query.pos = pos
results = query.execute()
for image in results:
image.cachedataset(filename="NVSSimages/%s.fits" % name)
query.pos = mysources['m101']
print query.getqueryurl()