+1 vote
428 views
I understand that one way would be to do a cross-match with the survey's catalog with a large enough radius. But what I have in mind is rather to visualise whether my source is inside or outside the DECALS contours shown e.g. here: https://datalab.noao.edu/survey.php
by maayane (130 points) | 428 views

1 Answer

0 votes

Thank you for reaching out.

There are several way to achieve this. One idea would be to check if your RA+Dec coordinates fall within a healpix ID that has any objects from the survey in it. This is of course approximative, but with nside=4096 the healpixels are only ~52 arcseconds along the side.

Example:

# imports
from astropy_healpix import HEALPix
from astropy.coordinates import ICRS, SkyCoord
from dl import queryClient as qc

# make hp instance with ICRS frame
hp = HEALPix(nside=4096,order='nested',frame=ICRS())

# instantiate a coordinate object from RA and Dec position
ra = 323.17
dec = -16.73
c = SkyCoord(frame=ICRS, ra=ra, dec=dec, unit='deg')
print('My coordinates:',c)

# compute healpix ID from coordinates
hpid = hp.skycoord_to_healpix(c)
print('Corresponding healpix ID:', hpid)

# query ls_dr8.tractor table to see if healpix ID is populated by any survey object
hit = qc.query('select exists(select nest4096 from ls_dr8.tractor where nest4096=%d)' % hpid).split()[1]
print('Coordinates in a populated healpix? -->', hit)

Output:

My coordinates: <SkyCoord (ICRS): (ra, dec) in deg (323.17, -16.73)>
Corresponding healpix ID: 198924159
Coordinates in a populated healpix? --> t

Try different RA/Dec:

# try other coordinates
ra, dec = 323.0, -16.87
c = SkyCoord(frame=ICRS, ra=ra, dec=dec, unit='deg')
print('My coordinates:',c)
hpid = hp.skycoord_to_healpix(c)
print('Corresponding healpix ID:', hpid)
hit = qc.query('select exists(select nest4096 from ls_dr8.tractor where nest4096=%d)' % hpid).split()[1]
print('Coordinates in a populated healpix? -->', hit)

Output:

My coordinates: <SkyCoord (ICRS): (ra, dec) in deg (323., -16.87)>
Corresponding healpix ID: 198924160
Coordinates in a populated healpix? --> f

Please let us know if this helped.
Robert

by robertdemo (7.9k points)
edited by 226

414 questions

430 answers

436 comments

635 users

Welcome to Data Lab Help Desk, where you can ask questions and receive answers from other members of the community.

Categories