Best way to crossmatch DECaLS photometry and SDSS spectroscopy?

Question

I am trying to download the DECaLS photometric outputs for every galaxy in the SDSS spectroscopy catalog. I've tried a couple of ways like: 

sdss = ''' SELECT t.ls_id, t.ra, t.dec, t.dered_flux_g, t.dered_flux_r, t.dered_flux_z, t.flux_g, t.flux_r, t.flux_z, t.shape_r, t.sersic, t.flux_w1, t.flux_w2, t.flux_w3, t.flux_w4, t.flux_ivar_w1, t.flux_ivar_w2,  t.flux_ivar_w3,  t.flux_ivar_w4, t.w1_w2, t.w2_w3, t.w3_w4 

FROM ls_dr9.tractor as t, sdss_dr17.x1p5__specobj__ls_dr9__tractor as s 

            INNER JOIN ls_dr9.x1p5__tractor__sdss_dr17__specobj as s

            ON t.ls_id = s.id1 

'''

response = qc.query(sql=neighbors, out='mydb://decalsdr9_specobjmatch', drop=True, async_=True)

I've also tried using ra/dec matching using q3c_join. None of the methods seem to work and I consistently get error messages. What is the best way to perform this crossmatch? Thanks

Answer 1

Hello, thanks for reaching out, and sorry for the long delay in responding. Your query is almost correct. I ran this one (using the 'datalab' command line client on my laptop), and it produced a file of over 4M rows in 2 minutes:

datalab query sql="SELECT t.ls_id, t.ra, t.dec, t.dered_flux_g, t.dered_flux_r, t.dered_flux_z, t.flux_g, t.flux_r, t.flux_z, t.shape_r, t.sersic, t.flux_w1, t.flux_w2, t.flux_w3, t.flux_w4, t.flux_ivar_w1, t.flux_ivar_w2,  t.flux_ivar_w3,  t.flux_ivar_w4, t.w1_w2, t.w2_w3, t.w3_w4 FROM ls_dr9.tractor AS t INNER JOIN ls_dr9.x1p5__tractor__sdss_dr17__specobj as s ON t.ls_id = s.id1" > xmatch.csv

You can install  the command line client with pip:

pip install --ignore-installed --no-cache-dir astro-datalab

Then, log in once (datalab login), and finally run your query.

Best,
Robert