photo-z in particular area

[Alina]

Hi,

I would like to ask again for a help, as I tried a few complicated ways to download data, but for sure it must be more efficient and better way to do it. I am interesting in photometric redshifts for particular area, the catalogues ls_dr9.photo_z and ls_dr9.photo_z_9p1.

Is any easy query where I could put ra dec and it gives me output photo-z for the object in this area?

thanks!

Alina

[Dustin Lang]

Hi,

The example query shown in the Datalab interface does an RA,Dec,radius query:

To get all sources within 5 arcminutes = 0.083 degrees of RA,Dec = 20, 30 degrees,

SELECT * FROM ls_dr9.photo_z WHERE 't' = Q3C_RADIAL_QUERY(ra, dec, 20.0, 30.0, 0.083)

If you have a list of RA,Dec position that you want to find photo-z values for, then you want to use the "cross-match" service.

cheers,

dustin

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[Jesse Palmerio]

Dear Dustin,

I have some follow-up questions/remarks, as I'm trying to do essentially the same thing and my attempts might be useful for other people.

I want to get the photo-z for sources in a given cone (ra,dec,radius) from a python script.

1) Your example uses the Datalab interface, if I've understood correctly, to perform the same query from a python script one would need to use the datalab client

2) The query does not work in your example, I think because the ls_dr9.photo_z table does not contain the ra,dec columns. My approach to solving this would be to query the ls_dr9.tractor table with the ra,dec,radius and get the release,brickid,objid for each object in the table to make a unique identifier hash as suggested, and then use this hash to query the ls_dr9.photo_z. Are you aware of a simpler way than this?

3) In general, is there a scriptable way to get the photo-z information for sources in the legacy survey Tractor or Sweep tables?

Cheers,

Jesse

PS: Thank you for all the work you've done for the community over the years, I particularly appreciate the astrometry.net service.

[Dustin Lang]

Hi Jesse,

Right, the photo-z table is an add-on table so you would need to search in the ls_dr9.tractor table and then join on the "ls_id".

I'm not familiar with the datalab python client but that sounds likely for how you would script this up in Python.

The photo-z and tractor catalogs are released as large files covering RA,Dec rectangles on the sky - either the raw "brick" formats, 0.25 x 0.25 degrees, or the "sweeps" that gather up tens of square degrees to make a more manageable number of files.

Alternatively... and I hesitate to point out the availability of this interface... you can use the service that the sky viewer uses to generate a JSON result... this is meant for use by web browsers so it may be trimmed to 1000 objects.

https://www.legacysurvey.org/viewer/photoz-dr9/1/cat.json?ralo=200.1378&rahi=200.1703&declo=-2.2986&dechi=-2.2801

cheers,

dustin

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[Jesse Palmerio]

Thanks Dustin,

I was able to get what I wanted.

Here is a minimal working example for those interested (note you need to have a logged in datalab account for this to work, follow the readme here):

from dl import queryClient as qc
result = qc.query(sql="""
SELECT TOP 10
    t.ls_id,
    t.ra,
    t.dec,
    p.z_phot_median
FROM
    ls_dr10.tractor AS t
JOIN
    ls_dr10.photo_z AS p
ON
    t.ls_id = p.ls_id
WHERE
    't' = Q3C_RADIAL_QUERY(t.ra, t.dec, 20, 30, 0.083);
"""

)

Cheers,

Jesse

[Dustin Lang]

Hi Jesse,

Thanks for the example code!  That's fantastic.

cheers,

dustin

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