help to download a catalogue with mags for a particular area

[Alina]

Hello,

It seems a bit primitive question, but I am stucked, as I am not sure how to write a query to 

https://datalab.noirlab.edu/  and I didn´t see any help in the example.

I need something similar to SDSS query when you put ra dec, radius of search and output is a the table with all sources.

For example, I have  ra dec  337.331 20.4484, I need all sources in the radius of 5 arcmin,  their ra dec and magnitudes g,r,z. Nothing more, 5 columns with this information.

Could you please let me know how to write such query or where I could find an example.

thank you a lot in advance!

Alina

[Dustin Lang]

Hi,

One option is to use the sky viewer -- eg

https://www.legacysurvey.org/viewer/?ra=337.3355&dec=20.4461&layer=ls-dr9&zoom=14&sources-dr9

and then click on the map and in the pop-up dialog box, select the "Data" link.  One of the links there is to download the catalog entries visible on screen.

Or in the NOIRLab Datalab query interface (https://datalab.noirlab.edu/query.php?name=ls_dr9.tractor)

Use a query like

SELECT ra,dec, mag_g,mag_r,mag_z,dered_mag_g, dered_mag_r, dered_mag_z FROM ls_dr9.tractor WHERE 't' = Q3C_RADIAL_QUERY(ra, dec, 337.3355, 20.4461, 0.083)

(the 0.083 is 5 arcmin/60 to give query radius in degrees)

There are observed and dereddened (adjusted for Milky Way dust) mags.

cheers,

dustin

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[Alina]

Great, Thank you a lot for help, I tried both ways.

The viewer of course is much more convenient, as for me,  but the problem is that the catalogue (tractor) has only fluxes, no mag values. Of course, you could convert, but to have this info already prepared, and even already dereded, is really helpful!

regards,

Alina

[Dustin Lang]

Hi Alina,

Glad it worked for you... the Datalab's added-value columns are certainly helpful :)

(For what it's worth, the mags would be

mag = -2.5 * (log10(flux) - 9)

for as-observed mags, and

mag_dered = -2.5 * (log10(flux / mw_transmission) - 9)

for dereddened.)

cheers,

dustin

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