taskGNU Astronomy Utilities - Tasks: task #15911, Query NED extinction calculator


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task #15911: Query NED extinction calculator

Submitter:  Mohammad Akhlaghi <makhlaghi>
Submitted:  Sat 06 Mar 2021 04:33:00 PM UTC
Should Start On:  Sat 06 Mar 2021 12:00:00 AM UTC Should be Finished on:  Sat 06 Mar 2021 12:00:00 AM UTC
Category:  Query Priority:  5 - Normal
Item Group:  New feature Status:  Done
Privacy:  Public Percent Complete:  100%
Assigned to:  makhlaghi Open/Closed:  Closed
Effort:  0.00

Sun 07 Mar 2021 02:43:07 AM UTC, comment #3: 

Fortunately implementing this in Gnuastro was easier than I thought, see Commit 03cd54d8692c, so I am closing this task.

Task #15912 has just been defined to enable reading and writing of VOTables in Gnuastro, once that is implemented we won't have to use the script inside the book for using the VOTable.

Mohammad Akhlaghi <makhlaghi>
Project Administrator
Sat 06 Mar 2021 06:15:38 PM UTC, comment #2: 

I just got an answer about the resolution question (in the P.S.):

I'll have to go through these papers later to find the respective values, but if anyone else has the time to do it, please post a comment here or let me know privately so I add it for you (in case you are not a member of Savannah).


The extinctions calculated by NED use reddening coefficients from 2011ApJ...737..103S (which are based on Appendix B of  1998ApJ...500..525S, where you will find information about the maps and their resolution) The brightness maps associated with the 2011 publication are fairly course by today's measure - the extinctions a bi-axial interpolation of extinction for the queried position within the map.

Rick Ebert
Segment Manager | Caltech/IPAC
Engineering Group Lead | NASA/IPAC Extragalactic Database
Mail Code MR-100 | Pasadena CA 91125

Mohammad Akhlaghi <makhlaghi>
Project Administrator
Sat 06 Mar 2021 04:49:41 PM UTC, comment #1: 

Just to make things easier to test, let me put an example of submitting a query to the extinction calculator below where the coordinates are shell variables (in degrees):



curl -oned.votable 'https://ned.ipac.caltech.edu/cgi-bin/calc?in_csys=Equatorial&out_csys=Equatorial&in_equinox=J2000.0&out_equinox=J2000.0&obs_epoch=2000.0&lon='$ra_deg'd&lat='$dec_deg'd&of=xml_main&ext=1'

But as they recommend, we should just not query in parallel or immediately after each other! It can be expensive on their server and cause disruptions for everyone! As they say, we should recommend the users to wait at least one second between every query.

This is fine for a small number of objects, but when there are (hundreds of) thousands of objects in a catalog and we want the extinction on each, it can be very slow and possibly expensive for NED also.

One solution may be this: people can define a grid over their field of view (for example with boxes of size 0.5 degree), query NED for the extinction at the centers of those boxes, then find the nearest box for each object in their catalog and use that value. We should later add an 'astscript-' to do this.

I will ask them if it is possible to get the spatial resolution of each extinction value to make the process above much more systematic.

Mohammad Akhlaghi <makhlaghi>
Project Administrator
Sat 06 Mar 2021 04:33:00 PM UTC, original submission:  

After contacting the NED helpdesk, I got the following reply (in P.S.) for automatically extracting the extinction tables for a certain coordinate.

We can easily automate this command in Gnuastro's Query program. What is a little complicated is that Gnuastro doesn't yet recognize the VOTable format (a format based on XML).

But the good thing is that XML is a plain-text format, so we can use things like AWK and SED to extract the necessary columns rows for now (until reading of VOTables is supported within Gnuastro).


You can call the following URL using curl (or wget, Python, whatever) to retrieve Galactic extinction estimates at a given input location on the sky. Since you are not converting the coordinate system type or equinox, in_csys and out_csys are set to the same value (Equatorial), and in_equinox and out_equinox are also set the same (J2000.0).

This example has input in decimal degree units ("d" after the digits):


The output is in VOTable (XML) format, which includes metadata describing the fields (columns), and you can extract the Galactic extinction values in bandpasses of interest such as this one:

<TR><TD>Landolt V</TD><TD>0.54</TD><TD> 0.562</TD><TD>2011ApJ...737..103S</TD></TR>

Here is an example with equatorial coordinates input in sexigesimal format:


Where in the VOTable you will find entries such as this:

<TR><TD>Landolt V</TD><TD>0.54</TD><TD> 0.140</TD <TD>2011ApJ...737..103S</TD></TR>

Please note our general Guidelines for Automated Queries, to limit the rate of your programmed queries so they do not inadvertently make NED unusable for others.

We hope you find this helpful.
Thanks for using NED.

Joe, for the NED Team

Dr. Joseph M. Mazzarella
NASA/IPAC Extragalactic Database (NED), Science Lead
MS 100-22, IPAC, Caltech
Pasadena, CA 91125

Mohammad Akhlaghi <makhlaghi>
Project Administrator


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