What is the astrometric precision of ls dr10 data?

Question

Dear all,

I have downloaded images from the LS DR10 but, being at a variety of RA,Decs, it is difficult to tell which sub-survey they have come from. Moreover, how can I find the astrometric precision of the data? (As I understand this to be different from the CDELT values in the image headers.)

Hope to hear from you soon,

Sarah

Answer 1

Hello Sarah,

For the first part of your question, can you share where you are downloading the data from? I want to download some of the data you are working with to give you a more helpful response. 

For the question about astrometric precision, according to the data release description of the Legacy Survey webpage,
"Each image is calibrated to Gaia Data Release 2, yielding an astrometric solution that is offset by the average difference between the position of Gaia stars at an epoch of 2015.0 and the epoch of the DR10 image. Source extraction is then fixed to the Gaia Data Release 2 system, such that positions of sources are tied to predicted Gaia positions at the epoch of the corresponding Legacy Surveys observation. Astrometric residuals are typically smaller than ±0.03″."

Does this answer your question?

Best,
Brian M.

Comments

Hiya Brian,

Many thanks for your answer! And yes, that helps with understanding the astrometry :)

Re: where I downloaded the images from -- I followed the instructions for obtaining FITS files, under the 'Image Cutouts' heading on this page: https://datalab.noirlab.edu/data/legacy-surveys  . All of my sources are at Dec < -76 deg, and I've found that some are not covered by LS DR10 at all (so I had to download DSS2 images instead).

Best wishes,
Sarah

Answer 2

Hi Sarah,
Thank you for reaching out and using our resources!

A general description of the Legacy Imaging Survey is provided in our own LS landing page. A more in depth description is in the LS DR10 website, that Brian already provided.
Taken from there, the general rule of thumb is that the Northern portion, considered Dec > 32.375, is covered by BASS (Bok telescope) in z and MzLS (Mayall telescope) in g,r,i, and the Southern portion, Dec <= 32.375, is covered by DECaLS (Blanco telescope) in g,r,i,z.

To complement what Brian mention about the astrometry, several columns for each object in the main catalog, ls_dr10.tractor, are related to the astrometry, as seen in its column descriptions in our data explorer (search for "astrometric"), or in the Legacy Survey catalog description as well.

Please let us if this helps!
Cheers,
David

Comments

Hiya David,

Many thanks for your response :)  Yes, that helps with understanding the origin for sources at different Declinations. The reason for my query is that I'm thinking of putting in a FORS2 proposal for the next ESO Call. On this page: https://www.eso.org/sci/facilities/paranal/instruments/fors/overview.html they say

"Highly accurate relative astrometry is required for any observing mode which in Phase 2 will make use of FIMS or blind offset acquisitions. The mask preparation with FIMS requires input images which are astrometrically corrected within the definitions and precision given below. DSS images will, in almost all cases, not be suitable for the task. In general the relative astrometry must be known to better than 1/6 of the slit widths all over the field of view. Relative astrometry here means that the slit positions must be known relative to those of reference stars in the field of view with the given precision."

(I'm planning to use a 1-arcsec slitwidth, so it sounds like I may be worrying for no reason... for the LS DR10 images, in any case!)

Best wishes,
Sarah