Galaxy Zoo Talk

What is a #primary?

  • suelaine by suelaine

    #skyserver calls this a #primary. What does Primary mean?


  • ElisabethB by ElisabethB moderator

    No idea ! Primary target ?


  • JeanTate by JeanTate in response to suelaine's comment.

    You mean in the box to the right of the "mode" (and above "flags")?

    As I understand it, the SDSS photometric pipeline (automated software which processes the imaging data; there's a separate one for spectroscopic data) 'finds' objects (which are called "photometric objects"), and fills several tables with data (values) on each one*. There are two "modes", PRIMARY and SECONDARY; these refer to how the pipeline classifies the object. The words refer to some sort of scientific value ... but what, exactly, and how an object gets to be PRIMARY and not SECONDARY, I do not know.

    Many, perhaps most, objects have at least one "Neighbor" which is SECONDARY; click on the link "Neighbors" in the left-hand panel to get a list of them. For this object, SDSS J024425.75+443054.8 (DR8 ObjId 1237661117555999457) is SECONDARY (the PRIMARY object is on the right):

    enter image description here enter image description here

    It looks the same, right? But the object on the right - the PRIMARY - is actually a different one! It's ObjId is 1237661121848018040, and its name is SDSS J024425.75+443055.1 Yep, SDSS names encode positions; the difference between the two names is just '5.1' - the last three characters in the PRIMARY's name - vs '4.8', meaning that the positions of the two differ by just 0.3 arcseconds in declination ... too small to show in these images.

    As far as I know, all SDSS objects in Galaxy Zoo are PRIMARY.

    *you can see many of the parameters, and the values, by clicking the PhotoObj link in the left-hand panel