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SkyServer Frequently Asked Questions
SkyServer FAQ

SkyServer Frequently Asked Questions

  1. What is the difference between Target and Best ?

    Because the survey imaging and processing is always being improved, the SDSS often obtains improved photometric measurements of objects AFTER they have been chosen ("targeted") for spectroscopy. However, it is important to keep a record of the photomeric measurements at the time objects were targeted. We therefore maintain two versions of the photometric catalog:

    1. Target:

      Known as TARGDR1 in the DR1 CAS, this database contains the photometric catalogs AS THEY WERE WHEN OBJECTS WERE CHOSEN FOR SPECTROSCOPY. This database contains the union of all target chunks. It may cover a slightly different area than Best; blended objects may be deblended differently; image quality may be worse; photometric calibration may be less accurate. However, if you want to see what the SDSS thought the magnitudes and other properties of an object were when it was chosen for spectroscopy, this is the place to look. Note that this database DOES NOT contain links from the photometric objects to the spectroscopy (you can always get the Target data for spectroscopic objects using the TargObjID field in the SpecObjAll table), nor does it contain the tiling information. This is because the Target database is intended to be a snapshot of the survey before any spectroscopy is done.

    2. Best:

      Known as BESTDR1 in the DR1 CAS, this database contains the latest, best versions of the imaging data, processed with the latest version of the photometric processing software, and with the most recent understanding of the photometric calibration applied. For any science based on object photometry, you will want to use the Best data. In addition, only the Best database contains all of the spectroscopy and tiling information.

  2. What is the difference between SpecObj and SpecObjAll? What does sciencePrimary mean?

    The SpecObjAll table contains ALL spectroscopic objects, regardless of their status in the survey. Queries on this table can produces unusual or undesired objects. Thus, we have created the SpecObj view, which contains data for ONLY those fibers defined as SciencePrimary. To be SciencePrimary, an object must meet all of the following criteria:

    1. It was targeted in the target skyVersion;
    2. The plate on which the spectrum was taken is the primary observation of that tile;
    3. The plate was a main survey plate (not part of the Southern survey or a special project);
    4. The objType is not QA, SKY, or SPECTROPHOTO_STD (these object types are repeatedly observed);
    5. The fiber was mapped correctly (this is the zWarning check)

    As a result, some plates may have many (or even all) of their fibers excluded from SpecObj. Some of these instances are:

    • They were targeted but are outside the official survey boundaries. These objects cannot be matched to a PhotoPrimary in the target. Plates 344-346, 348, and 364 (tileRun 6) have 0 SpecObjs. Other plates from tile runs 4 and 6 have reduced numbers of objects, especially 315 and 342. This could affect plates 266-315, 363, 361 from tileRun 4 and plates 342-348, 364 from tileRun 6.
    • Fibers where objType = 'SKY' do not show up in the SpecObj view. Some plates (for undetermined reasons) have large numbers of sky fibers: Plate 417 has 214 sky fibers, and they all seem to be on one half of the plate; Plate 595 has 91 SKYs and Plate 359 has 84 SKYs.
    • Fibers where objType = 'QA' also do not show up in the SpecObj view. Below is a table of the plates with more than 100 QA fibers and the number of such fibers:
      Plate# of QA fibers
      483174
      471136
      500125
      470123
      418120
      550108

      • Note: The definition of SciencePrimary relies purely on spectroscopic and geometric considerations. There are objects in SpecObj which do not have a corresponding Best photometric object (406 in DR1).

  3. What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?

    PhotoObjAll is a table in the Best and Target databases which contains all of the measured photometric quantities for all of the imaging objects. Because we measure so many different parameters for each object, this is a very large table, and queries can take very long to run.
    In an effort to speed up queries, we have created a table with only a subset of the parameters (a "thin table") that are requested most often. This table is called PhotoTag. If you have a query which uses and returns only values stored in PhotoTag, it will execute much faster than if you used PhotoObjAll.


    In addition, we have created a view ofPhotoObjAll which contains only those objects which are Primary or Secondary. This view is called PhotoObj. Because this view effectively contains fewer objects than PhotoObjAll, but all the measured quantities for these objects, queries will execute faster.


    Given the above, a user should:

    1. Query from PhotoTag if it contains everything you are looking for;
    2. Query from PhotoObj otherwise, UNLESS you are interested in data for objects which are neither PRIMARY nor SECONDARY. In that case, you will need to use PhotoObjAll.
    3. Importantly, the "shorthand" quantities u,g,r,i,z do not exist in the PhotoTag table. Instead, you must use ModelMag_[ugriz], which is indexed to make queries faster. HOWEVER, in PhotoObjAll and its views, only the u,g,r,i,z are indexed, and not the ModelMags!

    Because PhotoTag has many fewer parameters, larger portions of it can be cached, improving performance. We have found that for almost all queries which contain parameters fully in PhotoTag, it is faster. In the case of looking for objects that are detected multiple times, one could perform a join on PhotoTag with itself, requiring that one object be Primary and the other Secondary.

  4. How do I get photometry for spectroscopic objects? What is the SpecPhotoAll table?

    The SpecPhotoAll table is a precomputed join between the Best PhotoObjAll and SpecObjAll tables. It includes the most requested parameters from these two tables, as well as a few pieces of information about tiling. It also includes the TargetObjID, which allows user to retrieve the Target version of the photometry.

  5. What is the difference between SpecPhoto andSpecPhotoAll?

    As described above, The SpecPhotoAll is a precomputed join between the Best PhotoObjAll and SpecObjAll tables. This includes all non-science objects, and a variety of objects many users will not be interested in. The SpecPhoto view includes only those pairs where the SpecObj is a sciencePrimary (see the definition above), and the BEST PhotoObj is a PRIMARY.