Flags Tutorial

1. Use the Explore  tool to look at a single object. See the How do I get an image of my favorite object? tutorial to learn how to use the Explore tool.
2. Look at flags, immediately below the Imaging label. You will see the object's flags listed, one after another with one space in between. See the list of flags in the Schema Browser  for what they mean. Instructions for using the Schema Browser are in the next section, Interpret Flags.

1. Use the SDSS SkyServer Schema Browser  to study the list of the SDSS's flags.
2. Type flags into the Search for textbox and click Go.
3. Results of your search will show up in the right-hand section of the Schema Browser. Click on the very first PhotoObjAll link, next to the name flags. You will go to the schema browser entry for the PhotoObjAll table.
4. Scroll down through the table you will see a row with a name of flags. Click on the  link immediately to the right of the word flags.
5. The photoFlags Data values table shows the name, value, and description of each flag.
   • Name is the name of the flag; this is what you will see in the Explore tool
   • Value is the bitmask value for the flag
   • Description is a short description of what the flag means
6. The Access Functions, fPhotoFlags and fPhotoFlagsN, are needed to use the flag names in SQL queries. To find them, look under Functions in the left-hand menu of the Schema Browser.

You can search through flags as part of your searches with the Imaging Query Form . You can request that the tool return only objects that possess one or more flags, such as searching for moving objects by looking for the MOVED flag. Or you can search for only those objects that do not possess certain flags, such as searching for unsaturated objects by checking that the SATURATED flag is off.

1. Go to the Imaging Query Form . From the SkyServer home page, look under Search Tools. For more on how to use the Imaging Query Form, see the guide How do I find data for all objects in a given RA/Dec/Magnitude/Absolute Magnitude range?
2. Select any parameters you like on the form, then look at the Obj Flags drop-down menus at the bottom of the form.
3. To guarantee that your results will possess a certain flag, select the flag from the At least one of these flags ON menu. To select more than one flag to check, hold down the CTRL key while selecting.
4. To guarantee that your results will not possess a certain flag, select the flag from the All of these flags OFF menu. To select more than one flag to check, hold down the CTRL key while selecting.
5. The flags will not show up in the search results unless you specifically request them in the Imaging menu of the Parameters to Return section above. But the flags will be searched, and the search will return only objects that meet your flag criteria.
6. Click Submit Request to send your query to the database.

You can also search through flags as part of your searches using SQL. SQL searches will search for and return flags as long decimal numbers that have been converted from binary - but you translate between these numbers and the flag names with the functions dbo.fPhotoFlags and dbo.fPhotoFlagsN.

1. Go to the SQL Search  tool, under the Search menu. To get there from the SkyServer home page, click on SQL Search.
2. To constrain your search based on flags, add one of the following to the WHERE block of your query:
   1. To find only objects for which a certain flag is present:

      WHERE (p.flags & dbo.fPhotoFlags('flag')) != 0
      

      Replace flag with the name of the flag you want to search. For example, this query will find IDs and positions of the top 100 objects that have been flagged as having moved (be sure to use != 0 rather than > 0 in case integer values have rolled over):

      SELECT top 100 objid, ra, dec
      FROM photoTag
      WHERE (flags & dbo.fPhotoFlags('MOVED')) != 0
      

   2. To find only objects for which a certain flag is absent:

      WHERE (p.flags & dbo.fPhotoFlags('flag')) = 0
      

      Replace flag with the name of the flag you want to search. For example, this query will screen out IDs and positions of the top 100 objects that have been flagged as not being saturated:

      SELECT top 100 objid, ra, dec
      FROM photoTag
      WHERE (flags & dbo.fPhotoFlags('SATURATED')) = 0
      

By using several flags together, you can limit your searches to return only objects that have clean photometry, thereby ensuring that you have a good sample. The flags you use differ depending on whether you are looking at stars or extended objects.

The queries below run slowly. These are test versions that use TOP 10 to return only the first 10 matching objects. To return all matching objects, remove the top 10 and use the CasJobs  batch query system, which lets you run queries that take up to 8 hours. Also, note that lines that start with "--" are comments that are not executed as SQL statements.

For stars, use the following query to return IDs, positions, colors, and flags:

SELECT TOP 10 objid, ra, dec, u, g, r, i, z,
dbo.fPhotoFlagsN(flags) as flags
FROM star
WHERE ((flags & 0x10000000) != 0)  -- detected in BINNED1
AND ((flags & 0x8100000c00a4) = 0) -- not EDGE, NOPROFILE, PEAKCENTER,
-- NOTCHECKED, PSF_FLUX_INTERP,
-- SATURATED, or BAD_COUNTS_ERROR
AND (((flags & 0x400000000000) = 0) OR (psfmagerr_g <= 0.2))
-- not DEBLEND_NOPEAK or small PSF error
-- (substitute psfmagerr in other band as appropriate)
AND (((flags & 0x100000000000) = 0) OR (flags & 0x1000) = 0)

For extended objects, use the following query to return IDs, positions, colors, and flags:

SELECT TOP 10 objid, ra, dec, u, g, r, i, z,
dbo.fPhotoFlagsN(flags) as flags
FROM galaxy
WHERE ((flags & 0x10000000) != 0)  -- detected in BINNED1
AND ((flags & 0x8100000c00a0) = 0) -- not NOPROFILE, PEAKCENTER,
-- NOTCHECKED, PSF_FLUX_INTERP,
-- SATURATED, or BAD_COUNTS_ERROR
-- if you want to accept objects with
-- interpolation problems for PSF mags,
-- change this to:
-- AND ((flags & 0x800a0) = 0)
AND (((flags & 0x400000000000) = 0) OR (psfmagerr_g <= 0.2))
-- not DEBLEND_NOPEAK or small PSF error
-- (substitute psfmagerr in other band as appropriate)
AND (((flags & 0x100000000000) = 0) OR (flags & 0x1000) = 0)
-- not INTERP_CENTER or not COSMIC_RAY omit this AND clause if you
-- want to accept objects with interpolation problems for PSF mags.

You might also want to see SkyServer examples, such as Clean photometry with flags — Stars and Clean photometry with flags — Galaxies.

Finally, the photometric catalogs have a special flag called clean for each object, which contains an evaluation of whether the object is "clean" photometry. This definition is described in detail in our recommended flags documentation. (It differs slightly from the example directly above, an example of how different use cases can result in different decisions about what exactly is the set of "good" data).

Using clean is simple; to get a set of clean photometry you can just say in CASJobs or SkyServer:

SELECT TOP 100 ra, dec FROM photoObj
WHERE clean = 1