The Sky and Flux Indices

The Database names and the names of the Containers inside each Database are based upon the Object's coordinates and color properties. Before even opening a Database Container we already know in which region of the sky and in which domain of the color space the object returned will be. The Index is consulted  

The Sky Index

The Sky Index used in SDSS is two-fold use of the HTM. (See the HTM Homepage for a description of the HTM). The names of the level-5 leaf-nodes are used as the Objectivity Database names.

Database names are the level-5 HTM node names

In addition to that, every object stores its level-14 ID. This will enable us to cross-match with other databases (that also store the HTM) easily. See the HTM Homepage for details on how this is done.


The Flux Index

We use a k-d tree to index the u,g,r,i,z flux components of the SDSS filter data. See the description of the Color Partitioning Scheme by I. Csabai.

The Flux Index is a binary tree. At each step there is a certain split given by a plane in Flux Space (5-d space, using u,g,r,i,z as axes). The splits may be chosen differently for different regions of the sky, so we will have several Flux Indices. Since reddening is the major factor influencing the light from far-away objects, now we think of having 8 different stripes of Flux Indices each for north and south of the Galactic Plane. The mapping (which sky triangle is corresponding to which flux index) is also part of the Flux Index.

The first split is special and does not correspond to a plane but to a whole region of the flux space. The colors of the objects will usually fall into a very specific region. Only special objects will fall outside the scheme (like Quasars) and they will be sparsely distributed. All these outliers will have a separate index and the first split in the index indicates the split between outlier and normal objects.

Names of Flux Index leaves are given by the direction of their split and since it is a binary tree, we get names like "101011101". These names are used as container names in the databases.

During intersection, the Flux Index may alter a Flux Domain to simplify its structure.

Mapping of the Index Leaf Nodes to the BitList bits

The Flux Index Tree should be arranged in a way that all its Leaf Nodes are aligned in a single block following the root node (which is always at index 1). The i'th bit in the Flux BitList corresponds thus to the [i+2]nd node in the Flux Index.


For the Sky Index we cannot arrange the Tree in such a manner, so we need to generate an array where the IDs of the Sky Leaf Nodes are ordered in the same way as the bits in the Sky BitList. The i'th bit in the BitList finds the ID of its Sky Leaf Node through the i'th pointer in this array.