Changeset 40563 for trunk/doc/release.2015/ps1.datasystem/datasystem.tex

Timestamp:

Dec 3, 2018, 6:23:32 AM (8 years ago)

Author:

eugene

Message:

updates to the tessellation descriptions

File:

• trunk/doc/release.2015/ps1.datasystem/datasystem.tex (modified) (2 diffs)

trunk/doc/release.2015/ps1.datasystem/datasystem.tex

@@ -782 +782 @@
 grid laid out on the chips in the camera to a system of pixels with
 consistent geometry for a location on the sky.  The new image
-coordinate system is defined by one of a number ``tessellations'' of
-the sky.  Each tessellation specifies the way in which the sky is
-broken into individual images by defining a collection of projection
-centers.  For each projection center, positions on the sky are
-transformed to image pixels via a projection with a specified pixel
-scale and rotation.  In general, the pixel grid within the projection
-is defined as a simplified grid with the y-axis aligned to the
-Declination lines and no distortion terms.  The projection centers are
-typically separated by several degrees on the sky; for pixel scales
-appropriate to GPC1, the resulting collection of pixels would be
-unwieldy in terms of memory in the processing computer.  The pixel
-grid is thus subdivided into smaller sub-images called 'skycells'.  
-
-moves the data from a given exposure beyond
-away from being camera specific and towards a uniform sky oriented
-arrangement.  There are a number of ``tessellations'' defined and used
-by the IPP to define the extent and scaling of images on this uniform
-arrangement.  A tessellation can be defined for a limited region, such
-as M31 or other fields of particular interest that can be well
-described by a single tangent plane projection, or for larger regions
-which have multiple projection centers.  For the $3\pi$ survey, the
-\ippmisc{RINGS.V3} tessellation was used that arrange projection centers
-spaced every four degrees in both RA and DEC, with $0\farcs{}25$
-pixels.  These projections are further broken down into ``skycells''
-that form a $10\times{}10$ grid within the projection, with an overlap
-region of 60\arcsec\ between adjacent skycells to ensure that objects are not
-split on all images. 
-
-These tessellations are stored in the DVO format, with
-\ippdbtable{SkyTable} entries defining the projection centers and
-image boundaries for all the skycells.  The first step of the
-\ippstage{warp} stage is determining which skycells overlap with the
-input exposure.  These overlaps are determined by the
-\ippprog{dvoImageOverlaps} program, which compares the astrometrically
-calibrated catalog from the \ippstage{camera} stage to the
-\ippdbtable{SkyTable} entries.  The output of this command is used to
-populate the \ippdbtable{warpSkyCellMap} table in the database, which
-contains a row for each skycell and OTA that overlap.  This results in
-more rows than there are OTAs, as each skycell can contain
+coordinate system is defined by one of a number of ``tessellations''
+which specify how the sky is divided into individual images.  A single
+tessellation starts with a collection of projection centers
+distributed across the sky.  A grid of image pixels about each
+projection center corresponds to sky positions via a projection with a
+specified pixel scale and rotation.  In general, the pixel grid within
+the projection is defined as a simplified grid with the y-axis aligned
+to the Declination lines and no distortion terms.  The projection
+centers are typically separated by several degrees on the sky; for
+pixel scales appropriate to GPC1, the resulting collection of pixels
+would be unwieldy in terms of memory in the processing computer.  The
+pixel grid is thus subdivided into smaller sub-images called
+'skycells'.
+
+A tessellation can be defined for a limited region, with only a small
+number of projection centers (e.g., for processing the M31 region), or
+even a single projection center (e.g., for the Medium Deep fields).
+For the $3\pi$ survey, the tessellation contains projection centers
+covering the entire sky.  The version used to for the PV3 analysis is
+called the \ippmisc{RINGS.V3}.  In this tessellation, projection
+centers are spaced every four degrees in DEC and the RA spacing is
+approximately four degrees as well, adjusted to ensure an integer
+number of equal-sized regions.  \ippmisc{RINGS.V3} uses a pixel scale
+of $0\farcs{}25$ per pixel.  The projections subdivided into a
+$10\times{}10$ grid of skycells, with an overlap region of
+60\arcsec\ between adjacent skycells to ensure that objects of modest
+size are not split on all images.  The coordinate system used for
+these images matches the parity of the sky, with north in the positive
+$y$ direction and east to the negative $x$ direction.  The
+tessellations used by the IPP are stored in the DVO format (see
+Section~\ref{sec:DVO}), with \ippdbtable{SkyTable} entries defining
+the projection centers and image boundaries for all skycells.
+
+The first step of the \ippstage{warp} stage is to determine which
+skycells overlap with the input exposure.  These overlaps are
+determined by the \ippprog{dvoImageOverlaps} program, which compares
+the astrometrically calibrated catalog from the \ippstage{camera}
+stage to the DVO database defining the target tessellation.  The
+output of this command is used to populate the
+\ippdbtable{warpSkyCellMap} table in the database, which contains a
+row for each skycell and OTA that overlap.  Each skycell may contain
 contributions from multiple OTAs.
 
-Once this mapping has been defined, jobs to construct each skycell are
-run, passing the \ippstage{camera} stage catalog and the
+Once this mapping has been defined, jobs to warp the pixels onto each
+skycell are run, passing the \ippstage{camera} stage catalog and the
 \ippstage{chip} stage images (including the variance images and the
 updated masks) to the \ippprog{pswarp} program.  For details on the
@@ -830 +832 @@
 are the geometrically transformed images containing all input pixels
 warped to the common skycell pixel grid, which can subsequently be
-used for stacking and difference image analysis.  The image, mask, and
-variance generated at this stage will be available from the image
-extraction tools at the MAST archive at STScI as part of the DR2 data
-release.  A catalog is also generated containing the locations of
-sources from the input catalog that fall within area of the
-\ippstage{warp}.
-
-When the jobs have completed, an entry for the skycell is added to the
-\ippdbtable{warpSkyfile} database table, linked to the
+used for stacking and difference image analysis.  For the $3\pi$
+survey data, the signal, mask, and variance images generated at this
+stage are being made available from the image extraction tools at the
+MAST archive at STScI as part of the DR2 data release.  
+
+%% A catalog is
+%% also generated containing the locations of sources from the input
+%% catalog that fall within area of the \ippstage{warp}.
+
+When the \ippstage{warp} jobs have completed, an entry for the skycell
+is added to the \ippdbtable{warpSkyfile} database table, linked to the
 \ippdbtable{warpRun} entry by a common \ippdbcolumn{warp_id}.  An
 \ippmisc{advance} task again checks that all potential skycells have
 been generated.  At this point, the direct promotion of exposures from
 one stage to the next stops, as the logic for matching exposures for
-combination is more complicated than simply adding a single entry (as
-discussed above).
+other combinations is more complicated than simply adding a single
+entry.
 
 \subsection{Stack Combination}