Sorry to drop in out of ether, and perhaps in the middle of a discussion,
but I am quite interested in the issues raised by Serge and Jacek. If
the comments are clueless, please point me in the right direction and
I will be quiet for a while.
1) My first concern is whether the base camp catalog is the final version
of the catalog, or just an aide to other processing being done at
the base camp. I still have this serious mental issue when it comes
to the distinction between "detections" and "objects". To me, a
detection is a group of pixels that pass some sort of significance
test and thereby trigger additional processing for various bulk
parameters such as position, total flux, shape, etc. To me, an
object belongs to the physical universe and can be interpreted using
the laws of physics and having attributes like temperature, size, etc.
The mapping between detections and objects is an amazingly difficult
problem, and my feeling is that doing it more than once will result
in different catalogs. Not all detections are objects (airplanes
and Earth satellites are obvious cases), and not all objects can
be detected (otherwise we would have solved the Dark Matter problem).
There are additional issues such as variable seeing that would cause
two or more objects to appear as a single detection in bad seeing,
but as separate detections in good seeing. A similar confusion arises
for extended objects such as galaxies, variable stars, variable stars
in galaxies, etc., etc.
Again in my jaundiced view, the mapping of detections into new/existing/
spurious objects is quite subtle, and may require extensive processing
and database access that may be unreasonable to do at the base camp.
So to repeat my first sentence, what is the role of the proposed
in-memory catalog at base camp?
2) As is usual for an astrometrist, I am frightened of IDs based on
the measurement of position. Serge's note shows a sensitivity to
this issue in noting that the IDs need to remain fixed even if
subsequent processing changes the coordinates, but I still worry.
Many clever people have proposed single integers that are useful
indices for the 2-dimensional sky. Chunking of (RA,Dec) is certainly
one way to go, but nets and meshes of funny-shaped things (triangles
for instance) may be more useful.
I am a bit surprised that one needs to store the whole catalog in
memory no matter how it is organized. The small chunk needed
for a single LSST observation should be a relatively small and quick
read-only access to a database, and I would have thought that this
could be done in milliseconds, not seconds. The subtle issues
raised in (1) above would not preclude finding and matching enough
stars to compute a provisional WCS transformation, for example,
and perhaps this would be a useful thing to do for down-stream
processing.
This doesn't address the update issue, the what-to-do-with-extra-
detections issue, the is-this-interesting issue, and others.
So I think that I am getting back to my previous concern about what is
to be accomplished by the base camp correlation to an existing catalog.
Who is the consumer of this correlation, and what are the needs?
-Dave Monet is [EMAIL PROTECTED]
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