[Observation] relation to Dataset

[Douglas Tody]

On Thu, 21 Nov 2013, Douglas Tody wrote:
> But multiple table rows in ObsTAP may share the same Obs_ID, indicating
> that they are from the same observation.

Below is the relevant text we wrote for the ObsTAP doc (3.3.3),
detailing how multiple data products (datasets) may be associated
for an individual observation.

 	- Doug


ObsTAP and the Observation data model describe observations in a broad
sense (exactly what comprises an "observation" is not well defined
within astronomy and is left up to the data provider to define for their
data). ObsTAP also describes archive data products (e.g., actual archive
files). In general an "observation" may be composed of multiple
individual data products. In this case all the data products comprising
an observation should share the same observation identifier (obs_id).
The form of the obs_id string is up to the data provider so long as it
uniquely identifies an observation within the archive. The individual
data products comprising an observation may have different data product
types, calibration levels, and so forth. ObsTAP only directly supports
the description of science data products, i.e., data products which
contain science data having some physical (spatial, spectral, temporal)
coverage.

In general for instrumental data there are two different approaches for
exposing the data from an observation. One can either expose the
individual science data products comprising the observation, all sharing
the same obs_id, or one can expose the entire observation as a single
complex instrumental data product. Combinations of the two approaches
are also possible.

If the data products comprising an observation are exposed individually
then attributes such as the calibration level can vary for different
data products, e.g., the raw instrumental data as observed might be
level 1, a standard pipeline data product might be level 2, and a custom
user-processed data product subsequently published back to the archive
might be level 3. All such data products would share the same obs_id.

If on the other hand all data from an observation is exposed as a single
data product via ObsTAP this will likely be an aggregate of some sort
(tar file, directory, etc.) containing multiple files. This latter
approach is limited to instrumental data (level 0 or 1), even if objects
within the aggregate observation file are higher level. From the
perspective of ObsTAP this would be instrumental data, and it is up to
the user or client application consuming the data to interpret the
meaning of the data elements within the observation.