Observation grouping

Observation grouping can be very helpful to speed the analysis of data with similar characteristics. It is also essential in some cases, like the production of background models. In this case, the scarce statistic forces the grouping of observations taken under similar conditions in order to produce reliable models.

There are 2 classes in Gammapy that handle observation groups:

• ObservationGroupAxis is a class to define an axis along which to define bins for creating observation groups. The class offers support for dimensionless or Quantity-like parameter axes. In both cases, both, continuous and discrete variables can be used.
• ObservationGroups is a class that takes a list of axes (ObservationGroupAxis) and defines groups based on the cartesian product of the bins on each axis. The group definitions are internally stored as a Table object. This class has also methods to group the observations of a given ‘ObservationTable’ following the defined grouping.

Examples

Create an ObservationGroups object with three axes:

from astropy.coordinates import Angle
from gammapy.data import ObservationGroups, ObservationGroupAxis
zenith = Angle([0, 30, 40, 50], 'deg')
ntels = [3, 4]
obs_groups = ObservationGroups([
    ObservationGroupAxis('ZENITH', zenith, fmt='edges'),
    ObservationGroupAxis('N_TELS', ntels, fmt='values'),
])

The axes info is stored:

>>> print(obs_groups.info)
ZENITH edges [  0.  30.  40.  50.] deg
N_TELS values [3 4]

The observation groups are stored as a table (computed once on ObservationGroups object construction):

>>> print(obs_groups.obs_groups_table)
GROUP_ID ZENITH_MIN ZENITH_MAX N_TELS
            deg        deg
-------- ---------- ---------- ------
       0        0.0       30.0      3
       1        0.0       30.0      4
       2       30.0       40.0      3
       3       30.0       40.0      4
       4       40.0       50.0      3
       5       40.0       50.0      4

TODO: make this a real example … use the four Crab runs!

Apply the observation grouping to an observation list:

>>> obs_table = obs_groups.apply(obs_table)
>>> print(obs_table)

This would print an observation table with the format described in Observation index table and an extra-column at the beginning specifying the ID of the group to which each observation belongs.

Get the observations of a particular group and print them:

>>> obs_table_group2 = obs_groups.get_group_of_observations(obs_table, 2)
>>> print(obs_table_group2)

This would print the observation table corresponding to the group with ID 8.