Thursday, July 2, 2015

Mid-term summary

Mid-term has arrived and quite some work has been done for Gammapy, especially in the observation, dataset and background modules. At the same time I have learnt a lot about Gammapy, Astropy (especially tables, quantities, angles, times and fits files handling), and python (especially numpy and matplotlib.pyplot). But the most useful thing I'm learning is to produce good code via code reviews. The code review process is sometimes hard and frustrating, but very necessary in order to produce clear code that can be read and used by others.

The last week I have been working on a method to filter observations tables as the one presented in the figure on the first report. The method is intended to be used to select observations according to different criteria (for instance data quality, or within a certain region in the sky) that should be used for a particular analysis.

In the case of background modeling this is important to separate observations taken on or close to known sources or far from them. In addition, the observations can be grouped according to similar observation conditions. For instance observations taken under a similar zenith angle. This parameter is very important in gamma-ray observations.

The zenith angle of the telescopes is defined as the angle between the vertical (zenith) and the direction where the telescopes are pointing. The smaller the zenith angle is, the more vertical the telescopes are pointing, and the thinner is the atmosphere layer. This has large consequences in the amount and properties of the gamma-rays detected by the telescopes. Gamma-rays interact in the upper atmosphere and produce Cherenkov light, which is detected by the telescopes. The amount of light produced is directly proportional to the energy of the gamma-ray. In addition, the light is emitted in a narrow cone along the direction of the gamma-ray.

At lower zenith angles the Cherenkov light has to travel a smaller distance through the atmosphere, so there is less absorption. This means that lower energy gamma-rays can be detected.

At higher zenith angles the Cherenkov light of low-energy gamma-rays is totally absorbed, but the Cherenkov light cones of the high-energy ones are longer, and hence the section of ground covered is larger, so particles that fall further away from the telescopes can be detected, increasing the amount of detected high-energy gamma-rays.

The zenith angle is maybe the most important parameter, when grouping the observations in order to produce models of the background.

The method implemented can filter the observations according to this (and other) parameters. An example using a dummy observation table generated with the tool presented on the first report is presented here (please click on the picture for an enlarged view):
Please notice that instead of the mentioned zenith angle, altitude as the zenith's complementary angle (altitude_angle = 90 deg - zenith_angle) is used.
In this case, the first table was generated with random altitude angles between 45 deg and 90 deg (or 0 deg to 45 deg in zenith), while the second table is filtered to keep only zenith angles in the range of 20 deg to 30 deg (or 60 deg to 70 deg in altitude).

The tool can be used to apply selections in any variable present in the observation table. In addition, an 'inverted' flag has been programmed in order o be able to apply the filter to keep the values outside the selection range, instead of inside.

Recapitulating the progress done until now, the next steps will be to finish the tools that I am implementing now: the filter observations method described before and the background cube model class on the previous report. In both cases there is still some work to do: an inline application for filtering observations and more methods to create cube background models.

The big milestone is to have a working chain to produce cube background models from existing event lists within a couple of weeks.

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