distance limit in find_sources()

[jballet]

The find_sources() functionality of fermipy can take a very long time.
I would like to be able to shorten that time by restricting the area over which the search is carried out.

One possibility would be to define a maximum distance from the center of the RoI, inside which peaks are considered. This would then be an argument of find_sources() itself.

Since find_sources() works on the output of tsmap(), an alternative would be to restrict the area over which the TS map is computed. In that case, I presume the area would need to be a square rather than a disk, but the overhead due to the corners should not make a large difference.
This would then be an argument of tsmap() instead.

I do not know the fermipy code enough to volunteer to carry out this development myself.

[ndilalla]

Hi @jballet, I went through the code and I saw that this feature is already available. The SourceFind class has an option argument called search_minmax_radius (which is a 2-element list [min_radius, max_radius] in degrees) that limits which candidate sources are actually added during find_sources algorithm. It only has an effect when the other option search_skydir (which is a SkyCoord) is set.

The working principle is the following:

• For each candidate source, the code computes the angular separation (in degrees) from search_skydir to the source position.
• That separation is then checked with utils.apply_minmax_selection(separation, search_minmax_radius) and the candidate source is kept only if separation >= min_radius (or if min_radius is None, no lower cut), and separation <= max_radius (or if max_radius is None, no upper cut).
• If the separation fails this range, the source is ignored and you get a log message like: "Source … outside of search region. Ignoring it."

To use it, you need then to set both search_skydir and search_minmax_radius to define the search annulus.

Example (config):

sourcefind:
  search_skydir: !astropy.coordinates.SkyCoord
    ra: 83.0
    dec: 22.0
    unit: deg
  search_minmax_radius: [0.1, 2.0]   # only add sources 0.1–2 deg from that point

Example (keyword arguments)

gta.find_sources(
    search_skydir=SkyCoord(83.0, 22.0, unit='deg'),
    search_minmax_radius=[0.1, 2.0]
)

[jballet]

Excellent!
We will test that.
I guess that my request should be turned into a documentation issue then.