GALAPAGOS: From Pixels to Parameters
  To automate source detection, two-dimensional light-profile Sersic modelling
and catalogue compilation in large survey applications, we introduce a new code
GALAPAGOS, Galaxy Analysis over Large Areas: Parameter Assessment by GALFITting
Objects from SExtractor. Based on a single setup, GALAPAGOS can process a
complete set of survey images. It detects sources in the data, estimates a
local sky background, cuts postage stamp images for all sources, prepares
object masks, performs Sersic fitting including neighbours and compiles all
objects in a final output catalogue. For the initial source detection GALAPAGOS
applies SExtractor, while GALFIT is incorporated for modelling Sersic profiles.
It measures the background sky involved in the Sersic fitting by means of a
flux growth curve. GALAPAGOS determines postage stamp sizes based on SExtractor
shape parameters. In order to obtain precise model parameters GALAPAGOS
incorporates a complex sorting mechanism and makes use of modern CPU's
multiplexing capabilities. It combines SExtractor and GALFIT data in a single
output table. When incorporating information from overlapping tiles, GALAPAGOS
automatically removes multiple entries from identical sources. GALAPAGOS is
programmed in the Interactive Data Language, IDL. We test the stability and the
ability to properly recover structural parameters extensively with artificial
image simulations. Moreover, we apply GALAPAGOS successfully to the STAGES data
set. For one-orbit HST data, a single 2.2 GHz CPU processes about 1000 primary
sources per 24 hours. Note that GALAPAGOS results depend critically on the
user-defined parameter setup. This paper provides useful guidelines to help the
user make sensible choices.
