Welcome to legacysim’s documentation!¶
User documentation
Developer documentation
Introduction¶
legacysim (previously Obiwan 1) is a Monte Carlo method for adding fake galaxies to Legacy Survey 2 images, and re-processing the modified images with legacypipe. The legacypipe documentation is available here: legacypipe docs 3.
What for?¶
Targets for spectroscopic follow-up are selected among the sources detected by legacypipe. The target density includes cosmological clustering signal (to be measured) but is also impacted by so-called “imaging systematics”, due to the telescope, the opacity of the atmosphere, extinction and dust of the Milky Way, bias and variance of legacypipe. These systematics can be (partly) removed by regressing the target density against photometric templates (linear model, or neural nets 4), but these methods can only remove dependence of the target density on known systematics.
legacysim rather forward models the source detection and target selection processes, by injecting fake galaxies into raw images, running legacypipe and applying the target selection colour cuts.
legacysim was applied on eBOSS ELGs 5.
Quick start-up¶
For a quick start-up on NERSC, see Example on NERSC.
Acknowledgements¶
See the offical acknowledgements for the Legacy Survey.