LSSTScienceCollaborations / ObservingStrategy

A community white paper about LSST observing strategy, with quantifications via the the Metric Analysis Framework.
59 stars 75 forks source link

A metric for star/galaxy separation #81

Open ctslater opened 9 years ago

ctslater commented 9 years ago

Much of the Galactic structure science (with non-variable stars) will be limited in depth by our ability to separate stars from small unresolved galaxies. This is mostly dependent on seeing for morphological s/g separation and photometric precision for color-based s/g separation. We would like to be able to translate the conditions of numerous epochs into a limiting depth at which we experience e.g. 10% or 50% contamination by galaxies.

Suggestions for how to perform this exercise are welcome. My first thought is that this will likely require modeling how we plan to perform s/g separation (psf minus aperture mag is probably not the Theoretically Optimal solution), including a realistic background population of galaxies. We will probably want individual metrics for morphology and color separation.

ivezic commented 9 years ago

I have image simulations (based on gaussians, but good enough for this context) that I used to measure performance of various star-galaxy separators (including the SDSS method and the optimal Bayesian method). I also have a decent recipe for calculating galaxy size distribution vs. magnitude. I will cook up a metric that will give what you asked for: what fraction of galaxies are resolved at X% confidence level as a function of magnitude and achieved SNR at that magnitude. This morphological result can then be combined with color-based and other schemes. If we did this together, we'd reach the end result much faster, let's talk!

ctslater commented 9 years ago

Sounds perfect, we will definitely work on this back at UW.

dnidever commented 9 years ago

I have some good SMASH DECam data that you could use to play around with stellar locus cuts.

drphilmarshall commented 9 years ago

@ctslater I think @ivezic is offering to stay up all night pair coding with you. Carpe diem! :-)

drphilmarshall commented 9 years ago

@ctslater @ivezic How did the star-galaxy metric writing go in the end? It seemed like this would be relevant to the WL group ( @jmeyers314 ) as well as the Milky Way satellites people ( @bethwillman ). Are there white paper sections just waiting for this metric to get finished?

ivezic commented 9 years ago

Yes, Colin is on top of it. We think we can have a decent metric on timescale relevant for this white paper!

drphilmarshall commented 8 years ago

The primary mention of "Star / Galaxy Separation" in the current version of the white paper is in the MW Halo subsection of the Milky Way chapter's Future Work subsection, 4.4.2. @willclarkson shoudl we punt this issue to WP version 2 via the Tucson 2016 milestone, or are you hopeful of promoting the MW halo section to the results part of the chapter in the next week or two? Thanks!

willclarkson commented 8 years ago

@akvivas can comment more, but at this stage yes I think this can be punted to Tucson. I don't think that evaluation of the star/galaxy metric is likely by Friday.

akvivas commented 8 years ago

agree. @ctslater

ctslater commented 8 years ago

Agreed. I have a metric that works, but it is basically unverified and needs to be tested against existing data. That process is going slower than I had hoped.

drphilmarshall commented 8 years ago

Cool! Arizona here we come :-) Good luck with the testing, Colin! We're rooting for you :-)