# What are Stokes parameters for galaxy shapes, how do they compare to Stokes parameters in optics?

I remember Stokes parameters as a way to completely define the polarization state of a well-defined beam of light. However several questions here refer to Stokes parameters used in characterizing the morphology of galaxies.

In Optics it's $$S_0, S_1, S_2, S_3$$ and in Astronomy it's $$Q, U$$.

I'm guessing that these are not exactly the same set of parameters. but there might (or might not) be some connection either historically or in the way some observations were made.

1. How did they both end up being called Stokes parameters? Is there a relationship?
2. How are they calculated (equation and/or short explanation please, not just a link)

Related questions:

• There doesn't seem to be an appropriate morphology tag – uhoh Mar 1 at 5:22
• Can you give a reference for Stokes parameters (which refer to polarisation) being used to characterise galaxy shapes? Ah ok, I see one. It is just a way of characterising ellipticity (in both cases). – Rob Jeffries Mar 1 at 6:32
• @RobJeffries I thought that might be the case; it would be quite a coincidence if two different people named Stokes worked on two different ellipticity problems and came up with two different sets of parameters which were both independently named after them. – uhoh Mar 1 at 8:43