>This is a little bit confusing, since we normally think of the flux in a GSObject as being photons, so I'd recommend adding the following to help try to make it a bit clearer (assuming I got it right -- please correct if not):

>Typically, the SED describes the flux in photons per nanometer of an object with a particular 
magnitude, possibly normalized with the method `sed.createWithFlux` (see the docstrings in 
the SED class for details about this and other normalization options).  Then the `flux` attribute 
of the GSObject should just be the _relative_ flux scaling of the current object compared to 
that normalization.  This implies (at least) two possible conventions.
1. You can normalize the SED to have unit flux with `sed = sed.createWithFlux(bandpass, 1.0)`.  
Then the `flux` of each GSObject would be the actual flux in photons when observed in the given 
bandpass.
2. You can leave the object flux as 1 (the default for most types when you construct them) and 
set the flux in the SED with `sed = sed.createWithFlux(bandpass, flux)`.  Then if the object had 
`flux` attribute different from 1, it would just refer to the factor by which that particular 
object is brighter than the value given in the normalization command.

I inserted your text, which looks right.

>Also, I'm not wild about the createWith names. I think these might be better as normalizeFlux and normalizeFluxDensity. Similar to my comment above about the other create method. It seems overly verbose when all similar methods returna new object. No need to be explicit about that in the name of the method.

I'm happy to nix the `createWith*` names.  I'm slightly torn though between `normalizeFlux`, and just `withFlux`, as I think the latter reads better, but breaks the use-a-present-tense-verb pattern.  Below you suggest `atRedshift` for the SED class.  Do you prefer this to `setRedshift`?