Incorrect (TBC) read noise definition in Nemati.py optical class

[jruffio]

In Nemati.py optical class, the read noise appears to be defined as a variance instead of a standard deviation. This will make the config file inconsistent with typical expectations for read noise.

readout noise

_C_rn = Npix * inst["sread"] / texp

this is then simply added to other photon noise counts.

[dsavransky]

@CoreySpohn can you take a look please?

[CoreySpohn]

Sure, I was talking to JB earlier today about this. The basic idea is that the sread parameter in EXOSIMS is $\left(\sigma_\text{rd} / G\right)^2$ where $\sigma_\text{rd}$ is the read noise standard deviation and $G$ is the gain. Generally detectors will report $\sigma_\text{rd}$, so sread looks incorrect for people familiar with the detector parameters. So the fix here would be for an observing mode to define det_gain and det_read_noise_std. Alternatively we could allow a user to define either sread or det_gain and det_read_noise_std, then calculate sread during optical system processing of the observing modes.