Implement BRIGHT program exposure time model about desisurvey

[julienguy]

For the last point, one could use one of the redrock simulation runs.
For several definitions of SNR (r-band , around 4000A restframe ...):

• compute SNR on spectra of cframe files
• compute correlation coefficient of SNR with the redshift error reported by redrock

[moustakas]

I've generated a set of reference spectra (No 2, bullet two in @dkirkby's original post) for the BGS sample. See desihub/desisim#285 or just the notebook --
https://github.com/desihub/desisim/blob/bgs-refspec/doc/nb/bgs-reference-spectra.ipynb

The spectra themselves are at NERSC in
/global/project/projectdirs/desi/spectro/sim/bgs-refspec/bgs-refspec-v1.0.fits

Comments are welcome.

[dkirkby]

Here is a first look at the reference spectra provided by @moustakas (details in desihub/desisim#285).

The redshift distribution is reasonable (with some spikes due to realistic clustering). The magnitudes cover a range, so I am rescaling them all to DECam-r = 19.5 to define the "threshold" sample.

Here are the fiberloss fractions calculated for each target in the sample:

These are generally low compared with the DARK program targets because of the larger galaxy sizes. The strong correlation with redshift is because I scale disk and bulge sizes with DA(z), so closer galaxies have less overlap with the fiber:

disk half-light radius = 4.7" DA(0.1) / DA(z)
bulge half-light radius = 1.3" DA(0.1) / DA(z)

[moustakas]

I've begun characterizing the redshift efficiency of BGS targets under "nominal" observing conditions -- see desihub/desisim#286. This work is still preliminary because it's only based on 1000 spectra and also because the fiberloss fractions are not totally up to date with the ongoing work by @dkirkby (although I'm not sure in what sense). But it nevertheless gives a rough idea of how we should expect to perform with 5-minute exposures.

Comments are welcome here or at desihub/desisim#286.

[dkirkby]

The code and notebook used to develop this model is now checked into desihub/surveysim#56

[schlafly]

We've defined effective exposure times for the BRIGHT program, which is the extent to which desisurvey cares about this issue. I'm inclined to close. There's a PR to implement a better sky model in surveysim
desihub/surveysim#79
but that doesn't touch desisurvey directly.