definition of speed of light is not consistent within DESI

[londumas]

Doing the following simple grep on all the DESI packages, maybe there are other I have not thought of:

• grep '2\.99' -ri * --include \*.py
• grep '299' -ri * --include \*.py
• grep '3e5' -ri * --include \*.py
• grep '3e8' -ri * --include \*.py
• grep '299792458' -ri * --include \*.py
• grep '2\.99792458' -ri * --include \*.py

It shows that different definition of the speed of light is used in the overall desi packages.
It would be very useful to have every package use the same from scipy.constants import speed_of_light or c = 299792458.. This would allow consistency comparison and better definite results.
For Lyman-alpha this also has the possible impact of shifting the Lyman-alpha x QSO cross-correlation along the line-of-sight.

[edit] add option -i

[weaverba137]

astropy.constants would also work, but we should pick one or the other and stick with it.

Also, be sure to check for 3E5, or use grep -i!

[moustakas]

Nice catch.

In desitarget.mock.mockmaker we do

try:
    from scipy import constants
    C_LIGHT = constants.c/1000.0
except TypeError: # This can happen during documentation builds.
    C_LIGHT = 299792458.0/1000.0

[londumas]

The remaining, I could find, and I need to understand before changing are:

desi/code/desispec/py/desispec/fluxcalibration.py:    ZP_fnu = ZP_flambda * wave**2 / (2.9979e18)  # c in A/s
desi/code/specter/py/specter/throughput.py:        self._hc = 6.62606957e-27 * 2.99792458e8 * 1e10