Implement flexible sky model

[dkirkby]

We currently have fixed sky surface brightness lookup tables for "dark", "grey" and "bright" conditions. This feature request is to synthesize a more realistic sky based on the observing conditions, taking into account the relative positions of the observing field, the moon, the sun, the galaxy, etc.

[moustakas]

Another useful dimension would be all of the above (but especially grey & bright conditions) with cirrus. This is something the BGS Working Group has discussed, with the interested parties being Connie Rockosi and Jim Annis. Jim in particular sent around some broadband sky measurements as a function of moon phase and transparency (I think....) which might be useful. Besides BGS, this added realism would also be very relevant for the (bright-time) Milky Way survey group.

[dkirkby]

I guess cirrus contributes to the extinction and emission (via scattering) and imprints some degree-scale structure over the FOV? That sounds complicated, but its good to have it on the list if we are planning to observe in cirrus conditions.

[dkirkby]

@moustakas Is Jim Annis' cirrus sky info online somewhere?

[moustakas]

I don't believe so; I will reach out to him.

[crockosi]

There is an implementation of the Krisciunas & Schaefer (1991) lunar model in the idl version of quicksim. The output is a solar spectrum scaled to give the surface brightness moon value predicted by the model. I've verified that the code reproduces the K&S 1991 data more than 30 degrees from the moon. The code is in desimodel/pro/lunarmodel.pro

[dkirkby]

Here is a public link to the IDL code and the K&S paper is here.

[sbailey]

To make it an explicit part of the ticket: this flexible sky model should include the airmass dependency of the airglow, even when the moon is down. We should get this in before using specsim for the full survey data challenge.