Functional dependence of sky brightness on altitude seems to be wrong

In DC1 we are seeing some very strange behavior of the sky brightness as a function of altitude. It actually seems to get darker as you go toward the horizon. Below is a description and an experiment showing the problem. This is a blocker to us using phosim to simulate image for DESC data challenges.

The following email was sent by Chris Walter to the phosim mailing list in January. We are now seeing this problem again in the context of producing simulated data for DC1 for the Dark Energy Science Collaboration.

[Reposted from an email by Chris Walter (with his permission)]

I made an instance catalog with a single centered star using the release version of PhoSim 3.6. This instance catalog came from Scott who used CatSim to produce a set of consistent variables.

—

rightascension 31.1133844
declination -10.0970060
mjd 59797.2854090
altitude 43.6990272
azimuth 73.7707957
filter 2
rotskypos 69.0922930
rottelpos 0
dist2moon 145.1095257
moonalt -11.1383568
moondec -18.7702120
moonphase 59.6288830
moonra 230.9832941
nsnap 1
obshistid 99999999
seed 161899
seeing 0.7613760
sunalt -59.1098785
vistime 15.0
object 1046817878020 31.1133844 -10.0970060 22.0 starSED/phoSimMLT/lte033-4.5-1\
.0a+0.4.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0635117705 3.1

—-

I used the following for a config file:

—-

backalpha 0.1
backbeta 4.0
backgamma 1000.0
backdelta 1.0
activebuffer 600

raydensity 0.0
clearperturbations

—-

So clear all perturbations (so the star is really at the center of the sensor), turn off cosmic rays, and speed up background generation. I have also removed the quick background commands as a check and it doesn’t change anything.

This all works, but the sky level I get is about 65 counts/pixel for the 15 second exposure. If I change the altitude (and nothing else) I can vary the background level up to about 350 at the default value of 89 degrees. The results look like this:

Altitude 5 degrees:   36
Altitude 10 degrees: 49
Altitude 25 degrees:  60
Altitude 45 degrees: 65
Altitude 65 degrees: 102
Altitude 89 degrees: 351
Altitude 160 degrees: 58

Now, I know you are allowed to give non-self consistent results to PhoSim in terms of all of the parameters so you can look through the earth or something if you are not careful but I don’t understand what is going on here. I was wondering if you could help me understand what is happening in this particular example.

1) It seems that ‘Altitude’ takes precedence over the Dec value is that correct?

2) What “correct” sky level do you expect in the r-band? Other calculations I have seen give numbers more in the 300-500 range which is why I am asking but maybe 60 is correct.

3) Why is the level decreasing as you go towards the horizon (brighter straight up)? One thing I can think of is the moon but I think it should be below the horizon by the commands. Is there some other cloud or sky glow effect that I am not considering?

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