Weird behaviour of METIS WCU with and without pinhole mask

[oczoske]

What happened?

The primary concern is with the background level of the focal-plane mask (FPMask, e.g. a pinhole mask) compared to the open case without any light source. As the temperatures of the integrating sphere and of the pinhole mask are the same, they should give similar background levels (a slight difference may be due to differences in emissivity). With the short-L filter (chosen because it encompasses the 3.39 um laser line, although that is not important yet) the image planes have the following median values with all lamps switched off:

fp mask	median
open	384062.8
pinhole	22836.4

Both the integrating-sphere temperature (for "open") and the WCU temperature (for the pinhole mask) are at 300 K, so should give roughly the same background.

Maybe the integrating-sphere magnification acts on the background as well? That would be a misapplication of the model beyond its validity, as the integrating sphere without input is certainly not brighter than its surroundings.

Code to reproduce

cmd = sim.UserCommands(use_instrument="METIS", set_modes=["wcu_img_lm"])
cmd['!OBS.filter_name'] = "short-L"
metis = sim.OpticalTrain(cmd)
metis['wcu_source'].set_lamp("off")   # only look at background levels

metis['wcu_source'].set_fpmask("open")
metis.observe()
print(np.median(metis.image_planes[0].data))   # -> 384062.8

metis["wcu_source"].set_fpmask("pinhole_lm")
metis.observe()
print(np.median(metis.image_planes[0].data)) # -> 22836.4