Summary of OH Airglow/Cloud Signatures in Background Median Plots

The background median plots provide a useful, if qualitative, measure of the sky conditions in each scan, often allowing one to catch problems not visible even in jump counters. Here I have collected a number of illustrative images to show the kinds of things I look for.

About the plots

One can view scan backgrounds from the root night level in pipeline processed data by using the script: 
/home/hurt/bin/bkg NNN
where NNN is the scan number, or you can see all scan backgrounds sequentially using: 
/home/hurt/bin/bkgflash
Note that each plot shown below uses the same relative absolute ranges at J, H, & Ks (150, 700, 300 DN respectively) so the ranges (but not the absolute y values) are directly comparable to one another.
 

Normal backgrounds

As a reference image I include a typical normal boring background. "Normal" backgrounds can also include slow ramps and gradients, but generally not very much structure. It is possible to have steep gradients that do not create background artifacts so long as there isn't much wiggle to them.

OK background
 

OH Airglow

The following plots illustrate typical OH airglow signs. The first couple show "slow" variations, which can create artifacts if they are as strong as these. If the amplitude of variation is smaller than the first example then it usually doesn't leave many H background artifacts.

airglow slow
airglow slow

This next one shows very small amplitude variations but at a very high frequency. This is enough to trigger the jump counters in this scan and to create some stair-stepping artifacts in the background. Generally, the more rapid the change, the smaller the amplitude needs to be to cause problems

airglow fast
 

Clouds

Clouds have distinctive signatures very different from airglow (and often from one another). Below are several typical fields exposed to moving clouds. Note that in these standard stretches, J, H, & Ks are NOT varying in tandem with one another. Usually K shows larger jumps, relative to the absolute y ranges on these plots (this makes it easy to pick out).

clouds
clouds

In the following plot, the signs of clouds are quite weak, but the characteristic K spiking is seen and can often cause background problems.

clouds

The next one is interesting because of an H band drop as Ks jumps.

clouds

In the next two plots, cloud signatures are also seen, but in this case they appear to be cirrus on a winter night with a full moon, so the J band effects are most prominant.

clouds

clouds

The next plot shows a different kind of background animal, thus far noted on nights with a nearly-full moon. It leaves no significant artifacts in the backgrounds of the coadds and thus far we have treated it like airglow (even though the association with the moon might suggest a more cloudlike nature).

clouds

Robert Hurt
Last updated: 11/6/98