Noisy footprints in some spectral channels after gridding

[rfriesen]

Need to include a mechanism to check for noisy scans in reduction pipeline.

[low-sky]

Maybe this will do?

[rfriesen]

Certainly looks reasonable! We can re-run one of the DR1 sets that shows the issue through the gridder and see if that solves it.

[low-sky]

Okay, I (or someone else) can PR this snippet in soon (I'm on the way out the door). Do you have a region in mind?

[rfriesen]

All the (3,3) cubes in the DR1 regions show this issue, so probably NGC 1333 is the smallest? Doesn't need to be done today, I just wanted to post this since you had mentioned you had a possible solution! I'm happy to look into it, later.

[rfriesen]

I'm testing the Tsys check as implemented in the KEYSTONE pipeline, but find the same issue in the resulting L1688 NH3 (3,3) cube. It's often (but not always) an entire footprint that has a strong positive or negative offset in a single velocity channel (see image).

@jpinedaf , didn't we see something similar in one of the Orion A footprints? How did you resolve that?

[low-sky]

Thanks, @rfriesen

I've definitely seen evidence for this as well. Do you happen to have a session / footprint number that has the glitch? I can track it down, but if you already know, it would make it faster.

The issue appears to be a high spike in part VEGAS. It should be flaggable in the time series. Since we pass to the FFT domain for spectrum shifting, there could be a flagging step there as well.

[rfriesen]

@low-sky
I just took a look and the bad footprints for L1688 seem to all come from specific sessions:
The large negative features are in L1688-3,-6, and -9, observed in Session 42.
Positive/noisy features are in L1688-5 and -8, and were observed in Session 15.

[low-sky]

👍

Thank you. I'm working on GBT software stacks now.

[low-sky]

Addressed in #158

[low-sky]

Some progress: spikes are reduced but not eliminated in this approach. Baselines appear a lot better. This does a spectrum space flagging. I'll work on FFT flagging now that the grid-code refactor is approaching usability.