Reticles are off for non-square thumbnails posted to SP in broker mode

[dmitryduev]

We need a way to handle this situation:

These are events detected close to the CCD edge. They are always centered on the longer side. We need to use the (x, y) position of the transient to pad the shorter side in such a way that it always lands in the center.

A nice idea from @stefanv: make the thumbnails transparent where there is no data!

The relevant code lives here: https://github.com/dmitryduev/kowalski/blob/master/kowalski/alert_broker.py#L434.

[stefanv]

What information do we have available from the alert?

The latest thumbnails shown at https://fritz.science/alerts/ztf/ZTF21aamqgid seem to be padded.