Incorrect (col, row) returned for asteroid 4077 in sector 2

[barentsen]

Asteroid 4077 in sector 2 was reported by @christinahedges to display a strange kink in its (col, row) trajectory, which is almost certainly incorrect because its (ra, dec) trajectory on the sky is a straight line. It looks like the WCS transform can yield incorrect (col, row) values after an object has moved off the ccd.

I'm in the process of debugging this and adding a unit test.

Example bug:

[barentsen]

I'm somewhat stuck with this issue because it looks like the FFI WCS truly thinks the straight (ra, dec) trajectory of the asteroids translates into a weird curved path in (col, row) pixel coordinate space.

Example:

[christinahedges]

Yikes, this seems interesting!
What happens if you plot an image of the X, Y to RA, Dec, conversion something like:

Y, X = np.mgrid[:2048,:2048]
ra, dec = wcs.all_pix2world(np.vstack([X.ravel(), Y.ravel()]).T, 0)

plt.imshow(ra)
plt.imshow(dec)

Can you see that it looks funky in the corners?

[barentsen]

Thanks, that's a good idea!!

It looks like the RA=0 meridian runs through the image. This may be a clue...

[barentsen]

Update: tess-point agrees with the weird curve in the trajectory, so I'm really confused now...

[barentsen]

I am starting to think that there is no bug in tess-locator, because I am able to reproduce the funny kink in the trajectory using tess-point. My leading suspicion is that the stretch on the Y limit in the screenshot posted at the top of this issue made led me astray into thinking there is an obvious error in the WCS transform, but in reality, the kink is very subtle and visible in both (ra, dec) and (col, row) space.

Example: