Confirm that we can tilt the camera slightly and get a zero phase shift

[raacampbell]

If we are to obtain accurate absolute measurements of phase then we need to be able to take an image with a known curvature and re-use it later under different conditions. This will require the images to overlap as much as possible and so a translation correction will have to be made between the reference image and the current test image. We therefore need to check that this doesn't lead to any odd effects.

The test will be:

• Take a reference image
• Tilt the camera slightly
• Perform a rigid translation correction of the test image to the reference
• Calculate the phase shift. It should be negligible.

[raacampbell]

All this works in theory (7ddcb9301db038cf753861316322a1bd6885b322) but I need to test it practice as well.

[raacampbell]

I think this is trustworthy.