>(a) not something that needs to be done now -- better to wait until there is a specific use case for it that might inform how it should be designed, 

Off the top of my head:

1. To allow the user to pad with noise fields with other than Gaussian distributions in their values, in a general way.

2. Putting in real correlated noise from HST images and testing that the output galaxies are statistically indistinguishable from those using a self-generated field.

3. To allow the user to pad with noise fields that are not necessarily rectangular, e.g. trimmed at the corners to be circular.  This makes some sense if we know we are going to be randomly rotating the objects and want to simplify the problem of knowing later on which pixels in the final image have a correlated noise contribution to them.

4. To allow padding with a constant background, or absolutely anything they wish.  I don't see the problem.

These are all possible right now (not counting user preparation of course, but that's assumed) if the `__init__` constructor takes an `Image` for `noise_pad` (or whatever) and just sends it directly to `pad_image`.