[X] My Assistant

[JRehling]

I'm simply going to share a personal anecdote whose main point will be of no surprise to those with similar experiences:

I've tasked myself with making a lunar map using imagery that I myself gathered with a small telescope and a digital camera. In principle, this can be done with one image of the full Moon, but my goal was to get imagery with the terminator located at all longitudes across the lunar face, so that the merit of shadows revealing topography could be enjoyed. It is easy to find maps of this kind, but I thought it would be fun to make my own. And it is fun, but it's also a hell of a lot of work.

I had the extraordinary luck of clear California skies 13+ nights in a row precisely when I decided to begin the project. I got full-disk images 11 nights in a row, and then "all" I had to do was process the results. The process is still going.

Projecting a global image into a cylindrical map is extraordinarily sensitive, at the margins, upon having the geometry just right. (The pixel representing a pole becomes a line as wide as the map, whereas the pixel right next to it becomes something much smaller.) But despite clear skies, I had much more than one pixel of distortion across my images due to imperfect seeing. (That's an issue that spacecraft don't have to worry about, but they have other issues, such as motion.) This made it challenging to register my global images with adequate precision.

My whole pipeline, now in progress, is like this:

1) Take several images of the Moon.
2) Montage them into a global view for that night. (This is already challenging when seeing is bad.)
3) Geometrically align the global image into a view my map-projecting software (which I wrote) expects.
4) Using data on that date-time's lunar libration geometry, project it into a cylindrical map.
5) Resize and reorient this to "pin" it to an existing cylindrical map from LRO. This compensates for some error.
6) Select strips near the terminator from the 11 pinned maps.
7) Adjust brightness across the strips to achieve consistent illumination throughout.

I'm on (5) now, and may hit more unpleasant surprises as I go. The best I can achieve will still distort the positions of some features, but I hope to get a pretty attractive and pretty accurate map with about 2.5 km/pixel resolution covering about ±70° from (0°, 0°).

I could potentially expand this into a more detailed map, but the burdens would only increase.

I also hope to generate a map of Mars during the 2016 season next spring. That will be a much coarser-resolution product, but also a lot less post-processing work.

[Bjorn Jonsson]

Very interesting project. One possibility to compensate for the atmospheric distortions near the limb might be to warp the images to the 'correct' shape before reprojecting them (if the correct shape can be determined to high accuracy).

An additional problem that can get severe very near the limb is topography if you assume that the Moon is spherical or an ellipsoid. This is probably not a problem for the Moon unless the images are of really high resolution. In contrast, for bodies like Mimas (and especially Vesta and more irregular bodies) it is usually necessary to reproject the images onto a 3D shapemodel of the target.

[JRehling]

I think one of my most significant flawed assumptions was that the distance from the Earth to the Moon is essentially infinity. It's not, and so the limb isn't exactly at 180° from the sub-Earth point. This is most evident when a very thin crescent is observed, and the limb is illuminated somewhat less than 180° around.

The problem introduced by imperfect seeing is that, even though I shot video, no frame has an undistorted image of the whole Moon. Horizontal shifts in position occur continuously, like waves rippling across a surface. To get even one good picture of the Moon on a night like that, I'd need to perform elaborate composition of portions of different images. On ideal nights, this doesn't happen to a visible extent, but it probably take me years to gather all the imagery on perfect nights!

For a smaller object like Saturn, occasional frames turn out perfect even on a night with poor seeing, because the whole planet ends up being shifted in about the same way at the same time. (Although the vast majority do not.) But the Moon is far too big for that.

Gerald, that's an outstanding technique! I could probably benefit from that.