[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.

[scalbers]

I think with the scale of images you'd likely be working with that the error introduced in lunar images wouldn't be too bad. However it is possible to correct for this with the right equations added to the remapping procedure. I had used a reference called "Map Projections: A Working Manual" (1987) to learn about this.

When the angular diameter of the target increases to around 3 degrees this becomes more noticeable in cases I've worked with.

[JRehling]

Thanks for sharing your wisdom, Bjorn and Steve, in addition to those who commented before!

I'm en route to producing a passable map, but I can already see how a better version can result if I go back to scratch.

It's remarkable how much could go into making a good lunar map from telescopic images if one really pursued the project to its full extent. It would take mosaicking hundreds of images, for starters, whereas I'm trying to get away with only 35 or so (about 3.5 per night for 10 or 11 nights). And then very careful sampling of the frames to compensate for imperfect seeing. And then all of the other wisdom that people have shared.

[JRehling]

After much tinkering, I've realized a passable (to me) version of a map of the lunar nearside, made with my own imagery.

As mentioned earlier, the challenge was to composite many images so that locally, areas in morning shadow were used almost everywhere, making a much nicer product than one gets from projecting a full Moon onto a plane.

The challenges were many that are familiar to people who process space images, with some added bother due to atmospheric conditions. And, incidentally, a telescope so old that you couldn't even import things from China when it was made.

Here is a version jpg'd to fit on the board, with gridlines at 30° increments. Another version with major features labeled is in the works. Thanks to everyone for the tips offered earlier this year.

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