Intriguing features play hide-and-seek as the terminator crosses the lunar landscape

Whether you’re looking at the Moon through an eyepiece or via a camera sensor, the features you see will initially appear fairly static. But, if you pick a feature close to the terminator and monitor its appearance over 30-minute intervals, you will see changes. This stands to reason, because the view you get from one night to the next is different and changes must have occurred over the hours between views.

Each night the lunar terminator moves about 12˚ in longitude around the Moon’s globe. In this article we’ll look at how to capture a sequence of images to show the play of shadows around a lunar feature.

Look for high peaks

For the best results you’ll need to start observing a lunar feature when the Moon is visible for (at least) a few hours above the horizon. The Moon’s altitude needs to be 30˚ or higher for the most stable results. Try and pick surface subjects that have a good relief height as these will generate the most dramatic and fast-moving shadows. Look for flatter surrounding areas as these are better than complex, contoured ones for showing faster moving shadows.

Mountain ranges make excellent subjects for this as many border the relatively flat lava surfaces of adjacent lunar mares. High peaks will cast dramatic pointed shadows across a mare’s surface and these appear to move surprisingly fast, certainly quickly enough to show the sort of motion we are after here.

Choose a night which looks as if it will give you a few hours of clear sky. There’s nothing more annoying than having to terminate a sequence due to cloud or high haze. The latter will also affect the appearance of your results and, if creating an animation, will cause unwanted frame flickering which will detract from the overall effect.

Shadow movement animations need magnification for the best results. Low image scale shots can show shadows change over time, but it takes longer for the movement to be revealed. But a high-resolution, highly magnified image can show impressive shadow movement over a short time.

High frame rate captures work best for this, as they give the sharpest results. Use a high frame rate camera and capture around 1,000 frames for each shot. Process the results with your preferred registration-stacking software (eg AutoStakkert!) combining the final results into your animation.

Be sure to maintain a constant interval between sequences, or the shadow movement will appear to judder. Start with 30-minute intervals to give a decent number of frames to animate. If you like the end result, consider doing animations using shorter time intervals to give more frames.

Equipment: a telescope fitted with a high frame rate camera, plus a polar aligned, driven tracking mount

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Step by step

STEP 1

The best results will come from using a telescope with a high frame rate imaging camera. If using an infrared sensitive mono camera, consider fitting an infrared pass filter (pictured), which will help stabilise the view, delivering sharper results. Use a polar-aligned, driven equatorial tracking mount.

STEP 2

While low image scale is easier to capture, it’s less adept at showing shadow movement over short time intervals. High image scale requires more imaging skill and higher telescope precision, but even short capture periods should show good movement. A focal length of 4,000mm or more is recommended.

STEP 3

Ideally, select a night when the Moon will be higher than 30° altitude for a few hours and the weather forecast shows good clarity for that period. Your capture attempts should be taken under dark-sky conditions to avoid contrast variation. A gap of 30 minutes between shots is best to start with.

STEP 4

Choose a relief target near the terminator, preferably showing a lot of height variation, such as a mountain. If the Moon is waxing the target slowly reveals over time. If waning, the target slowly disappears. Rotate the camera so slewing in RA (Right Ascension) moves the target parallel to the bottom edge of the imaging frame.

STEP 5

Re-centre the target and focus accurately. Next, make a note of features near the corners of the frame. These can be used to re-frame should the telescope drift position between captures. Adjust camera settings to optimise exposure while maintaining a decent frame rate. Capture 1,000 frames and process the result.

STEP 6

Repeat and process after each capture. When the run is complete, load all the frames in order into a layer-based editor, one result per layer. Align all layers and create an animated sequence if your software allows animation. Or save the frames individually and use a program like PIPP to create a sequence.

Send your images to: gallery@skyatnightmagazine.com

ALL PICTURES: PETE LAWRENCE