Photographing the Moon and Planets: Why Video Beats a Photograph

Point a 150mm telescope at Jupiter, take a photograph, and you get a small cream-colored disc with maybe a hint of a belt. Then you see what someone else got with the same telescope: the Great Red Spot, four distinct bands, and structure at the poles.

The difference is rarely the telescope, the camera, or twenty years of experience. They took a video and you took a photograph.

The atmosphere is the problem

At high magnification you are photographing a planet through roughly a hundred kilometers of moving air.

That air is not uniform. It has cells of slightly different temperature, and so slightly different refractive index, and they drift and churn on a timescale of tens of milliseconds. Each cell bends the light passing through it by a small angle. The image of Jupiter arriving at your sensor is being smeared, wobbled and reshaped by all of them at once.

Take a one-second exposure and you have averaged over maybe fifty different states of the atmosphere. The result is the blur of all of them. It will be soft however good the optics are.

The atmosphere is not equally bad at every instant, though. Every so often, for a fraction of a second, the air along your line of sight settles and the image snaps into focus. Anyone who has spent an evening at the eyepiece has seen it: mush, mush, mush, then for half a second the planet is there, sharp and detailed, and then it is gone.

Those moments are what you want. You cannot predict them, so you record continuously and find them afterwards.

Lucky imaging

Record a video of several thousand frames at a few milliseconds each. Grade every frame for sharpness, keep the sharpest, and align and stack those.

That is the whole technique, and it is why planetary imaging looks nothing like deep-sky imaging even though both involve stacking.

The proportions are unfamiliar if you come from deep sky. There you might reject 10% of 300 frames. Here you routinely keep 5% of 4,000 and throw away the rest. Those 200 frames were taken during still air; the other 3,800 were taken through soup, and averaging them in would drag the result back toward the mush.

How many frames to keep depends on the seeing. Older guides suggest keeping the best 5% to 25%; on the captures I measured, keeping far more than that gave the same sharpness with less noise. Akastroid keeps the frames that score close to the best, never fewer than half, and shows the grades so you can cut harder on a turbulent night. (Two real captures, measured.)

What you need

Less than you might expect.

A telescope, ideally with some focal length. Planets are small. 1000mm is workable and 2000mm is comfortable. A Barlow lens multiplies what you have, and planetary work is one of the few places where cheap magnification really helps.

A camera that shoots fast video. A dedicated planetary camera is ideal: small sensor, high frame rate, uncompressed output. But a phone held to the eyepiece has produced perfectly good lunar work, and any mirrorless camera shooting 4K video will do for the Moon.

Tracking is optional, strictly speaking. For the Moon at short focal length you can let it drift through the frame. For Jupiter at 2500mm you want a tracking mount, or you will spend the session re-centering. It does not need to be a good tracking mount, because errors that would ruin a 300-second deep-sky sub are irrelevant across 5 milliseconds.

You do not need a cooled camera, darks or flats. The Moon is bright, so thermal noise never gets a chance to matter.

This is the cheapest serious astrophotography there is. It is also the only kind that works from a city center, because a planet is bright enough that light pollution does not matter.

Recording, practically

Format. SER if your capture software offers it, uncompressed AVI otherwise. Not MP4: its compression throws away exactly the fine detail you came for, before you get a chance to grade anything.

Exposure. Short, in milliseconds. Watch the histogram and keep the brightest part of the planet around 70-80% of full scale. Clipping the bright limb loses detail you cannot get back.

Frame rate. As high as the camera and your storage allow. More frames means more chances at a still moment.

Duration. For the Moon, as long as you like. For Jupiter, keep it under about 90 seconds; it rotates fast enough that a longer capture smears surface detail. Saturn tolerates two or three minutes. Mars is about the same as Jupiter.

Focus. At high magnification focus is unforgiving, and it drifts as the tube cools. Refocus between captures. It has more effect on the result than anything else here, and it costs nothing.

Then grading, stacking and sharpening

These three steps are where the software does its work.

Grading means measuring sharpness on every one of those thousands of frames and ranking them. Akastroid shows you the ranking with a thumbnail of each graded frame. The difference between the best frame and the median one is usually startling, and it shows better than any explanation why the technique works.

Stacking aligns the kept frames and averages them. Planets drift and wobble between frames, so alignment is done per frame. Averaging 200 frames cuts noise by roughly a factor of fourteen, which is what lets the stack survive the next step.

Sharpening is where planetary images are made. Wavelet sharpening amplifies detail at chosen spatial scales, and on a stack of 200 good frames you can push it far harder than any single frame would tolerate. It is interactive and worth learning by feel: too little and you have wasted the stack, too much and you get bright rings around every edge. (What each slider does, and the mistake to avoid.)

Start with the Moon

If this is new to you, do not start with Jupiter. (The other daytime option is the Sun, which uses the same technique and has safety rules you cannot skip.)

The Moon is bright, large and forgiving, and it looks good at any focal length. A 60-second video of the terminator, the line between lit and unlit where the shadows are longest, will give you a first result that looks like the pictures that got you interested.

Jupiter is small, dim by comparison, needs accurate focus and only really rewards good seeing. It is the natural second target.

What this does not do

Multi-video derotation, where you record several videos over an hour and de-rotate them onto a common surface to get past the 90-second limit, is a real technique and Akastroid does not do it. If you get far enough into planetary work to need it, WinJUPOS is the tool, and it is free.

None of this will rescue a video shot out of focus either. Grading finds the best frames you took. It cannot find frames you did not take.

Once you have the frames, processing Moon photos in true color covers what to do with them. The mineral color is real, and it is already in your data.

Summary

• Seeing, not optics, limits planetary detail. Video beats a photograph because it samples the atmosphere thousands of times.
• Keep far more than the old 5-25%. Akastroid keeps at least half; cut harder only if the grades show a long tail of very soft frames.
• SER or uncompressed AVI. Never MP4.
• Under 90 seconds for Jupiter, because it rotates.
• Focus, then refocus between captures.
• Start with the Moon. It works the first time.