How Many Frames Should a Planetary Stack Keep?

Every planetary tutorial gives a number. Keep the best 25%. Keep the best 10%. Keep 500 frames. The numbers disagree with each other because they were each right for the capture the author had in front of them, and nobody says which kind of capture that was.

The honest answer is that the right fraction is a property of your capture - specifically of how quickly the frame quality falls off from the best frame to the median one - and that this is something you can measure rather than guess. This post shows the measurement on two real captures, because they came out differently and the difference is the whole point.

What you are trading

Lucky imaging works because the atmosphere is not equally bad at every instant. Keeping only the sharpest frames buys resolution. Averaging more frames buys signal-to-noise. Those pull in opposite directions: every frame you add past the sharpest few is a little softer than the ones before it, and averaging a soft frame into a sharp stack does not average away - it low-passes the result, exactly as a blur would.

So there is a knee. Above it, adding frames costs more sharpness than it returns in noise. Below it, the stack is grainy and the wavelet sharpening amplifies the grain. Where the knee sits depends on how the quality curve is shaped, and the quality curve is shaped by the seeing.

Two captures, graded

Akastroid grades every frame - fine-scale structure relative to the frame’s own noise, so that a noisy frame does not score as sharp - and normalises the scores so the best frame is 100. Here is where the top of each capture’s ranking sat:

Fraction kept	Jupiter, steady air (11,473 frames)	Saturn, rough air (17,934 frames)
top 1%	91	72
top 5%	85	62
top 10%	82	56
top 25%	75	48
median frame	65	40

Read the rows. On the Jupiter night, the 25th-percentile frame scored 75 - three quarters as sharp as the best moment of the evening. On the Saturn night, the 25th-percentile frame scored 48, and the median frame 40. A “best 25%” stack of that Saturn is averaging in frames less than half as sharp as the best ones.

The stacks

Each capture was stacked at 25%, 12% and 6%, everything else identical, and the results measured for edge energy - how much fine detail survives at the belt and ring boundaries - and for residual noise on the disc.

Jupiter. Indistinguishable. Same edge energy, same noise, within measurement error at all three fractions. When the top quarter of the capture is nearly as good as the top frame, it does not matter much where you cut.

Saturn.

kept	frames	edge detail	noise
25%	4,484	0.38	1.50
12%	2,152	0.41 (+8%)	1.56 (+4%)
6%	1,076	0.45 (+18%)	1.73 (+15%)

Going from a quarter to an eighth bought 8% more edge detail for 4% more noise - a trade worth taking, and the noise increase is smaller than what the sharpening adds anyway. Going on to a sixteenth bought more detail again but at a noise cost that starts to show on the globe. The knee is around 12% for that capture, and it is not close to 25%.

What Akastroid does with this

The keep fraction is read off the capture’s own quality curve. The rule: keep the frames that are within a quarter of the best score (the best being the 99th percentile, so one freak frame does not set the bar), never more than the conventional quarter of the capture, never fewer than a stack’s worth of frames.

On the Jupiter capture that rule keeps 25%. On the Saturn capture it keeps 12.8%. The number is shown next to the selection so you can see it, and the slider still does what it always did if you disagree.

It is worth saying what the rule does not do. It does not keep 5% of a steady-air capture because someone once said 5% - on a steady night the extra frames are nearly free and the noise reduction is real. And it does not keep 25% of a rough-air capture out of habit, because on that capture a quarter of the frames are not lucky.

How to read your own capture

The frame browser shows the graded frames in order. Scrub from the best frame toward the median and watch the belts. On a steady night the first few hundred look alike and the softening is gradual. On a rough night the drop is visible within the first hundred, and by the median frame the planet is a smear.

That is the quality curve, seen rather than plotted, and it tells you the same thing the rule reads from the numbers: how deep into the capture the luck actually extends.

The short version

• The right fraction depends on how steeply your capture’s quality falls off, which the seeing decides and you can measure.
• Steady air: a quarter is fine, and cutting harder gains nothing.
• Rough air: a quarter averages in soft frames; somewhere around an eighth is where the trade stops being free.
• Akastroid reads the fraction from the curve and shows it. Override it if your eye disagrees with the number.