STACKING

Photographers often describe a stacked photograph as “sharper”, but what is actually happening is more complicated. To understand it, separate these concepts:

Stacking improves the visibility of legitimate detail because random noise is reduced. For moon and planetary photography, stacking also allows software to select the moments when atmospheric turbulence temporarily produces the clearest view — a technique often known as Lucky Imaging.

The most important photograph in a stack is every photograph.

If the subject is badly out of focus, heavily blurred or incorrectly exposed, combining more photographs will not magically recover missing detail. Your individual frames should already be technically sound. Stacking takes good photographs and extracts more information from them — it does not turn bad photographs into good ones.

The Moon is extremely bright compared with most astronomical subjects, meaning photographers can use fast shutter speeds and capture large bursts of photographs. This makes the Moon ideal for stacking.

Focus is everything.

Do not simply rotate the lens to the infinity symbol and assume the image is focused. Infinity markings are not always perfectly calibrated and temperature can also affect focus.

Recommended method

More frames give the software more opportunities to find moments of excellent atmospheric seeing. However, quality matters more than blindly stacking every frame. Sometimes stacking the sharpest 20% of 500 frames will produce a considerably better result than stacking all 500.

Milky Way stacking works differently from Moon stacking because Earth rotates beneath the stars. Even when the camera remains completely stationary, the stars move slightly from one exposure to the next. Therefore, specialist software must align the stars before averaging them.

Do not trust the infinity mark.

The traditional 500 Rule: 500 ÷ focal length = approximate shutter speed in seconds.

But the 500 Rule was developed in an era of lower-resolution photography and is often too generous for modern high-resolution cameras.

The NPF Rule accounts for aperture, pixel pitch, focal length and the declination of the stars. When using modern high-resolution cameras, inspect stars at 100% — if stars appear elongated, shorten the shutter speed.

One of the strongest techniques is to treat the sky and landscape as completely separate photographic problems.

The sky moves. The Earth does not appear to move relative to your tripod. Therefore the optimum exposure for each is different.

Stack the sky separately. Process the foreground separately. Blend both carefully using masks.

Deep-sky photographers often measure total exposure in hours rather than individual shutter speed.

Multiple landscape frames can be stacked to reduce noise. Photograph a dark woodland scene from a tripod, capture 10 identical frames, align them and average the images. The resulting photograph may contain considerably cleaner shadow information than a single photograph.

This can be particularly useful for:

• —Blue hour
• —Woodland
• —Caves
• —Waterfalls
• —Architecture
• —Night cityscapes
• —Low-light landscapes

Yes — sometimes. Stacking works best when the subject remains extremely still.

Examples: owls perched motionless, sleeping animals, stationary reptiles, frogs, insects at rest, birds sitting calmly, macro subjects.

For difficult subjects, manually mask the strongest subject frame over a stacked background if necessary.

Modern AI denoise software is extremely impressive, but stacking and AI denoising solve noise differently.

AI denoise estimates what clean detail should look like based on learned image patterns. Stacking uses actual information recorded repeatedly by the camera. For critical detail, capture more real information whenever possible.

Before frames can be averaged, the same physical feature needs to occupy approximately the same location in every image.

Misalignment creates softness, double edges, ghosting, blurred stars and false detail. Different software uses different alignment systems — global alignment, star alignment, surface alignment, feature detection, optical flow and control points.

For lunar images, specialist software may divide the surface into many small alignment regions to compensate for local atmospheric distortion.

The reason stacked files tolerate aggressive sharpening is that the underlying noise level has been reduced. However, sharpening should still be controlled.

Methods to understand: capture sharpening, deconvolution, wavelet sharpening, unsharp mask, high-pass sharpening, texture and local contrast.

Blur in an optical system can be described mathematically using a point spread function. Deconvolution attempts to reverse some of that blur, recovering apparent fine detail more intelligently than basic contrast sharpening.

However, it requires clean data. Stacking therefore creates an ideal foundation for deconvolution.

Photographers often obsess over cameras and lenses while forgetting that kilometres of moving atmosphere may sit between the camera and subject. For lunar photography, atmospheric turbulence can become the limiting factor.

Best conditions often include stable air, the Moon high above the horizon, minimal heat shimmer, a clear atmosphere and low wind at different atmospheric layers.

Photographing the Moon low on the horizon means looking through much more atmosphere — more turbulence, more haze, more colour dispersion and less fine detail.

A 60MP, 100MP or higher-resolution camera cannot record detail that never reaches the sensor cleanly. Final detail is limited by lens resolving power, diffraction, focus, atmospheric seeing, motion, sensor sampling, exposure and noise.

Stacking improves the quality of captured information, but it cannot exceed every physical limit of the imaging system.