In astrophotography stacking, the debate between taking many photos with short durations versus fewer photos with longer durations is complex, and the "better" option often depends on various factors. Both approaches have their distinct advantages and disadvantages.
Many Photos with Short Durations (Short Exposures):
Advantages:
Mitigates Tracking Errors: Even with the best tracking mounts, minor deviations can occur. Shorter exposures minimize the impact of these errors, resulting in sharper stars and less trailing.
Reduces Overexposure: Bright objects (like bright stars or the core of some nebulae) can easily be overexposed with long exposures, leading to a loss of detail. Shorter exposures help preserve detail in high-dynamic-range objects.
Increased Flexibility and Error Tolerance: If a single short exposure is ruined by a plane, satellite trail, or sudden atmospheric turbulence, it's easier to discard that one frame without significantly impacting the overall data.
1 With many frames, you have more redundancy.Reduced Thermal Noise (for uncooled cameras): Shorter exposures mean the camera sensor heats up less, which can reduce thermal noise, especially in DSLRs or mirrorless cameras without active cooling.
Better for Fast-Moving Objects or Poor Seeing: For objects like planets or the Moon, or in conditions with turbulent atmosphere (poor "seeing"), very short exposures are crucial to "freeze" the image and capture sharp details. While deep-sky objects are much slower, shorter exposures can still help mitigate atmospheric blurring.
Easier on Mounts: Less demanding on tracking accuracy, especially for less expensive or less precisely aligned mounts.
Disadvantages:
Higher Read Noise Contribution: Each time an image is read out from the sensor, it introduces a small amount of "read noise." With many short exposures, the cumulative read noise can become more significant.
More Files and Processing Time: A large number of short exposures means more individual files to manage and process, which can be computationally intensive.
May Not Capture Faint Details: If individual short exposures are too short, the signal from very faint objects might not be strong enough to rise above the read noise in a single frame, even with stacking.
2 You need enough signal in each sub-exposure to make the stacking effective for faint targets.
Fewer Photos with Longer Durations (Long Exposures):
Advantages:
Better Signal-to-Noise Ratio (SNR) for Faint Objects: Longer exposures collect more light (signal) from faint deep-sky objects, allowing their signal to rise more prominently above the noise floor (especially read noise). This leads to clearer, smoother, and more detailed images, particularly in low-light areas.
Less Read Noise: With fewer exposures, the overall read noise contribution is reduced because it's incurred once per frame.
Less Processing Required: Fewer frames generally mean a simpler workflow and less computational demand.
Captures More Photons per Frame: This directly translates to more signal from the target, which is essential for revealing dim structures and colors.
Disadvantages:
More Susceptible to Tracking Errors: Any movement, drift, or periodic error in the mount becomes more apparent and can lead to star trailing or blurring.
Higher Risk of Overexposure: Bright stars or nebula cores can easily be blown out, losing all detail and color.
3 Less Forgiving of Mistakes: If a single long exposure is ruined by a plane, wind gust, or mis-tracking, you lose a significant amount of valuable integration time.
Increased Thermal Noise (for uncooled cameras): The sensor heats up more during longer exposures, which can increase thermal noise and hot pixels.
4 Requires More Precise Equipment: Demands a highly accurate and well-aligned equatorial mount, and often autoguiding, to prevent star trails.
Which is better?
There isn't a universally "better" option; the optimal choice often involves a balance and depends on:
Your Equipment:
Mount Accuracy: If you have an excellent, precisely aligned, and well-guiding mount, longer exposures become more feasible.
Camera Sensor: Cameras with very low read noise can benefit more from shorter exposures, as the read noise penalty is less significant. Cooled astrophotography cameras help mitigate thermal noise in longer exposures.
5
The Target:
Bright Objects: For bright nebulae or star clusters, shorter exposures can prevent saturation.
6 You might even combine different exposure lengths (HDR stacking) for objects with extreme dynamic range (e.g., Orion Nebula).7 Faint Objects: For very faint galaxies or nebulae, longer individual exposures are often preferred to ensure enough signal is captured in each frame to rise above the read noise.
8
Sky Conditions:
Light Pollution: In light-polluted skies, shorter exposures might be necessary to avoid quickly saturating the sky background.
9 Atmospheric Seeing: If the atmosphere is turbulent (poor seeing), shorter exposures can "freeze" the turbulence, resulting in sharper stars. In excellent seeing, longer exposures are less problematic.
General Rule of Thumb:
Many experienced astrophotographers aim for individual sub-exposure lengths that are long enough for the signal from the faintest details of the target to be significantly above the camera's read noise, but not so long that bright stars are saturated or tracking errors become apparent. Then, they take as many of these optimal-length exposures as possible to maximize the total integration time and further improve the signal-to-noise ratio.
In essence, stacking is always beneficial as it averages out random noise. The question is about the length of each individual exposure ("sub-exposure"). For deep-sky objects, it's generally a compromise between minimizing read noise (favors longer) and mitigating tracking errors/overexposure/hot pixels (favors shorter).
