Is there are rule of thumb for determining how long to capture frames of a celestial target before the limits of the processor no longer increase output quality?
Yes, there is a general rule of thumb for determining the optimal acquisition time for capturing frames of celestial targets, which is tied to the concept of Signal-to-Noise Ratio (SNR). Here are some key points to consider:
Stacking Frames: The quality of the final image improves with the number of frames stacked. Stacking multiple frames helps reduce noise, which is randomly distributed across each frame, while the signal from the celestial object remains consistent1.
SNR Improvement: The reduction in noise achieved through stacking increases with the square root of the number of subframes. For example, stacking 4 frames reduces noise by a factor of 2, and stacking 9 frames reduces noise by a factor of 31.
Acquisition Time: Doubling the acquisition time increases the SNR by a factor of about 1.4 (the square root of 2), or by 40%. Therefore, longer acquisition times generally lead to better image quality1.
Practical Example: For wide-field astrophotography, the "500 Rule" can help determine the maximum exposure time to avoid star trailing. The formula is: $$\text{Shutter Speed (SS)} = \frac{500}{\text{Crop Factor (CF)} \times \text{Focal Length (FL)}}$$2
Environmental Factors: Sky brightness and light pollution also affect the optimal exposure time. Under bright urban skies, shorter exposures are needed, while under truly dark skies, longer exposures are possible2.
By considering these factors, you can determine the optimal acquisition time for your astrophotography project to achieve high-quality images.
