Silent and Mechanical Shutter Comparison

To stack lunar and planetary images, you need lots of low noise images taken as fast as possible. Video cameras are often used for this purpose. A still camera like the Sony a6300 offers capture modes that trade off field of view, resolution, image capture rate, and signal to noise ratio. It is difficult to understand how these tradeoffs interact to obtain the highest quality final image.

With a 1300 mm focal length and 24 x 16mm APS sensor, still image modes are required to capture the entire lunar disk at full resolution. Still images can be captured at up to 11 frames per second with the a6300. Electronic first curtain shutter prevents vibrations from the shutter opening from disturbing an image, but the mechanical shutter closing can cause vibrations that blur the next image.

The silent shutter option prevents mechanical vibrations by using an all electronic rolling shutter mode. Unfortunately, this introduces electronic readout noise that can increase noise in the images. Noise can be reduced by stacking additional images. If silent shutter allows images to be taken more quickly, it may result in lower noise in the final stacked image.

I've taken two series of images on the same night for this experiment. One series with electronic first curtain mechanical shutter and one with the fully electronic rolling shutter. Mechanical shutter images can be taken at up to 10 frames per second and silent shutter images at up to 3 frames per second on the a6300. To minimize vibration with the mechanical shutter, both sequences of images were remotely triggered at a rate of about 1 frame per second. 80 images were taken with each shutter mode, with the top 10% stacked and processed with the same work flow: stacking with 8 parameter affine transform in Nebulosity, with LR and wavelet deconvolution in Lynkeos. All steps were the same and processing parameters very similar although still optimized for each individual stacked image.

A 100% crop image the lunar Alps with Plato, the Alpine Valley, Aristoteles, and Eudoxus. This processed image, taken with the mechanical shutter, shows some loss of sharpness compared to the silent shutter image below.

The silent shutter image shows noticeably sharper detail without much increase in noise.

Even with images taken on the same night, here are still too many sources of variation to make any final conclusions yet. Without multiple comparisons, it may have just been a change in seeing in the couple of minutes between the two series of images. My working theory is that the most likely cause of the loss of detail is vibration from the mechanical shutter closing from the previous frame. If so, shooting with a 11 fps continuous shooting mode should make this much worse. Shooting more slowly or with a more rigid mount should reduce any vibration.

Silent shutter images can be taken at up to 3 fps on the a6300. I expect no difference in noise or vibration for continuous shooting silent shutter shots. For the mechanical shutter, I assume that doubling the interval between shots to two seconds will eliminate any blurring due to vibration. I expect that it should be possible to acquire five or more times more silent shutter images than mechanical shutter images in the same session time. I expect this to improve final stacked and processed image quality and reduce noise.

I'm very encouraged that electronic rolling silent shutter mode appears to be a good choice for full disk stacked lunar images. As I have time, I plan to continue experiments comparing these techniques and understanding the tradeoffs better. Other comparisons that I've done have shown good results with the electronic silent shutter. For now, silent shutter has become my preferred mode for high resolution, full disk, lunar images.

Content created: 2016-09-22




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