Techniques for imaging the ICE GIANTS
The ‘high gain, short exposure’ developments described opposite open the door to a method of planetary imaging that maintains image brightness and reduces atmospheric smearing through muchreduced exposure times. Instead of exposures of 100200msec you can now have suitable image brightness with much more reasonable exposures of 10-20msec. With short exposures, however, comes another challenge. The low surface brightness of Uranus and Neptune means that the number of photons landing on each pixel of the camera sensor becomes painfully small, giving rise to high levels of ‘shot noise’.
Shot noise is the main source of noise in planetary imaging and stems from the particle nature of light. Photons from an object arrive at random intervals, so the number captured by a pixel over the length of a frame also fluctuates randomly. Since each image pixel varies in this way, the planet ends up looking noisy. The fewer photons, the worse the shot noise relative to the signal. In fact, there’s a square root relationship between the two. So, for example, a frame exposure that is 9x shorter, the frame’s shot noise (relative to the image) will be 3x worse.
But don’t be too concerned about noise in individual frames: what matters is the noise in the final stack. Remember, with shorter exposure times you can capture more frames in a given period, and with a 9x shorter exposure you should have 9x as many frames to stack. Because a square root relationship exists between the number of frames in the stack and the noise, the 9x increase in the number of frames reduces the noise in the stack by 3x – exactly compensating for the 3x worse noise in each individual frame.
This balancing act between exposure time and stack size, which fixes the overall level of shot noise, gives us a key principle of modern planetary imaging. From the perspective of noise in the stacked image, it’s not the gain and exposure used for each individual frame that matters, but the total accumulated exposure time of the whole stack.
Keeping focused
The shorter exposures will allow you to gather more frames in your recording to make up for the high frame noise, as long as you maintain the same overall video duration. With Uranus and Neptune being so dim, however, don’t expect to get nice smooth images showing details with a one-minute video – you will need to stack a total of at least 5 to 10 minutes’ worth of data for the signal to start to dominate over the shot noise.
As mentioned earlier, the high shot noise in individual frames can lead to focusing problems. One technique to help here is to carefully swap your IR filter for a luminance filter of the same glass thickness. Once you’ve dropped the gain, the increase in brightness will mean a much better signal-to-noise ratio, allowing you to focus more accurately. Don’t forget to switch things back after focusing and before hitting the record button.
A second focusing solution uses the live stacking preview feature of FireCapture, a camera control program. In this, the last eight or so frames are continuously aligned and stacked. This significantly reduces shot noise but overexposes the preview image, so drop the gain before attempting to focus.