To capture the best planetary images these days, the preferred technique is known as “lucky imaging.” This method records thousands of frames in a high-speed video stream, which you can later sort for the best frames to stack into a final high-resolution image.
Do you want to know what High-resolution planetary imaging is all about;
Aperture and Optical Quality
The bottom line for high-resolution planetary photography is the quality of the seeing. Seeing describes how much the image of a celestial object is affected by turbulence in the Earth’s atmosphere. With good seeing, the image is steady, revealing more detail.
If you don’t have good seeing, it doesn’t really matter if you have a terrific-quality large-aperture scope and perfect photographic technique. With bad seeing you’re just going to get bad images.
Seeing is different than transparency, which describes how clear the atmosphere is. The two are not really related for planetary photography. You can often have great seeing with poor transparency and vice versa. The Clear Sky Chart gives generalized seeing predictions in addition to cloud cover predictions. MeteoBlue also which gives very detailed seeing predictions (be sure to change to your observing location, this link is for mine).
Local seeing, such as observing over heated roof tops or hot asphalt, can also adversely affect your images. Even heat coming off your body can get in the light path of an open tube scope and ruin an image.
Also watch out for the jet stream. When it is overhead, it is notorious for creating bad seeing.
2. Aperture and Optical Quality
To do world-class high-resolution planetary imaging, you need the world’s best seeing, plus a large telescope (11 – 16 inches aperture) with excellent optical quality.
Aperture determines how much fine detail your scope can resolve. The larger the scope, the smaller the detail it can resolve. But, the larger the scope, the more difficult it is to get it acclimated. And the larger the scope, the more expensive top-notch optical quality will cost.
You can still do planetary imaging with any scope, but the image scale will be smaller with a small scope, and smaller scopes will resolve less detail. You can still take some very nice images with apertures as small as 5 inches as seen below.
Most scopes these days are of decent optical quality, but reflectors and Schmidt-Cassegrains must be collimated correctly to perform at their best. Collimation is incredibly important and frequently neglected, especially by SCT aficionados. For the absolute best results, you should collimate on a star near your target every time you shoot.
I know one world-class planetary photographer who will only shoot planets on one side of the meridian with his SCT because he doesn’t want the pain of having to re-collimate after the mirror shift due to a meridian transit or mount meridian flip.
Acclimation means letting your telescope cool down to get as close as possible to the ambient air temperature.
If the optics of your scope are not at the air’s temperature, the temperature difference will cause local seeing effects in the air – optics boundary and cause blurriness that affects the resolution of the detail you can resolve on a planet.
It is very important that your scope cools down to ambient temperature, otherwise heat plumes off the primary and tube currents will wreck the image. If you don’t use active cooling fans, this process can take an hour to several hours depending on the mass of your scope. Larger scopes have more massive optics and it takes longer for them to shed the heat they hold.
If the temperature is dropping throughout the night, such as in the winter, the optics may never reach equilibrium and even if you have good seeing in the atmosphere, you won’t be able to maximize the resolution of detail you are trying to capture because of scope seeing effects.
Spend the time to really nail the focus.
Focus on a nearby star instead of trying to focus on low-contrast planetary detail. Use software-assisted metrics for focusing that give an objective readout of star size in Full Width Half Maximum or Half Flux Diameter.
If the temperature is changing during the night, you may need to refocus periodically.
6. Correct Sampling
You will need to magnify the image so that it is large enough to be sufficiently sampled by the pixel size in your camera to record fine details.
If the seeing and your scope can produce 1 arc-second detail, but your pixels only cover 1 arc second in image scale, you won’t record any detail at that scale – all you will have is a single square illuminated pixel.
The amount of magnification used should be based on the seeing and the pixel size of your camera. Magnification is determined by the focal length. The aperture is fixed, so increasing the focal length also increases the focal ratio (the focal length divided by the aperture).
Since the linear diameter size of the Airy disk, which determines detail, is solely dependent on the focal ratio, use a high-quality Barlow or eyepiece projection to adjust the size of the Airy disk to match the seeing so that it is sufficiently sampled by your pixel size. For planetary work, you need more than the traditional 2x Nyquist sampling. 3.5x is usually considered an absolute minimum to capture high-resolution extended detail.
A simple rule of thumb for high-resolution work is to multiply your pixel size by 3x to 7x to get the focal ratio at which you should work.
On the nights of the good seeing, use 5x the pixel size. For example, if you have a 4.3 micron pixel, 4.3 x 5 = 21.5, so you would want to work at about f/20.
If you have a night of superb seeing and a scope with great optics you should push the magnification up to 7x and work at f/30.
On nights of mediocre seeing, you can use less magnification and get a wider field, but expect to record less detail.
7. Lucky Imaging
Lucky Imaging means recording video to capture hundreds or thousands of frames. Special software is then used to examine each frame and pick out the best ones that were “lucky” to be sharp from moments of better seeing.
Using a camera with a high framing rate will allow you to capture more frames and have a better chance of getting more frames with good seeing.
After the software examines the best frames, it stacks them together to improve the signal-to-noise ratio so they can be sharpened by sophisticated sharpening algorithms such as wavelets in RegiStax.
Get out there every clear night. You won’t really know what the seeing is until you look at it at magnification… trying to judge the seeing by just looking at the naked eye seeing is not really going to tell you what the seeing is like at high magnification.
Likewise, practice your image processing. Sharpening, with wavelets in particular, is really more of an art than something you can do by some type of rote formula. You won’t know if you have the best results unless you try different combinations of wavelets during your processing.
Like any other skill set, you will get better with practice, and the more the better.