Photographing planets

Planets are small and bright. Jupiter is at most about 50 arcseconds across, which is under 3% of the width of the Moon, but it shines bright enough to need only milliseconds of exposure. The problem is not a lack of light. It is seeing: pockets of warm and cool air above you bend the light and make the image shimmer and blur several times a second.

The method is called lucky imaging. You record a video of a few thousand frames, each short enough to freeze the air. Some frames catch a moment when the air was steady and look sharp, most are blurred. Software scores every frame, keeps the best few percent, lines them up and averages them. The averaging cuts noise, and sharpening then brings out detail that a single frame could not show.

The upshot: planetary imaging suits people with bright skies, a short window of free time, or a telescope with a lot of aperture and focal length. It rewards patience at the focuser and in processing, not hours of integration. See the comparison of all the types for how it fits next to Milky Way, wide-field and deep-sky work.

Telescope

Aperture sets the finest detail you can resolve, and focal length sets how big the planet looks. Long, slow telescopes do well here: Schmidt-Cassegrains and Maksutovs, Dobsonians with a Barlow, and long refractors or Newtonians. The fast astrographs that suit deep-sky work (f/2 to f/5) are a poor fit unless you add a strong Barlow. A reflector needs good collimation, and any scope needs time to cool to the outside air, which can take an hour or more for a big one.

Camera

• Frame rate matters. Planetary cameras read out a small crop at 100 frames a second or more over USB 3, so a short video collects thousands of frames.
• Small pixels, low read noise. Cameras with pixels around 2 to 4 microns work well. The ZWO ASI 462, 678 and 585 families, the Player One Uranus and similar are popular examples.
• Cooling is not needed. Exposures are too short for dark current to matter, which is why these cameras are cheap and uncooled.
• Color or mono. A color camera is easiest. A mono camera with RGB filters gives sharper results and allows infrared channels, at the cost of more runs and derotation.
• An existing DSO camera can work if it offers a region of interest and a fast readout. Its large, slow sensor is the weaker choice.

Barlow or amplifier

A Barlow lens multiplies the focal length, typically 2x to 5x. It gets a short scope to the focal length a planet needs. A good Barlow sits close to the sensor, and the real multiplication depends on the distance between lens and sensor, so read its specification for the spacing it was designed for.

Filters

• UV/IR cut: keeps a color camera from picking up near-infrared, which blurs and shifts color. Useful with refractors and color cameras.
• IR-pass (685 nm and up): on a mono camera, infrared light is less disturbed by the atmosphere, so it gives a sharp luminance layer, and it is useful on the Moon and in poor seeing.
• RGB filters: for a mono camera, one run per color.
• Methane (around 890 nm): shows Jupiter and Saturn cloud-top structure.
• Atmospheric dispersion corrector (ADC): two small prisms that cancel the red and blue fringe on a planet low in the sky. A big help when the planet is under about 40 degrees.

The filters guide covers the deep-sky filters. Light-pollution and narrowband filters do not help for planets.

Mount

At high magnification a planet drifts out of a tiny field in seconds, so you need tracking. A decent equatorial or GoTo alt-az mount is fine. Autoguiding is not used, and polar alignment can be rough, because each capture lasts only a minute or two. What counts is stable tracking that does not vibrate when you touch the focuser. A Dobsonian without tracking can be used if you are patient, but each frame run is a struggle.

The goal is to sample the planet finely enough to keep the detail your aperture gives, without spreading the light over more pixels than it needs. A common rule of thumb is an f-ratio of about 3 to 5 times the pixel size in microns. For a 2.9 micron camera that is roughly f/9 to f/15, and for a 3.76 micron camera f/11 to f/19. With a seeing-limited night, the low end is enough. Add a Barlow to reach the f-ratio you want.

Here is how that looks in pixels. Jupiter near opposition is about 45 arcseconds across. A 2.9 micron camera at 2000 mm focal length gives 0.30 arcseconds per pixel, so Jupiter spans about 150 pixels. At 1000 mm that drops to about 75. Saturn’s disk is around 18 arcseconds and its rings about 40, so it needs a similar scale to be more than a few dozen pixels wide.

Use the FOV calculator to see the image scale and field for a camera and scope with a Barlow. Pick a reducer or Barlow in the list, or enter a custom focal length. The detail check tells you how many pixels fall across a target.

1. Cool the scope and check the air. Set up an hour early. Forecast seeing helps, but a steady star at high power is the best test.
2. Wait for altitude. Image when the planet is as high as it will get, ideally above 40 degrees. Low planets cross more air and smear.
3. Frame and crop. Center the planet and use a region of interest just larger than the disk. A small crop lets the camera run at its top frame rate.
4. Set exposure and gain. Use the shortest exposure that gives a good signal, often 5 to 20 ms. Raise gain before you lengthen the exposure. Keep the histogram away from the right edge so no highlights clip.
5. Focus carefully. Refocus often as the scope cools and the air changes. A Bahtinov mask on a bright star is good for a first approximation, then fine-tune on the planet itself. See the focusing guide.
6. Record in short runs. Save as SER if the software offers it, uncompressed. Capture 1 to 3 minutes for Jupiter, because it rotates about once every ten hours and features smear in longer runs. Saturn and Mars allow somewhat longer.
7. Repeat. Take many runs. The seeing changes minute to minute, and the best one is rarely the first.

Free capture software includes SharpCap, FireCapture and the camera maker’s own tool. Pick the one that supports your camera and a SER file format.

Deep-sky processing is about pulling a faint signal out of noise. Planetary processing is about picking the best moments and then sharpening, because the signal is already strong. The deep-sky processing guide does not apply step for step.

1. Stack the best frames. AutoStakkert! (free) scores each frame, aligns them on many small patches to cancel the distortion, and averages the best 10 to 30 percent. Fewer frames are sharper but noisier.
2. Sharpen with wavelets. RegiStax and similar tools split the picture into layers by detail size, which lets you strengthen fine detail without boosting noise as much. Deconvolution is another route.
3. Derotate, if needed. For separate filter runs on a mono camera, WinJUPOS can rotate each result back to a common time so the planet lines up. It can also combine several runs.
4. Combine the colors. Merge R, G and B, then align them. If you used a color camera, you already have a color image. Adjust the balance by eye or with a reference.
5. Finish gently. Check levels, color, and a touch of noise reduction in an editor. Overdone sharpening leaves ringing and a harsh, plastic look.

Flat frames help to remove dust shadows, but planets do not need dark frames or heavy stretching. A planetary image has a bright subject on a black background and sits in the middle of the histogram from the start. See the stretching guide for what you do not need here.

• Jupiter (about 30 to 50 arcseconds): the easiest and most rewarding. Belts, the Great Red Spot and its four big moons change from night to night.
• Saturn (about 15 to 20 arcseconds, rings about 40 across): rings, Cassini’s division and the larger moons. The ring tilt changes slowly over the years.
• Mars (about 4 to 25 arcseconds): tiny most of the time. It is worth the effort only for a few weeks around opposition, every two years or so, when dark markings and polar caps can show.
• Venus (about 10 to 60 arcseconds) and Mercury: show phases. Image them in daylight or twilight with an infrared filter, since they stay low and close to the Sun. Never point the telescope near the Sun without a proper solar filter.
• Uranus and Neptune (about 2 to 4 arcseconds): small blue-green disks. A record shot, not a detailed picture.
• The Moon: the same method gives superb close-ups of craters, and it is a great place to practice.

Dark Window’s planner is built around deep-sky nights, but a night with steady air and no cloud is a good night for planets whatever the Moon is doing. Check the hour-by-hour cloud forecast on the planner, then look up where the planet is in your sky with a planetarium app.

Can I photograph planets with my deep-sky setup?

Yes, if the telescope has enough focal length. Add a 2x to 3x Barlow or amplifier, swap the cooled camera for a small-pixel planetary one or use its crop or region-of-interest mode, and capture video. A short refractor around 300 to 500 mm will show a planet as a tiny disk, so expect a small result rather than a detailed one.

Do I need a mono or a color camera?

A color camera is simpler and gives a good result in one run. A mono camera with red, green and blue filters is sharper and lets you add infrared or methane bands, but it takes several runs per planet and the planet turns between them, so Jupiter and Saturn need derotation. Most people start with color.

What aperture do I need?

Resolution grows with aperture. A 4 to 6 inch scope shows Jupiter's belts, the Great Red Spot and Saturn's rings. Eight inches and up starts to show finer cloud detail, Cassini's division and Martian features when Mars is close. A smaller scope still gives a worthwhile picture on a steady night.

Why are my planet pictures soft?

Usually seeing, focus or collimation. Check the air before you set up, let the scope cool to the outside temperature, focus on the planet itself with a fine adjuster, and collimate a reflector. Then stack only the sharpest portion of your frames. A planet low in the sky also smears into color fringes, which an atmospheric dispersion corrector reduces.

Why not just take long exposures of a planet?

A planet is bright, so there is no need for long exposures, and a long exposure blurs it. The air above you shifts constantly and distorts the image many times a second. A very short exposure freezes one moment of that, and stacking picks the best moments out of thousands.