Photographing each planet has their own challenges.  I find it useful to divide the planets into three sets: Inner Planets (Venus and Mercury), Outer Bright Planets (Mars, Jupiter, and Saturn), and Outer Small Planets (Uranus and Neptune).  I’ve already covered Venus and Mercury.  Right now, we are in Mars, Jupiter, and Saturn season.

Motion blur is caused when the subject moves more than a pixel at a rate that is faster than the exposure time.  The greatest challenges with the outer bright planets are capturing details that are blurred by seeing and the motion blur caused by the rotation of these planets.  Multiple pixel rotation blur can be reversed using a program called WinJUPOS.  Derotation allows the integration time to be multiplied, but there are limits to how far the derotation is effective.  So it turns out that there are three exposure times that must be considered when capturing the outer bright planets: per-frame exposure, sequence time, and total integration time of all the sequences.

Per Frame Exposure

The per-frame exposure time should be fast enough to freeze the motion caused by seeing (atmospheric scintillation).  Unfortunately, for most backyard astrophotographers in the northern hemisphere, this is very difficult to attain.  The best photos are captured in an area where the planets rise above 60° altitude with great seeing and transparency.  Of course, these challenges don’t stop the rest of us from trying.  That said, if you live in an area as I do, where the seeing is almost never great and the skies are rarely transparent, don’t expect your photos to look as good as those from others that were captured under better skies.  All you can do, is all you can do–but all you can do is still far better than what was available just 10 years ago.

Guidelines for the per-frame exposure time:

1. Maximize the frame rate settings.  In SharpCap, use small Capture Area (ROI or Region Of Interest), set Frame Rate Limit to Maximum, Turbo USB to 100, and High Speed Mode to On.  My computer has an i7 processor, USB 3, and an SSD for maximum data throughput.
2. Most astro-cameras perform best at 12dB gain or higher (ZWO users set your gain to at least 120), and can become excessively noisy and loose too much dynamic range above 36dB (ZWO users should not exceed 360 gain).
3. The frame rate should be at least 25 frames per second.  This means that the slowest exposure time is 40ms.  (Under my skies, I try for 10ms for Mars and Jupiter, and 15ms for Saturn at high gain.)
4. Based on those guidelines, adjust the gain and exposure time to achieve a 40% to 80% peak histogram, at a fast frame rate.

Fast frame rates allow a greater number of frames to be captured per sequence.  This is most important with Jupiter, as it has the fastest rotation rate, and thus, the shortest sequence time.  I find that I need at least 1,500 frames in a sequence to maintain good resolution with a 1.5x drizzle in AutoStakkert! 3.  The ability to drizzle means that the final image is 1.5 times larger.  With only 30 seconds per sequence, I need at least 50 fps to capture 1,500 frames.  It’s personal preference, but I prefer a larger image and need faster frame rates.

Also, capture at the highest bit depth of your camera.  The ZWO cameras capture 12, 14, or 16 bits.  Keep in mind the histogram is only one bit less dynamic range at 50% than 100%.  We can afford to loose a bit, if we capture full bit depth.  This is why I recommend exposure at 40 to 80% histogram, rather that recommending a higher histogram percentage.  If the choice is faster exposure, less gain, or lower histogram, choose the lower histogram.  The data is in there, you may need to increase the screen setting brightness to see it.  By this, I mean in SharpCap, Image Controls, Brightness or modify the Display Histogram Stretch, or in AutoStakkert! select 2x under Display Options.  I do not mean to noodle with the controls on your computer monitor!

Sequence Exposure

To minimize sequence motion blur, the length of a given sequence (a sequence is a run of frame captures) is limited by the time required for the planet to rotate one pixel at its equator.  This time is computed from several parameters: the effective magnification of the telescope and camera, and the size and the rotational rate of the planet.  The effective magnification of the optics and camera is solved with this formula:

Magnification in arc seconds per pixel = 206.3 / focal length * pixel size

For example, the EdgeHD 14 with a 2x TeleVue PowerMate in the TexasStarCave has a theoretical focal length of 7810mm.  However, the measured focal length (by WinJUPOS) is 8320mm.  Typical planetary cameras (ASI290, ASI178, ASI462, etc.) have a pixel size of 2.9 microns.  The effective magnification is 206 / 8320 * 2.9 = 0.072 arc sec/pixel.

The seconds/pixel rate formula is expressed as:

Time in seconds per pixel = (Rotation in hours / 2 * 3600) / (Planet Size / Magnification)

Mars, for example rotates every 24.6 hours, and is about 23 arc seconds at opposition.  Thus, (24.6 * 3600) / (23 * 0.072) = 138 seconds.  The size of the planet as seen by the camera is simply the size of the planet in arc seconds divided by the magnification.  Here are my values for the TexasStarCave (planet sizes are arc seconds at opposition):

Table 1: Sequence Length

The last two columns of Table 1 are my suggested starting numbers for my equipment.  Continuing with our Mars example, round-off the 138s/px sequence length to 120s and use a 480 x 480 Capture Area (ROI).  With the ZWO ASI462 camera at 480 x 480, the frame rate tops off at 143 frames per second.  The inverse of 143 is about 0.007.  If possible, I would set the exposure to 7ms or faster, assuming the gain can be kept below 36dB (ZWO gain 360), and achieve a 40% to 80% histogram peak.  At the 143 fps rate and 120s fro Mars, each sequence is about 17,000 frames.

For this configuration, the diameter of the planet Saturn is 264 pixels.  However, Saturn has rings!  So, I use the same Capture Area width as Jupiter for Saturn.  At 800 width ROI my frame rate tops out at 87 fps for Jupiter and Saturn.  The inverse of 87 is 0.011, or 11ms.  Rounded to 10ms, this exposure works well for Jupiter.  Since Saturn is over a stop dimmer Jupiter, it usually requires a least 2x longer exposure, 2x more gain, or a combination of exposure and gain adjustments.  Saturn usually has very little surface details, so less frames is not as much a problem.  The detail on Saturn that most astrophotographers try to capture is the Polar Hexagon.  However, it is not near the equator of Saturn, so is not as limited by the planet rotation.  These numbers are good starting points for my system at 8320mm, f/23, and a camera with a quantum efficiency of >80%.  Your mileage will vary.

Total Integration

Lastly, effectively longer total exposure time (integration) is achieved with multiple sequences.  But how much?  I find that WinJUPOS does a great job of derotating up to 4 pixels.  It is capable of derotating pixels forwards or backwards.  Thus, we capture nine sequences, and use the middle sequence as the derotation reference.  This gives us four pixels to be derotated forward and four pixels derotated backwards.  More of by best practices with WinJUPOS coming later.  Here are my settings, sequence, and integration times (number of sequences) for the EdgeHD 14, 2x PowerMate, ZWO ADC, Astromonics IR642 filter, and ZWO ASI462 camera.

Table 2: Typical Settings for the Texas Star Cave

There is plenty of advice available on how to capture the outer bright planets, but much of the advice is tribal knowledge that can usually be traced back to people with large telescopes in ideal locations.  With the formulas and analysis given here, it is possible to optimize the best settings based on your actual equipment and sky conditions.  For instance, with the EdgeHD 8 and a 2.5x PowerMate (5,000mm at f/25), Jupiter sequences could be up to 45 seconds each for a total integration of up to 20,000 frames in 6.75 minutes.  Use this spreadsheet to determine the optimized sequence length for you: Mars, Jupiter, Saturn Sequence and Size.  Enter your telescope focal length in millimetres and camera pixel size in microns.  The results are guidelines to help determine a good starting point.

There are many other things to consider when capturing these planets not covered here: how to focus, finding and guiding (yes, I said guiding), atmospheric dispersion, using multiple instruments, one-shot color vs. RGB, infrared filters, and much more.  Soon, I plan to release a series of videos on how to capture and edit Mars, Jupiter, and Saturn.  Please let me know your thoughts and questions below. Thanks!