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High-Resolution Planetary Imaging, Part 1: Telescope and Mount Selection

Intricate details inside the Great Red Spot imaged a very high resolution of 0.08 arcseconds / pixel. Credit: Rouzbeh Bidshahri

Planetary and deep sky imaging have little in common, with different telescopes, cameras, software, and techniques required. Planetary imaging does have some advantages, though. Planetary imaging is virtually immune to light pollution, making it accessible to those in brightly lit cities. Cost is usually less as well, with telescopes and cameras typically less expensive, and mostly free imaging capture and processing software.

Capturing simple images of planets is fairly easy and quick, but my goal here is to help imagers get started with what is commonly referred to as “High-Resolution Planetary Imaging.” It’s a rapidly expanding field, with amateur astronomers now capturing details on planets that were beyond large professional Earth-based telescope not many years ago.

What follows, in four parts, is some essential information about equipment, software, and technique based on years spent refining my planetary imaging setup and honing my skills:

Part 1: Telescope and Mount Selection for Planetary Imaging

Part 2:  Designing the Imaging Train

Part 3: Planetary Data Capture Session Preparation

Part 4: Planetary Imaging Software and Processing

 

Aperture Matters

Even an optically perfect 4- or 6-inch triplet refractor simply does not have sufficient resolution and light gathering power for very high-resolution imaging.

The small apparent size of planets is one of the main challenges to overcome. The largest planet, Jupiter, is between 30 and 50 arcseconds in angular diameter. That’s 36 to 60 times smaller than our far smaller but much closer Moon.

To capture fine details, we need high resolution, which is largely determined by the telescope’s aperture. An approximation to the maximum resolution for a given aperture is:

Resolution (max; arcseconds) = 120 / aperture (mm)

A 4-inch (100mm) refractor will therefore have a maximum theoretical resolution of 120/100 = 1.2 arcseconds. That’s the smallest detail it can resolve. A 14-inch reflector can resolve 0.33 arcseconds (i.e, capture much finer detail).

For most amateurs, planetary imaging apertures are in the 8- to 16-inch range, with the sweet spot between 10 to 14 inches. I get very good results with my 14-inch telescope.

A simulation of Jupiter’s image size with a top-end refractor (Takahashi TOA 150mm) compared to a Celestron C14 SCT, both at F/22. Credit: Astronomy Tools.

Telescope Selection
With aperture being key, reflectors are a better choice for planetary work than refractors.

Good candidates include Newtonian, Classical Cassegrain, Dall-Kirkham, and the popular Schmidt Cassegrain (SCT) designs. 

A mid-sized 12-inch Newtonian can be very long, as seen above. And this is short for planetary imaging at f/6. Credit: Parallax Instruments.

 Newtonian Telescope Advantages:

  • Very good imaging telescopes
  • Simplest and most cost-effective design
  • Longer f-ratio telescopes are the better choice because of their smaller secondary mirrors

Newtonian Disadvantages:

  • High quality optics and mechanics not usually found in off-the-shelf products
  • Long focal length Newtonians are not commercially available unless custom designed
  • Large aperture Newtonians are physically bulky and very long, requiring very large mounts
  • Internal tube currents can degrade the image in closed tubes and mirror cells
Carbon fiber Classical Cassegrain from CFF telescopes. Credit: CFF telescopes

Classical Cassegrain and Dall Kirkham Advantages:

  • Very well suited for planetary work
  • Native long f/ratio of f/15 to f/20 ideal for planetary work
  • Small secondary mirrors
  • Do not require image amplification by a Barlow or Powermate
  • More compact than Newtonians

Classical Cassegrain Disadvantages:

  • Very few off the shelf options available, especially in large apertures
  • Often more expensive than alternatives
  • More complex than Newtonians and SCTs
The Takahashi Mewlon Dall-Kirkham 210mm Telescope. Credit: Takahashi

 Dall Kirkham Advantages

  • Similar to Classical Cassegrain, very well suited for planetary work
  • Optics slightly cheaper to produce

Dall Kirkham Disadvantages:

  • Very few off the shelf options available, especially with large apertures
  • Smaller corrected field of view though this doesn’t matter for planetary work where only the center of the field is used 
The author’s SCT ready for a night of planetary data capture. Credit: Rouzbeh Bidshahri

Schmidt-Cassegrain (SCT) Advantages:

  • Compact
  • Cost effective
  • Easy to collimate and use
  • Relatively long native focal ratio
  • Readily available in many sizes
  • Thinned mirror reduces weight and thermal mass

 

Schmidt-Cassegrain (SCT) Disadvantages:

  • “Premium” exotic optics not common
  • Corrector plate susceptible to dew
  • Requires thermal management (adequate cooling to minimize air movement inside the closed tube)

Other Telescope Designs

Other telescope designs like Ritchey-Chretien (RC), Maksutov, and refractors can also be used but are more of a compromise for high-resolution planetary imaging.

  • RCs use hyperbolic mirrors that are difficult to figure, making them more expensive. Importantly, near-perfect optical alignment (collimation) of the mirrors (necessary for capturing fine details) is a lot more challenging to achieve than with most other reflector designs. Furthermore, most RCs and similar deep sky astrographs (widefield imaging scopes) have very large secondary mirrors, up to half the diameter of the primary mirror. This is a major cause of low contrast in the image.
  • Maksutovs, on the other hand, usually have a smaller secondary obstruction, but the very thick and heavy front glass meniscus traps heat in the sealed tube, causing internal tube currents that can seriously degrade the image. The problem only gets worse as the aperture increases, and large aperture is what’s needed for planetary work.
  • Refractors typically lack the necessary aperture. There are some very large refractors, 8-inch and up, but they can be prohibitively expensive ($20,000 and up). They are also very long, making them unwieldly.

 Mount

Unlike long exposure deep sky imaging that requires perfect tracking for extended periods, planetary imaging mounts need not be as high end because the exposures are very short. Most mounts will suffice, as long as they can carry the telescope with good stability. The cost can be a lot less without the need for a premium mount with encoders or direct drive motors.

An Alt-Az mount will also work but will suffer from field rotation. Field rotation can be corrected with a field de-rotator like the Pegasus Falcon reviewed here: https://astrogeartoday.com/pegasus-astro-falcon-rotator-review-slim-lightweight-cost-effective/

I prefer a German equatorial mount that can track the planets well at high magnification for an extended period, but for different reasons than when imaging deep sky objects. Tracking and imaging planets for an hour or two on either side of their highest point in the sky allows you to sift through all the data later and find the times when the seeing was calmest. This can greatly increase your ability to take advantage of the best seeing.

If your aperture is 8 to 12 inches, aim for a mount like the Losmandy G11, I-Optron Gem60/70. For 12 inches and up, its best to go for something like a Celestron CGX-L, Losmandy Titan, or Skywatcher Eq8 and up.

You’ll also want to have a very well-aligned finder scope. The field of view of a planetary imaging camera is tiny, so the mount may not always place the planet into the camera’s field on the first try. The finder scope can help avoid a lot of frustration when trying to find planets.

Mid-sized mounts can carry apertures of up to 12 inches (Losmandy G11 class, right). Large mounts will be required for 12 inches and up (Losmandy Titan class, left). Credit: Rouzbeh Bidshahri

 In Summary

Choosing a telescope can be tricky, with availability, mount capacity, and budget all a part of the decision.

While there is no perfect telescope, the choice for most successful planetary imagers has been the humble SCT. SCTs offer a good compromise of availability, aperture, price, and quality. I’ve had very good results with my Celestron SCTs.

Mount selection is relatively easy for planetary work since very short exposures are less critical of tracking errors.

In Part 2, Designing the Imaging Train, we turn our attention to setting up the imaging train to match the optics, telescope focal ratio, and camera specifications.

 

More from the High-Resolution Planetary Imaging Series

Part 2: Designing the Imaging Train

Part 3: Imaging Session Preparation

Part 4: Imaging Software and Processing

 

Useful Links:

Author’s gallery with more example: https://www.astrobin.com/users/Rouzbeh/

Celestron: https://www.celestron.com/

CFF Telescopes: https://www.cfftelescopes.eu/

Takahashi: https://www.takahashiamerica.com/

 

 

 

 

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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