Equatorial platform

Introduction: The Dobsonian Telescope

Dobsonian telescopes ("Dobsonians") were popularized by San Francisco-based astronomer John Dobson in the 1970s. A Dobsonian consists of a Newtonian telescope (with a large parabolic primary mirror and a small, flat secondary mirror) mounted on a simple altazimuth base. It usually has no built-in motors to track objects in the sky. Everything is reduced to a minimum with the only expensive part being the primary mirror.

Fig. 1 - 16-inch f/5 Dobsonian made by Dieter Martini in Germany with its Carl Zambuto quartz mirror.

Manual Tracking Challenges

If an observer wants to follow heavenly objects, they have to push or pull the Dobsonian telescope. If low magnifications (below 150x) are used, moving the telescope is straightforward. But at high magnifications (above 150x), manually tracking objects becomes cumbersome. As a planetary observer, I often use magnifications of 350x-450x (moon, planets) and up to 1200x (double stars).

Fig. 2 - The disassembled Dobsonian telescope.

Tom Osypowski’s Platform

Tom Osypowski from EquatorialPlatforms.com produces equatorial platforms that can be placed beneath nearly any Dobsonian telescope and will track celestial objects for approximately 80–85 minutes. After a short reset, the platform tracks for another 80-85 minutes and so on. The platforms are custom-made. Depending on the latitude of the observer, the weight of the Dobsonian telescope, the position of its center of gravity and the dimensions of the rocker box, the platform will have a different geometry (though an existing platform can be adopted to carry different telescopes).

Fig. 3 - The Tom Osypowski dual-axis aluminum equatorial platform.

Contacting Tom

In April 2010, I contacted Tom Osypowski via email. I was fascinated by the images on his website taken through Dobsonian telescopes on equatorial platforms and by the observing reports. Tom replied to my email on the same night but it wasn't until mid October 2010 that I decided to order one of his dual-axis aluminum platforms after trying to observe Jupiter at 350x for a couple of hours on different nights.

Will the platform have an impact on the stability of the Dobsonian telescope? Will it track accurately enough for high magnifications? Will it take a lot of time to get ready for observing? Will it add a lot of complexity to a beautifully simple system? These were some of the things I was worrying about. I trusted the advertisement on Tom's website. The images of the moon and the planets and especially of deep sky objects were just too tempting.

First Impressions and Setup

In December 2010 - much earlier than anticipated, a huge box with the equatorial platform arrived (the biggest and heaviest box I've ever received in my life). "Wow, this platform looks awesome!" was my first thought. This was my very first encounter with an equatorial platform. What a piece of exquisite craftsmanship.

Fig. 4 - The 16-inch Dobsonian riding on the Tom Osypowski dual-axis aluminum equatorial platform.

In January, I put the platform to to its first test. On the very first night, my 16" f/5 Dobsonian followed planets and stars seamlessly. No vibrations were detected, even at 1000x-1200x while observing multiple stars such as Almach (Gamma Andromedae). The Dobsonian was as stable as without the platform and tracked as perfectly as any equatorial mount at the highest powers. What a walk in the park! During the following nights, I've observed more and more objects and was impressed by how well the platform performed at very high powers.

Astrophotography Begins

How about astrophotography? After a few weeks, I've organized an off-axis guiding system, an autoguider, and a coma corrector. In April, my Canon 20Da had a ‘first light’ through the 16-inch f/5 Dobsonian. The results of my first astrophotography attempts can be seen below.

Fig. 5 - Messier 66 and Messier 65 in Leo on April 8/9, 2011. 4 x 8 min & 1 x 3 min at ISO 1600. Canon EOS 20Da, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian, dual-axis Tom Osypowski equatorial platform.

Fig. 6 - Messier 27 in Vulpecula on August 20, 2011. 3 x 8 min at ISO 1600. Canon EOS 20Da, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian, dual-axis Tom Osypowski equatorial platform.

Fig. 7 - NGC 6905 in Delphinus on September 1/2, 2011. 2 x 8 min & 1 x 9 min at ISO 1600 . Canon EOS 20Da, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian telescope, dual-axis Tom Osypowski equatorial platform.

Fig. 8 - NGC 7331 in Pegasus on October 23 & 27, 2011. 5 x 8 min & 4 x 15 min at ISO 1600 . Canon EOS 20Da, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian telescope, dual-axis Tom Osypowski equatorial platform.

Early Imaging Challenges

I faced some challenges during the first few months of imaging. The challenges didn't concern the equatorial platform but the pitfalls of astrophotography:

- A faulty position of the pick-off prism of the off-axis system caused some vignetting in the first batch of images.

- It took me some time to figure out the best settings of my DSLR camera. I worked with too high ISO values (ISO 1600) and too long exposure times (10 - 15 minutes). These settings led to washed-out star colors and blown-out highlights and all kinds of noise (banding, pattern noise). After a year I reduced the ISO to 800 and shortened the exposure times to 3-5 minutes (settings for the Canon 20Da and the Canon 450Da) with better results.

- The light shroud of the Dobsonian telescope blocked part of the light path and caused diffraction effects on bright stars.

- My first DSLR (Canon 20Da) suffered from purple amp glow in the image corners. It wasn't until I switched to the Canon 450Da that these problems fully disappeared.

- Vignetting of the camera body and the optical elements caused some problems, especially with post-processing of extended deep sky objects. After a few years, I purchased a flat field panel for the 16-inch telescope, making post-processing easier.

- Centering an astronomical object in the camera viewfinder proved a bit more difficult than anticipated. Even though the object appeared in the center of the viewfinder, it turned out somewhere else in the final image: It is rather difficult to judge whether an object is perfectly centered in the viewfinder in darkness. It wasn't until I started using an eyepiece with illuminated crosshairs to center objects (prior to attaching the camera) that these issues vanished.

I reprocessed the first images from the years 2011 and 2012 - which were taken under the circumstances described above (see images on this page) - in 2020. I leave them on the website mainly for nostalgic reasons.

Fig. 9 - Messier 51 in Canes Venatici on May 26, 2012. 19 x 3 min at ISO 800. Canon EOS 20Da, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian telescope, dual-axis Tom Osypowski equatorial platform.

Fig. 10 - NGC 6960 in Cygnus on July 24/25, 2012. 15 x 3 min at ISO 800. Canon EOS 20Da, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian telescope, dual-axis Tom Osypowski equatorial platform.

Fig. 11 - Messier 57 in Lyra on October 2, 2011. 6 x 4 min at ISO 1600. Canon EOS 20Da camera, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian telescope, dual-axis Tom Osypowski equatorial platform.

Fig. 12 - Messier 42 & Messier 43 in Orion on October 23, 2011. 1 x 80 s, 1 x 4 min, 1 x 8 min at ISO 400. Canon EOS 20Da camera, TeleVue coma corrector, Zambuto 16-inch f/5 Dobsonian telescope, dual-axis Tom Osypowski equatorial platform.

Switching to the Ninja Telescope

Since 2013, I've been working with a new telescope, a 16-inch f/4.5 Dobsonian called 'Ninja' and sold by Kasai Trading (unfortunately, the Ninja telescopes are not produced anymore). The closed tube and the excellent weight balance convinced me to switch. Tom's platform now carries the Ninja. No changes to the platform were necessary. My work with the new Ninja telescope on the equatorial platform can be found here and here.

Fig. 13 - The new 16-inch f/4.5 Dobsonian telescope 'Ninja' on Tom Osypowski's dual-axis platform, while taking images of the planetary nebula Messier 27. My work with a cooled ASI 2600MC and the new Ninja telescope on the equatorial platform can be found here and here.

The Reality of Astrophotography

Astrophotography combines two captivating disciplines - astronomy and photography. If everything works out as planned, it can be very thrilling and satisfying. It's a multi-step procedure from the point when you have decided to drive up a mountain for an astrophotography session to the point when the image of a remote galaxy is on the memory card. If any of these steps fail, astrophotography for that night will likely be canceled. A missing or broken cable, a misplaced or loose screw, a misaligned optical element, dew accumulation, low battery levels, a short circuit, a missing memory card — even a curious little fox — can ruin an imaging session.

Equatorial Platform vs. German Equatorial Mount

I did astrophotography with roll film and a 10" f/6 reflector on a German equatorial mount back in the 1980s. It was challenging back then and I feared it would get even more difficult with the platform. So how about astrophotography with the platform? In short, it's as easy or difficult as with a "normal" German equatorial mount. After arriving at the observing site, it takes me about one hour to set up and align the platform with a bubble level and the polar finder, put together the telescope, collimate the optics, connect all cables, reset the platform for an 85-minute guiding session, search for the first object, focus the camera and autoguider and find a guiding star and push the remote release. If anything goes wrong, it will take more time. Since the primary mirror needs to cool down for at least one hour, I don't see this as a restriction.

There are things which are easier with the platform: E.g. it's much easier to balance the Dobsonian telescope on the platform. And it will stay balanced during the exposure. Balancing a large Newtonian telescope on a German equatorial mount can be very challenging, as torque shifts during the guiding process. On the other hand, wind is the enemy of the Dobsonian on the equatorial platform. If there is wind, it can ruin the guiding quite easily. I work at focal lengths of 81" (Ninja) and 90" (Zambuto), so the guiding must be extremely precise. (Note: the TeleVue coma corrector slightly increases the focal length of the 16" primary mirror).

Reflections

Astrophotography is a demanding task but the results are rewarding and exciting. I've always felt it's worth the effort. I could likely achieve more resolution out of a 16" mirror by putting it into a solid tube and onto a heavy-duty German equatorial mount. But such a mount would be very expensive (costing at least three or four times as much as the platform) and it wouldn't be a mobile solution anymore. I feel the equatorial platform is one of the most significant innovations in amateur astronomy since the invention of the Dobsonian telescope. It doesn't make the classic German equatorial mount obsolete, but it transforms a simple Dobsonian on an altazimuth mount into a fully-fledged observatory with exciting capabilities. I've mainly ordered the platform for observing. I love to sketch at the telescope and without the tracking capabilities things get difficult. Now I can even take pictures with the telescope.

A New Era of Astrophotography

In 2021, I retired my Canon 450D and replaced it with a cooled ZWO ASI2600MC camera. The significant leap in quantum efficiency (80% vs. 30%), the greater bit depth (16 bit vs. 12 bit), and the ability to actively cool the sensor—greatly reducing thermal noise—combined with calibration frames I created for the first time, allowed for much deeper exposures, revealing fainter stars, intricate structures within deep-sky objects, and far more vibrant colors. The improvement in image quality was truly striking, and I found myself wanting to re-photograph every object using this new technology—though I haven’t yet had the chance to revisit most of them due to some ongoing health-related limitations.

The Limits of Resolution and the Beauty of Observation

When shooting long exposures with a modern CMOS camera equipped with 3.76 µm pixels through a Dobsonian telescope with a 2.1-meter focal length, you enter an oversampling zone. Due to atmospheric turbulence, long exposures are typically limited to a resolution of about 2 arcseconds. This means that you can hardly achieve more resolution than with an 8-inch telescope at a 1.2-meter focal length. Only near the zenith, under exceptionally calm skies, can slightly better results be attained.

From a practical standpoint, it would be wiser to pair the Dobsonian telescope with an equatorial platform and focus on "Lucky Imaging" of the Moon, planets, and bright deep-sky objects with exposures shorter than 1 second — the domain where this configuration truly shines.

Still, the ability to conduct visual observations with a 16-inch telescope and then capture images of those same objects on the very same night has gifted me with countless magical evenings under the stars. Unfortunately, recurring back issues over the years have made it increasingly rare for me to set up the 16-inch Dobsonian for observation. And yet, the memories it gave me remain luminous.

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