What Are Digital Setting Circles?

What Are Digital Setting Circles

We're a company that primarily makes digital setting circles - but we've never really offered a clear explanation about what they are (unless you count a short explanation in our brochures). This has been a mistake on our part, considering that digital setting circles can be an astronomer's most-used accessory and have benefited anyone who's ever used them.

We believe that it's about time we sat down and wrote about the products we make and sell. Below you'll find explanations about DSCs and a little history lesson about them.

First of all - what are digital setting circles (DSCs)?

DSCs are little computers that show you where to point your telescope to locate your astronomical target. You input the target's coordinates (or find it in the DSC's catalogue) and the DSC will then show you a readout of numbers that represent the angular distances to your target, your goal being to move your telescope until those numbers are as close to zero as possible. At zero (or close to), your scope will be pointing at your target object. Think of them as a GPS for your telescope.

Digital setting circles are useful for:

  • finding faint objects
  • finding objects in light-polluted skies
  • identifying objects
  • preserving an astronomer's patience

Each DSC comprises of:

  • 2 rotary encoders
  • mounting hardware
  • and the DSC unit itself

The encoders can vary in resolution - the higher the resolution, the more accurate your DSC will be. Most telescopes have encoders that have a resolution of 8,000 - 10,000 steps per revolution, and most DSCs can't accurately read higher than 40k steps. Encoders are attached to your telescope's axes and sense the scope's position as it is moved. The encoders send pulses to the DSC unit, whose computer will convert them to angular values and calculate where the telescope is pointing. All of this is done after a quick two-star alignment to determine the telescope's location. It generally takes about a minute to set up a DSC for a viewing session.

Overall, these units have progressed so much that many telescope manufacturers now sell DSCs as a pre-installed addition to their telescopes. (Smaller telescope manufacturers such as SDM and Teeter's Telescopes have been selling modern DSCs on their scopes for years now, whilst larger companies such as Sky-Watcher are now starting to do so as well.)

But what prompted the invention of digital setting circles?

Before Digital Setting Circles (B.D.S.C)

Some setting circles can be quite aesthetically pleasing - via Optic Smart

Eleven years ago, Sky & Telescope offered a concise and accurate description of the analogue setting circles on equatorial telescopes:

Nearly every telescope on an equatorial mount comes with setting circles. In theory, they show the right ascension and declination to which the telescope is pointed, making it simple to aim at any object whose coordinates you look up. In practice, experienced observers generally regard setting circles as decorations to help sell telescopes, as a source of false hope for beginners, and possibly useful as makeshift frisbees.

Using these setting circles is a hard task because your alignments and adjustments have to be very precise to get an accurate result. The process of calculating and adjusting ends up making the margin of error bigger and bigger, resulting in inaccurate readings. Using the analogue circles usually amounts to a great deal of frustration.

It's now pretty easy to see why astronomers often didn't use the analogue setting circles on their telescopes.

But what was the olden-day (read: 30+ years ago) way of finding specific astronomical objects? Astronomers used star charts and would star-hop (which is the practice of using a brighter star/object as a guide to finding fainter objects - astronomers will generally start off with a bright object close to their target and then hop object-by-object until they land on their target). You can use either of these options by themselves or together, with the star charts guiding your journey if you're a beginner and don't know the sky too well yet.

It's important to note that many astronomers still use the above methods because they enjoy the adventurous aspect of star-hopping. Some older astronomers also consider computerised (go-to) telescopes to be "cheating". (But laziness wins out in the end, with some of the most seasoned astronomers loving their computerised scopes now.)

The Dawn of Digital Setting Circles

via Astromart

Digital setting circles were invented in the 1980s (Rick McWilliams of Tangent Instruments is credited with inventing them) and started becoming commercially available to amateur astronomers in the late 1980s. The first producer of DSCs was a company called Tangent and their DSCs were marketed under lots of different brand names such as Celestron, Lumicon, Jim's Mobile, Tele Vue, and Orion. There are probably more Tangent DSCs in the field than DSCs from other manufacturers combined.

The earliest DSCs weren't much better than analogue circles as all they showed was a digital read-out for the right ascension (RA) and declination (dec.) axes. They lacked the intuitiveness of modern DSCs, and the only useful part was that their red display was easy to see in the dark.

Several factors contributed to DSCs not being very popular when they first came out:

  • These early DSCs were only slightly more reliable than analogue setting circles
  • They were expensive
  • Their release coincided with the introduction of the first computerised (go-to) telescopes, which astronomers preferred to use instead of fiddling with DSCs. Go-to telescopes already have in-built DSCs, rendering standalone DSCs useless for a portion of the market

Evidencing that early DSCs weren't all that great, it's documented in David A.J. Seargent's book, The Greatest Comets in History: Broom Stars and Celestial Scimitars, that a comet hunt in 1998 was ruined due to inaccurate digital setting circles.

Until DSCs became more intuitive and added more features, amateur astronomers weren't really interested in them. The game really changed in the early 2000s as more manufacturers emerged with newer and better DSC units.

A Timeline of DSCs


via Astromart

Tangent Instruments starts selling their DSCs, branding them under different names. The reason why most DSCs of the following decade were so similar is because Tangent produced most of them. Telescope makers such as Celestron and Tele Vue would sell their branded DSCs as an addition to their telescopes, but haven't done so for a long time. You can now only buy their branded DSCs used - mostly from astronomers who have upgraded to bigger and better DSCs.

The earliest DSCs were expensive and generally not worth the money - a 1989 issue of Sky & Telescope advertises a DSC by Jim's Mobile (pg 17 of 121) selling for US$899, which calculates to US$1,740 in 2016.

1990 - 2000

via Discmounts

During the '90s, companies continued to produce DSCs but the technology mostly stagnated for a decade (most likely due to the lack of interest displayed by amateur astronomers). It wasn't until the 2000s that DSCs grew more popular as other manufacturers entered the market to continue improving them.

One of the most popular DSC in the '90s was Sky Engineering's Sky Commander DSC, first introduced in 1991. The Sky Commander had a catalogue of 16,000 objects, whilst previous DSCs averaged around 12,000.

Until the early 2000s, Sky Engineering & Tangent Instruments continued to be the biggest producers of DSCs.

Considering that Tangent produced most DSCs, it's interesting to see the differences in models: a 1993 issue of Sky & Space reviews three DSCs under different price points, all of which are made by Tangent.

2001 - Modern DSCs enter the market

via Wildcard Innovations

A new, much improved DSC emerges. Manufactured by Wildcard Innovations, the Argo Navis stands out due to its design and functionality. It hosts 29,000 objects in its catalogue with the ability to add 1,000 more. It's one of the first DSCs to allow the user to update its software.


Astro Devices BETI

Astro Devices releases the BETI, the first ever device to provide a wireless interface to encoders. Not being a standalone device, BETI allowed its user to connect their phone or PC via Bluetooth and use the phone/PC as a DSC display. Planetarium software such as Sky Safari is used to interface with BETI.

The Sky Commander XP4 is released as an update to the original Sky Commander, now holding 30,000 objects in its database and using higher-resolution encoders.


Astro Devices Nexus

With the advent of wireless internet and the technological maturity of mobile devices, Astro Devices releases the Nexus, the first ever device to provide a WiFi interface to encoders. It functions much the same as BETI, but uses WiFi rather than Bluetooth.

Astro Devices releases the Nexus-II in 2016 as an update to the Nexus, featuring less ports and a smaller body. The Nexus-II is particularly popular in Japan.

2014 - Nexus DSC releases

Astro Devices Nexus DSC

In 2014, Astro Devices introduces their first ever standalone DSC: the Nexus DSC. The Nexus DSC revolutionises the technology, introducing a host of new features that never existed before: a catalogue boasting almost 2 million objects; supporting high-resolution encoders (with speeds up to 500k steps per second); a 64GB microSD card that enables users to add an almost unlimited number of catalogues; a fully numeric tactile keyboard; software that is updated via USB; a GPS receiver.

The Nexus DSC is currently the most advanced DSC on the market.

Where Are We Going Now?

Astro Devices Nexus DSC

Digital setting circles continue to improve as consumer demand for the best value rises. The two biggest names in DSCs are currently the Argo Navis and the Nexus DSC. Both devices have added huge value to the field and have heightened consumer expectations.

We at Astro Devices pride ourselves on our ability to evolve and we're constantly updating our DSCs' technology. Our Nexus DSC has had several new features added as our customers request them, and our newest software update will allow for the ability to easily correct telescope pointing errors to ensure complete accuracy when using the Nexus DSC.

We're also currently improving our range of products and now sell encoders and encoder installation kits for astronomers who wish the install their DSCs themselves.

We hope you enjoyed this little history lesson from us, and that you learnt a few new things about one of our favourite telescope accessories.

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