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Portable planetarium peers into space with low-cost sensors, GPS

Posted: 10 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:celestial viewing device? Skyscout star gazing device? Celestron SkyScout stargazing?

Though I can't distinguish Scorpius from Sagittarius, I do enjoy perusing a star-filled sky. To help folks like me, as well as those who know more about stargazing, Celestron has created the SkyScout. This handheld viewing device can be used to identify or locate more than 6,000 celestial objects.

Named among the "Best of Innovations" at the 2006 Consumer Electronics Show, the SkyScout shipped midyear and remains on my personal "most interesting" list for innovative use of low-cost components. While some products that go through the teardown process reveal design approaches that more or less deliver what's expected, the SkyScout was a surprise in implementation as well as simplicity.

Design surprises
When it was first announced, my first guess was that the SkyScout might work by using pattern recognition and database matching of observed star formationsa challenging design task, and in hindsight a foolish guess on my part. The device does indeed use a database of celestial objects, but the database just holds a map, showing what stars and formations lie where in the three-dimensional space of space itself. More on that in a minute.

Usable as a standalone device or as an attachment to a telescope, the SkyScout measures about 18-by-10-by-7cm and consists of the product enclosures, electronics and a built-in sighting apparatus.

The device has no magnifying power. Instead it lets the user view the night sky through a set of two windows that are aligned along the long axis of its rubberized case. To aim the SkyScout, the user centers the star or constellation in the targets located on the front and rear sight windows. The operator can then push a simple "identify" button. An included headset lets the user listen to a description of the sky targeted by the SkyScout.

Alternatively, users can elect to find a celestial body. After the user chooses a star or constellation of interest from the extensive set of database options and presses the "locate" button, the SkyScout guides the user to the selected star(s) by a series of LED lamps that ring the viewfinder. As the device is moved around, the LEDs begin to blink more slowly (as the image moves further away) or more quickly (as motion brings the target closer within the viewfinder).

As it turns out, this fascinating tool uses no image recognition at all. Rather, the SkyScout simply needs to sense "where, when and in which direction." In this way, the objects seen in the viewfinder can be identified from and matched to the previously mentioned location-only database.

Celestron Skyscout

SkyScout can identify or locate over 6,000 celestial objects.
Click image to view teardown diagram

Virtually a planetarium
Think of the database as a virtual planetarium. The electronics determine where the device is pointed within the full sphere of the planetarium.

The trick then is to render the location, time and direction information without breaking the bank.

The first two pieces of information come from the SkyScout's built-in GPS. A PG4200 from Freescale contains the GPS receiver (and an associated 256Kbyte Samsung SRAM memory) to receive coordinate information and a time stamp. GPS chipsets have become quite inexpensive, and by leveraging the processing power of the Samsung S3C2410 host CPU, the receiver-only circuitry of the Freescale part adds little to the total BoM.

With the "where and when" taken care of by GPS, the "which direction" relies on the earth's magnetic fields. One magnetoresistive sensor from Honeywell, the HMC1022, covers x and y. A second magnetoresistive chip (HMC1041Z), from the same company, provides angle of incline. Much like a compass with an added inclinometer, the two sensors finalize the overall (electronic) pointing vector into the planetarium stored in the database. Combining that information with the time and place on the earth's surface, the SkyScout can give you a report on what you are looking at.

For location vs. identification, all this works in reverse: The SkyScout guides the user to a selected feature in space by monitoring closure between the known target point and the pointing vector.

Accelerometer mystery
Dual-axis accelerometers supplied by Analog Devices join with the Honeywell sensors, with one accelerometer located on the board shown and a second located elsewhere in the system. We are guessing that the accelerometers are used either to determine rate of closure on a desired pointing direction for the "locate mode" or to help remove error through tracking of shake and vibration, but those theories are speculative. Readers' thoughts in this regard are welcome.

Other parts in the system are used for analog signal amplification, analog signal multiplexing and analog-to-digital conversion, but none is particularly complex or expensive. An AKM Semiconductor AK4366 audio codec interfaces the headphone jack to let the user hear descriptions of the objects being viewed. The Samsung processor is joined by 32Mbytes of NAND flash, presumably holding both the system code and the celestial database, and 8Mbytes of SDRAM for working memory.

The processor also handles all user interface, speech output and display driving for the 240 x 80pixel monochrome LCD.

Despite the SkyScout's fairly amazing capabilities, the design relies fundamentally on low-cost sensors and ever-cheaper GPS location to get its bearings in three-dimensional space. Once that task is taken care of, a modest processing engine and an abundant collection of celestial data accomplish the rest.

The total cost of goods sold for the SkyScout accounts for a small fraction of the $399 retail price for the product, but don't let that keep you from buying it if you find the concept interesting. Celestron's premium here is the company's reward for clever design and a rich database of space.

Happy stargazing!

- David Carey
President, Portelligent




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