Plus: Very good main optics; the smartphone-aided pointing works!
Minus: Poor Amici prism diagonal and eyepieces; somewhat shaky mount
Summary: The StarSense technology provides computer-aided pointing for the Celestron StarSense Explorer DX 102AZ, without the cost and complexity of a GoTo telescope.
Who Is It For? All beginners, including kids ages 10 and up, and also experienced amateur astronomers looking for a second grab-and-go telescope or mount.
When Celestron introduced their StarSense telescopes in early 2020, I was skeptical. Surely this was another gimmick. How could your smart phone take images of the sky good enough to provide accurate pointing?
Well, it works. And amazingly well. For this review I tested the DX 102AZ, a 102mm f/6.5 achromatic refractor. The other telescope offered on the same DX mount is a $400 130mm f/5 Newtonian reflector. The series also includes an 80mm f/11 refractor and a 114mm f/9 reflector, both for $180 on the lighter LT mount. The StarSense app also lists 5-inch and 6-inch Schmidt-Cassegrains, models still to come I presume.
Smart Phone Aiming
What is unique to the StarSense scopes is that they use your smart phone (most recent Apple and Android phones are compatible) mounted in a bracket so the phone’s camera looks into a mirror. That way it can see the sky while remaining horizontal and convenient to view and touch. The StarSense app uses the camera to capture images of the sky to “plate solve,” jargon for figuring out where it is aimed from the pattern of stars it sees.
I found that the app could find enough stars to provide accurate pointing even in deep twilight and on bright moonlit nights with some clouds drifting through. It performs its plate solving in seconds every time you go to a new target, not just for an initial alignment as with GoTo telescopes.
The app requires that you first adjust the bracket to place the camera so it sees as much of the mirror as possible. Then at night it needs a simple alignment, first by aiming at a bright light in the distance so it is centered in the eyepiece of the telescope. In the phone’s image you then drag crosshairs over that light. It’s all quick and easy, with the on-screen instructions stepping you through the process.
On subsequent nights you might not need to perform that phone-to-scope alignment again as the phone will likely go back into the bracket in the same position. But the app asks each time it starts up if you wish to align or not.
The app then displays arrows to show you how to move the telescope to center a target, picked either by tapping on an object on the screen, or by picking from the list of Tonight’s Best, from the Messier or Caldwell catalogs, or lists of double stars and asterisms. Plus the moon and planets. Missing, as of v1.0.5, is the full NGC/IC catalog.
When the app indicates it’s on target I found that the object was indeed in the field of the low-power eyepiece, consistently and reliably. It really was a pleasure exploring around the sky. The bonus is that the app provides narrated explanations, images, and pages of information about the objects. But just to be clear: the phone does not control the telescope, through WiFi or any other connection. It’s a digital finder that shows you where to move the scope. The scope has no motors and so cannot track the sky.
Should your phone die (in winter its battery could fail) you still have a good telescope you can use to find things the old-fashioned way, using the red dot finder.
The 102mm Optics
Being a fast f/6.5 achromatic, the DX 102 does show some false color, with bright magenta halos inside focus and cyan halos outside focus. However, in focus the limb of the Moon and the disk of Jupiter looked surprisingly color-free and sharp. But colored double stars like Gamma Andromeda did take on a magenta hue.
In a star test at high power using a high-quality eyepiece and star diagonal, the main lens proved free of image-blurring spherical aberration and astigmatism. The telescope can deliver satisfying views of the planets, with the following caveats.
The included optical accessories are designed to keep costs down. The 25mm and 10mm Kellner-class eyepieces are decent enough to start with, providing powers of 26x and 66x, so there are no inflated claims of “450x Power!” here, a good thing.
The star diagonal, as is now often the case on beginner scopes, uses an Amici prism to give an erect image. That avoids new users thinking their telescope is broken because the image is the wrong way around. However, at high power the Amici prism adds diffraction that blurs images and surrounds bright objects with a spike of light. The included diagonal also vignettes the field of the low-power eyepiece.
For better optical performance buy an astronomical star diagonal for $40, then upgrade to better Plössl or wide-field eyepieces, and perhaps a good 2x Barlow. While the DX102’s focuser will accept 2-inch accessories, they would have to be light weight.
The DX Mount
The alt-azimuth mount works well, with slip clutches so it can be pushed around the sky without fussing with axes locks, which it lacks, yet it stays firmly in place. The slow-motion controls allow precise aiming, and they remain in the same location relative to the eyepiece no matter where the scope is aimed, a nice convenience.
The downside is that the mount is shakier than ideal, with vibrations taking about 3 to 4 seconds to die down after touching the focuser or mount. I’ve seen worse, so this is about par for a lightweight beginner mount. The mount uses a standard Vixen mounting bracket, so users could buy a DX model just to use its mount and StarSense pointing with another scope, such as a small apochromatic refractor. With the shorter tube of an 80mm f/6 apo vibrations damped down in just over 2 seconds.
I tested the cheaper LT 80AZ earlier this year for the May-June 2020 issue of the Canadian SkyNews magazine, and found it similar for accurate pointing, good optics, but also so-so fittings and a shakier, but usable, mount.
From my testing of the two refractors, I think the StarSense telescopes represent a breakthrough in beginner scopes. They provide accurate computer-assisted pointing without the potential for errors when entering site information and the fuss of star alignments needed by more costly GoTo telescopes. The StarSense Explorers are just fun to use. I soon found myself enjoying my test nights by forgetting to test and just looking at lots of objects that were now so easy to find.
All images by Alan Dyer.