Wednesday, February 28

Celestron StarSense Explorer 254mm Dobsonian – Astronomy Now

Celestron’s StarSense Explorer 254mm Dobsonian facilitates plate solving using your mobile-phone camera and an app. All images: Ade Ashford.

Celestron’s new StarSense Explorer 203mm and 254mm instruments may seem like the company’s first foray into larger aperture Dobsonians, but those with long memories will recall their StarHopper range of the mid-to-late 1990s available in apertures up to 444mm – behemoths that would still turn heads today. However, Celestron’s StarHoppers of a quarter of a century ago were classical Dobsonians very much in keeping with the late John Dobson’s eponymous telescope design, so I wonder what he would make of the instrument reviewed here with its radically new alignment and navigation system based on a smartphone. One thing’s for sure – the StarSense Explorer 254mm is not your father’s Dobsonian!


The telescope comes in two double-boxed cardboard shipping containers. The largest contains the pre-assembled optical tube, accessories and instructions, and grosses 21 kilograms, so you’ll need an extra pair of hands getting it indoors. The smaller box tips the scales at 15 kilograms. The latter contains the instrument’s alt-azimuth mount made of plastic-coated particleboard (chipboard) in flat-pack form, so it pays to have some proficiency with Allen keys, a screwdriver and spanners (all supplied as part of the kit). Thankfully, Celestron’s eight-page A4 quick set-up guide is profusely and clearly illustrated with well-written instructions, but be sure to give yourself plenty of floor space and set aside about an hour to put it together.

All sides of the particleboard components, including the edges, are faced with the same durable black plastic coating for improved water resistance. Cheaper Dobsonians often leave bareedged particleboard hidden during assembly that swells over time due to moisture ingress from proximity to dewy grass. Particleboard is quite dense, so the mount has strategically positioned cut-outs to save weight without compromising its strength and rigidity. These apertures double as convenient hand grips in addition to the sturdy carrying handle on the mount’s front panel.

The StarSense Dobsonian employs a silky-smooth, single-speed Crayford focuser.
Down the barrel of the Dobsonian tube, looking at the primary 254mm (measured) mirror.

Rather than relying on just three radial Teflon pads for the azimuth-bearing surface, StarSense Explorer Dobsonians feature a large central Teflon washer surrounding the azimuth pivot bolt that helps achieve the correct level of ‘stiction’ on this axis. Vertical slots on either side of the mount accept hand-knobs threaded into the circular side-hubs on opposite sides of the optical tube, the hubs riding on two altitude-bearing cylinders on each side of the mount. The optical tube merely lowers into the mount and the left-hand knob acts as a variable friction brake to control the altitudeaxis stiction.

An Apple iPhone 6s Plus in the dock attached to the top of the StarSense Explorer’s telescope tube. Through an ingenious mechanism, a pair of knurled knobs will position your phone’s camera over a mirror through which it can see the stars.

First impressions

There’s no denying that the fully assembled Celestron StarSense Explorer 254mm Dobsonian is an aesthetically pleasing instrument. Its rolled steel tube in a silver-grey finish stands out in the dark even when bathed in only starlight, so you’re not going to blunder into it on the observing field. The 112.5cm long tube has 29cm-diameter curved alloy cells at either end, while the curvaceous lines of its mount, reminiscent of Sky-Watcher and Orion Dobsonians, reveals their common Synta Technology Corporation heritage. Fully assembled, the instrument tips the scales at a little under 25 kilograms.

The assembled mount is 67cm high and the base is 54cm in diameter, so it will fit through most doors. The front panel of the base is where you’ll find a rack for a 50.8mm and three 31.75mm push-fit eyepieces. As for the 13 kilogram optical tube, Celestron should be lauded for adding a stout carrying handle on the top at its centre of gravity. It is a simple but very welcome feature that greatly aids set-up and is infinitely preferable to cradling a slippery, dew-covered Dobsonian tube close to your chest at the end of an observing session.

A closer look at the front of the StarSense Explorer 254mm Dobsonian. The phone dock is situated between the red-dot finder and the focuser. Note the scoop-shaped light-guard on the dock.

The mount’s altitude axis is around 58cm above the ground, placing the eyepiece 82cm high when the tube is horizontal, or 113cm high at the zenith. Hence the StarSense Explorer 254mm Dobsonian is best used in a seated position by most adults; I knelt on the grass most nights. The single-speed Crayford focuser is smooth and very precise in operation, ergonomically inclined to the horizontal for comfortable use. The drawtube accepts two-inch push-fit eyepieces, but you’ll need the supplied extension tube and a two-inch to 1.25- inch adaptor to use the 25mm focal length (48× magnification) Omni Plössl provided.

The instrument’s optics were almost perfectly aligned out of the box, but Celestron have thoughtfully provided a small collimating cap, with detailed online instructions found on their support pages. A laser collimator confirmed that the 64mm minor-axis secondary was optimally positioned, while just the merest tweak of the primary mirror’s collimation hand and lock knobs were required. The latter are recessed into the rear of the telescope, so you can conveniently stand the optical tube on its end when not in use.

A conventional red-dot finder is supplied, although its position on the side of the tube, almost 90 degrees from the focuser, is a little inconvenient. However, most times you will be using your smartphone in the StarSense dock that lies on the top of the tube. So, without further ado, what is StarSense and how do you use it?

The process of using the StarSense to find your target, in this the Hercules Cluster, M13. Selecting the object within the app will show you a sky-map with animated arrows telling you where to push the telescope (left). As you near it a reticule will appear around it as the app guides you in (middle). Once you are on your target the app will prompt you to look through the eyepiece (right).

A StarSense primer

Analysing star patterns in a night-sky image captured by a camera as a means of determining where a telescope is pointing is called ‘plate solving’. It’s a technology that’s filtered down from professional instruments, but it was Celestron who first commercialised self-aligning technology with their SkyProdigy range in 2011, followed by the StarSense AutoAlign accessory as an add-on to most of their computerised telescopes two years later.

StarSense uses a so-called ‘Lost in Space Algorithm’ (LISA), much like that used by artificial satellites to orient themselves in space, patternmatching stars against a vast internal database to determine where the camera is pointed. If the camera’s orientation relative to the telescope is also known – asimple one-off alignment procedure – then the camera can orientate the telescope.

What Celestron’s engineers, in conjunction with software partner Simulation Curriculum (creator of SkySafari), have ingeniously achieved is to distil StarSense’s AutoAlign LISA algorithm, deep-sky object databases and Solar System body modelling into a smartphone app with a gorgeous graphical user interface. The app uses the phone’s camera to capture real-time star fields via an inclined mirror, and these are plate-solved to precisely determine the handset’s position relative to the telescope it’s mounted upon.

The StarSense Explorer smartphone app is available for iOS and Android. Any iPhone 6 or newer is compatible, as are smartphones running Android 7.1.2 (or later) and manufactured after 2016. If you’re unsure that your smartphone makes the cut, Celestron have a convenient online checker – just point your phone’s web browser to At the time of writing, StarSense v1.17 was available from the Apple App Store, which I installed on a seven-year-old iPhone 6s Plus. The first time you run the app, you need to enter a unique alphanumeric code from a card supplied with the telescope. You can unlock the app on up to five different devices per telescope.

Given the age of my iPhone, I was sceptical that its 12MP rear camera would be sensitive enough or have the resolution (~1 arcminute) to capture enough stars in the app’s ‘night mode’. However, in tests conducted under a Bortle 4 sky on 10 August 2022 at 10pm BST, deep in nautical twilight with a 13-dayold Moon low in the south-southeast, I could still see stars as faint as fourthmagnitude when zoomed in on the app’s alignment screen.

The app warns you if the Moon will interfere with star recognition, but in practice this is not a hindrance: no-one looks for deep-sky objects next to a full Moon and you always have the red-dot finder to fall back on. Observing from the rural edge of a small market town, I found that the app could always plate solve in nautical twilight and darker skies. Basically, if you can see more than a few faint stars it will work, which is most impressive.

The StarSense app will determine where the telescope is pointed in the sky based on the patterns of the stars in an image taken by your phone. Note the red-screen night-vision mode. The list of categories of object in the StarSense database. Annoyingly, comets and asteroids do not have their own category, nor are they under the ‘Moon & Planets’ category, but they are in the database if you search for their individual designations.

Once aligned, the app shows the simulated night sky in real-time based on the phone’s internal clock and its GPS-derived location. You can select some ‘best of’ deep-sky objects from the app’s screen, or search from a list of the Moon and planets, bright stars, double stars, deep-sky objects from all the Messier, Caldwell, NGC and IC catalogues – and some obscure ones too. It’s impressively comprehensive, though I was initially disappointed that comets and asteroids weren’t listed under the Solar System category, but out of curiosity I entered a few of their official designations into the search box and most were there. For nearly every object there is a detailed description, observing tips and observational data screens – you can even have the app talk about the object as you observe it using the Celestron audio feature! Don’t worry that you may be observing from some remote location without a mobile signal or Wi-Fi because the app is entirely self-contained. In fact, everything works even if the phone is in airplane mode.

Once you’ve selected an object, the app captures a night-sky image to calculate the telescope’s position, then shows animated direction arrows to where you should move the instrument to find it. The app automatically zooms the view as you approach your target, showing fainter field stars and more deep-sky objects in its vicinity. As you near your goal, the app requests that you pause the telescope while another plate solve refines its pointing. By this iterative process the on-screen reticle soon displays corners around your targeting graticle showing that you’re on target, and there it is in the eyepiece!

Celestron claim that the StarSense Explorer app has a quarter-degree pointing accuracy, and I would wholeheartedly concur with that based on real-sky testing. Provided that I had carefully aligned my phone with the telescope at the start of an observing session, my selected targets were always within the field of view of the supplied 25mm (48x) eyepiece, and most times in the onethird degree field of my medium-to-high power 7.2mm (167x) eyepiece.

StarSense dock

Every StarSense Explorer telescope possesses a dock to hold your smartphone firmly in place. Given the huge variety of handsets, Celestron pulled off a neat trick devising a quick-release, onesize-fits-all adjustable holder.

Removing a clip-on plastic cover from a scoopshaped lightshield on the front of the StarSense dock exposes a first-surface mirror that lies at 45 degrees to your smartphone’s camera. The mirror permits your smartphone to access alignment stars high in the sky while its screen remains at a convenient viewing angle. To align your phone entails the use of an ingenious mechanism.

On the underside of the dock, perpendicular to your phone, is a short stalk with two knurled grip knobs that operate two geared pinions meshing against a pair of racks and sliders at right angles to each other. Judicious use of both knobs will, to sub-millimetre precision, move your phone so that its camera is positioned over the mirror.

While it may sound complicated, the StarSense dock is very intuitive to set-up and adjust, and the app’s user-friendly software makes it just as easy to align your phone with the telescope as any conventional optical finder. In fact, I would argue it’s considerably easier. Just like an optical finder, you only need to align it once and then you can forget about it unless you accidentally dislodge your phone or re-collimate your telescope.

Optical performance

The summer heatwave of 2022 afforded several opportunities to use the review instrument under clear, if somewhat turbulent, skies. However, the early hours often delivered some tolerably good seeing in which to observe Jupiter and Saturn close to opposition. The supplied 48× eyepiece was too low a power for the gas giants, so I used my own 7.2mm eyepiece at 167×. Saturn was magnificent, the Cassini division crisply rendered when seeing permitted. The planet’s retinue of inner moons were also clearly on view, notably Tethys.

A red-dot finder is provided, even if it is almost redundant thanks to the StarSense app. Its location almost 90 degrees from the focuser is also inconvenient.

The double shadow transit of Jupiter’s moons Io and Ganymede in the small hours of 9 August 2022 made for an attractive show. The difference in the size of the shadows was obvious, Ganymede clearly being the larger moon. The end of Io’s transit of Jupiter on 11 August shortly after 12am BST was also well observed, the little moon looking like a bright bead – adistinct disc – superimposed on the planet’s western limb.

Jupiter’s Great Red Spot, close to transiting Jupiter’s meridian on 20 August, was obvious even with the supplied eyepiece at 48×. The seeing was average, but using my own 7.2mm eyepiece revealed fleeting glimpses of fine surface detail when the view crystallised into clarity. Incidentally, the instrument’s Dobsonian base is exceptionally stable: vibrations induced by a sharp tap on the tube die away in a second.

A 254mm-aperture telescope gathers sufficient light to reveal a great many deep-sky objects in considerable detail under dark, moonless skies, and I never found the StarSense Explorer Dobsonian wanting in this regard. From my lightpolluted observing site I could hold Messier 13 even under direct vision, as indeed I could the central star and core of the Cat’s Eye Nebula. The latter had a distinctly greenish tint. Messier 27 was lovely, the diaphanous portions of the Dumbbell Nebula revealing glittering stars of the rich Milky Way backdrop.

A look at the 254mm primary mirror, wirh the secondary mirror and spider reflected in it. The instrument barely needed collimating out of the box.

Concluding thoughts

The fusion of computerised star recognition and object location with the elegant simplicity of a Dobsonian telescope may appear to be the antithesis of what John Dobson was striving for, but to my mind it’s a marriage made in heaven. The intuitive alt-azimuth motions, stability and ‘biggest bang for your buck’ in optical performance and aperture afforded by a Dobsonian wedded to the easy-touse, reliable and virtually foolproof StarSense system is a winning combination.

Celestron’s 254mm StarSense Explorer Dobsonian retails for about the same price as competing bare-bones 300mm models, but the former’s intuitive and very well executed object location system more than makes up for the smaller light grasp.

But this is mostly about the StarSense Explorer app and the way that it will revolutionise your observing sessions. Its beautiful graphical user interface (which, incidentally, you can redden to preserve your dark adaption) invites you to explore a constellation or region of sky in greater detail, making serendipitous finds here and there. The way that it uses any compatible phone to reliably plate-solve and find objects means that you don’t need great swathes of clear sky to seek out alignment stars, as is demanded by traditional computerised GoTo mounts – indeed, it effectively renders such methods obsolete. Make no mistake, the StarSense Explorer app is a game changer that will appeal to seasoned observers and smartphone-addicted millennials alike.

I felt a distinct pang of regret when the courier arrived to take it back to First Light Optics; I miss it already.

At a glance

Optical design: Newtonian reflector

Aperture: 254mm

Focal length: 1,200mm

Focal ratio: f/4.72

Focuser: Single-speed Crayford, 50.8mm with 31.75mm adaptor

Mount: Dobsonian with variable friction brake (altitude axis)

Object location: StarSense Explorer app (user supplied phone); red dot finder

Eyepiece: 25mm Omni Plössl (48×)

Accessories: StarSense Explorer dock, eyepiece rack, collimation cap, carry handles for OTA and base, pointing knob, dust covers

Tube length: 112.5cm

Tube weight: 13.25kg

Base weight: 11.6kg

Price: £849


Ade Ashford has travelled the globe writing about astronomy and telescopes, serving on the staff of astronomy magazines on both sides of the Atlantic.