Telescope review
This telescope’s innovative technology provides the simplest and quickest solution yet to finding objects to observe
There are four instruments in the Celestron StarSense Explorer range: an 80mm refractor, a 114mm reflector, the 102mm refractor under test and a 130mm reflector. All these instruments are standard members of the Celestron range and are available in other mounting packages. In this range, the 80mm and 114mm are supplied on lighter weight mountings, while the 102mm and 130mm have the same heavier mounting. The alt-azimuth mounting means you can move the instrument around two axes – up and down and from side to side. This is the simplest form of mounting, and is particularly suitable for a beginner. The alternative equatorial mounting, with one of its axes tilted at the same angle as your latitude, not only confuses many beginners, but has the drawback that it involves an overhanging load usually balanced with a counterweight, reducing the portability.
The mounting of the DX 102 is completely manual, with flexible rods to allow what are called slow motions, providing a finer control over the movement than simply pulling and pushing the instrument. The extra ingredient over a standard manual mounting is the attachment for a smartphone, directly fixed to the central assembly.
This is not to be confused with smartphone attachments that simply enable you to take photos of bright objects through the eyepiece, as found on some other beginners’ instruments.
This is an achromatic refractor with a 102mm doublet lens composed of two different types of glass. It has a focal ratio of f/6.5, which means that it has a comparatively short tube for its diameter
– a situation that provides a wide field of view and bright image with a fairly compact instrument. Achromatic implies a freedom from the false colour that bedevilled refractors in the very early days of telescopes, but this is a comparative term. Like other achromatic refractors, the instrument still has some false colour, which shows as coloured fringes around the edges of objects, particularly at high magnification, and the short tube results in inherently more false colour than an achromatic instrument with a longer focal ratio. To completely overcome false colour you need a considerably more expensive apochromatic refractor or a reflector. However, refractors are traditionally more robust than reflectors, so there is no ideal instrument to suit everyone.
The package comes with two eyepieces, giving two useful magnifications of 26 and 66, and what is termed an erect-image diagonal. A star diagonal is standard with astronomical refractors, and reflects the beam from the focuser through 90 degrees to allow a much easier observing position than crouching on the ground looking up through the tube, but usually it has the drawback of reversing the image as in an ordinary mirror. The diagonal provided by Celestron use a roof prism rather than a simple mirror, giving a non-reversed image so you can use the instrument by day as well as by night. It also avoids the confusion of viewing the moon’s features or star fields back to front. Finally,
“Provides a wide field of view and bright image with a fairly compact instrument”
there is a standard red-dot finder to help in pointing the telescope at objects. This is necessary when aligning the telescope, as you need to point it at a recognisable object at the start of observing.
The instrument comes in just two main parts – the tripod and the tube assembly – so it seems straightforward. The accessory tray that sits in the middle of the tripod legs, however, needs to be attached by fiddly little thumb screws that are already in place, but the wrong way round in their holes! This is poor design, as these screws will quickly get lost. A simple slot-and-turn system would be much less aggravating, unless you intend to use the instrument solely as a piece of furniture permanently set up in the corner of the room – or do without the tray altogether.
The telescope attaches to the mount using a standard dovetail, but as it came out of the box the clamp assembly was twisted through 180 degrees, with the locking screw at the bottom. The instructions tell you to turn it, but this is easier said than done. Many beginners will wonder if they are meant to be forcing this round at all. In fact, you do need to apply a lot of force to twist it. Confusingly, the instructions then refer to the knob at the bottom of the clamp.
Once set up, you need to align the red-dot finder with the main instrument by day. Unfortunately the
adjustment range on the device supplied did not allow it to be aligned accurately, and for the test we used another that we already had.
The final step is to remove the cap from the mirror on the StarSense dock – again courageously pulling it off with some effort – and fitting your smartphone into the spring-loaded slider. Bear in mind that not all smartphones will work, and check your model against a list on the Celestron website before taking the plunge. It must have a camera sensitive enough to image the brighter stars using a night setting.
We’re pleased to report that loading and using the software on a Moto G8 was easy and quite intuitive. This was fortunate, as the quick start manual ends at this point. All the following stages are accomplished using the StarSense app that you need to unlock using a code supplied with your instrument, which will work with up to five devices.
The camera views the sky through a mirror, enabling you to view the phone’s screen at a convenient angle. The StarSense app displays the camera’s view of the night sky using a time exposure if necessary, so it may take some seconds to appear or refresh. The method of alignment is to aim the telescope at a chosen object – which is where the red-dot finder comes in handy – and then move the red crosswires on the app to the same object shown on the phone view. You can zoom in for better precision.
Once complete you are in business, and you can choose from a list of objects observable with this instrument at the time, graded according to whether they can be seen from the city or from a dark sky. Once chosen, you’ll see a set of arrows on the screen that tell you which way to move the telescope to get to that object. Within seconds you are on target, and the crosswires turn green. Moving from one object to another takes just seconds – the quickest finding system we’ve seen.
Some drawbacks did become evident. The finding system is not always spot on, so you need to be prepared to search for objects around the edge of the field of view. The clutch that holds the telescope in altitude, on the test model at least, was fierce, so to move it you need to grasp the tripod firmly with one hand and wrench the tube up or down with the other. The altitude slow motion is equally stiff to turn, while the azimuth control had an annoying backlash.
One major source of problems was the mirror, which is crucial for finding objects. The software constantly checks the alignment of the instrument, complaining if it can’t see the sky, so snapping on the cover in between observations is not easy or ideal. The mirror can become dewed up within minutes on some nights – often the clearest. Unlike household mirrors, its reflective coating is on the front surface rather than the back, so regularly wiping it clear could result in damage to the coating. Unwary users are likely to try polishing it with glass cleaner or even abrasive solutions.
Optically the telescope is well up to standard for a short-focus achromatic refractor, and is well suited to deep-sky observing. The eyepieces supplied, although in lightweight plastic barrels, are of good optical quality. Boosting the magnification using a Celestron 2x Barlow lens revealed false colour in lunar and planetary images, so this is not an ideal high-power or planetary instrument. The erect-image diagonal, which allows the instrument to focus, introduces very little distortion, only marginally noticeable on a star image using a magnification of 130x.
Once you’re accustomed to the instrument, finding objects is extremely quick and fun. The optical quality is good for a telescope of this type, but for planetary observing a different instrument would be preferable, such as the 130mm reflector from the same range. The clunky movement and delicate mirror could limit the usefulness of the instrument for more detailed observations, but the system certainly does get you observing within just a few minutes, unlike more advanced instruments, thus overcoming one of the greatest hurdles that beginners face.
The pricing of this instrument is not significantly less than that of the Celestron NexStar 102 SLT, the same instrument on a fully motorised mount, which has the great advantage of tracking objects for you. Whether you buy this model or opt for the full Go-To version, or indeed the NexStar 130 SLT reflector, is a decision that you will have to make depending on your observing interests.
“Loading and Using the software on a Moto G8 was easy and quite intuitive”