We’re all familiar with Neil DeGrasse Tyson’s Cosmos: A Spacetime Odyssey, a reboot of Carl Sagan’s Cosmos: A Personal Journey, which was originally broadcast in the 1980s. Now thanks to Celestron there’s a telescope in association with the TV series, offering skywatchers the chance to make their own tours of the universe.
The Celestron Cosmos 90 GT comes very well equipped, ensuring that the beginner has everything they need to get started in skywatching. Two good quality 1.25-inch Plössls — a 25 mm and 10 mm — are supplied, working with the refractor’s optical system to provide magnifications of 36x and 91x respectively.
Boasting a light-gathering capability that’s 165 times better than the human eye, Celestron promises clear, sharp views thanks to fully-coated optics that ensure good light transmission.
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Optical design: Refractor
Aperture: 90 mm (3.54")
Focal length: 910 mm (35.83")
Focal ratio: f/10
Eyepiece 1 focal length: 25 mm (36x)
Eyepiece 2 focal length: 10 mm (91x)
Total kit weight: 14.2 lbs. (6.44 kg)
Mount type: Computerized alt-azimuth single fork
Cosmos 90 GT: Design
- High-quality build
- GoTo doesn’t drain batteries
- Useless without WiFi or handset
- Steady SkyQLink WiFi
- Good alignment tech
On unboxing the telescope, we didn’t take much time in putting the instrument together. The final set up boasts a high-quality telescope. On seeing the single fork arm alt-azimuth mount and the battery fitting, which holds eight AA batteries, we immediately saw comparisons with the Celestron NexStar 6SE.
However, despite the Cosmos 90 GT’s similarities with the NexStar 6SE, the telescope isn't supplied with a handset to assist the beginner with finding their way around the night sky. Instead, they need to use either an internet-enabled smartphone or tablet to locate or identify targets of interest.
Some may be put off at this point but let us assure you: connecting the telescope’s built-in SkyQLink Wi-Fi network to a smartphone or tablet and downloading the free Cosmos Navigator app (available on iOS and Android) is incredibly straightforward with the provided manual and support.
Operating the telescope using our device is intuitive as we navigated the mount using the on-screen control buttons. Tapping an object and selecting the GoTo option caused the Cosmos 90 GT to slew to the target ready for us to observe — a highly impressive function of this refractor and something that beginners to observing will be grateful for. Chosen targets appeared roughly at the center of the field of view.
The connection between our smartphone and the SkyQLink WiFi network is very steady during our test of the instrument. On a couple of occasions, we manually disconnected from the WiFi to see if the telescope is capable of picking up from where it had left off during an observing run.
We weren’t disappointed in this respect — the Cosmos 90 GT connected quickly and didn’t need realignment, meaning that we could go back to viewing the last target we were observing. A useful feature should the connectivity drop.
When the power switches off, however, this is a different story and you will have to go through the realignment process again. This is nothing major, especially given the time it takes to set the telescope up ready for observations. The Cosmos 90 GT's computerized mount doesn't drain batteries quickly either and we were still going strong even after a continuous three hours of slewing and observing.
Celestron has ensured that this telescope is accessible for those who don’t have a tablet or smartphone — the Cosmos 90 GT can be operated using a NexStar handset (sold separately) by plugging it into the aux port on the single-arm mount. It is important to keep in mind that the telescope is pretty much useless without a handset or WiFi to control it.
Many skywatchers are all too familiar with artificial light that ruins our dark-adapted vision during observing, so we were pleased to find that the Cosmos Navigator app comes with a red light function to ensure that our night vision is preserved.
Cosmos 90 GT: First light and functionality
- Good clear views with decent contrast and clarity
- Offers good views of solar system and deep-sky targets
- Some evidence of color fringing
- Red dot finder allows for easy navigation
Early April saw us take advantage of Venus and Jupiter, which were both visible in a south-western sky. Venus dazzled as a bright featureless point of light, while pleasing views of Jupiter and four of its largest moons — Ganymede, Io, Europa and Callisto — were picked out by the Cosmos 90 GT with ease.
Color fringing is evident but nonetheless, the Cosmos 90 GT offered a very good performance for an entry-level telescope. As promised by Celestron the optical system offers good clear views with decent contrast and clarity.
Slewing to the moon, the craters and lunar are a stunning sight through this telescope. Since a large proportion of the surface was illuminated by the sun, a great deal of detail was washed out, so we targeted the more prevalent features such as the craters Tycho and Copernicus as well as the lunar seas Oceanus Procellarum (Ocean of Storms) and Mare Imbrium (Sea of Showers). Contrast again is found to be excellent and sure to delight beginners.
Waiting for a further few nights, we took advantage of a change in phase and we were not disappointed — a superb contrast between lunar night and day was evident through the Cosmos 90 GT and we enjoyed beautifully presented craters along the terminator. Again, the Cosmos 90GT offered magnified views of these lunar surface features and no blurring could be seen except the rippling of the atmosphere.
The Beehive Cluster (NGC 2632) in the constellation Cancer appears as a white smudge to the unaided eye, but the Cosmos 90 GT quickly transformed the open cluster into a swarm of pin-sharp stars, glistening with a steady white-blue. The Beehive fits reasonably well in the telescope's field of view.
Turning our attention to the brightest star in the constellation of Leo, Regulus, we were able to resolve it into a double star — Regulus A, which shines in a brilliant blue-white compared to its companion, the sun-like and golden star Regulus B — are excellent targets easily picked up by the refractor. The pair are easy to resolve through binoculars, provided you’re able to keep your arms steady and gain a decent amount of focus, but the view was all of the more pleasing through the Cosmos 90 GT.
Slewing over to Gamma Leonis, another double star within Leo, we pushed the magnification of the telescope to 91x using the supplied 10 mm eyepiece. Diffraction rings appeared around the binary’s components and we realized at this time, we’d hit the limit of detail the Cosmos 90 GT is able to show. These rings are interference in the instrument’s optics, caused by starlight hitting the telescope’s objective lens.
Since we were already in Leo, we decided to make the most of the constellation’s famous collection of galaxies. Intermediate spirals Messier 65 and Messier 66 offered good views under magnifications of 35x and 86x, with some dust lanes, glowing bulges and spiral arms visible.
A warning though: while the telescope’s database offers 40,000 solar system and deep-sky objects to observe, the 3.54-inch (90 mm) isn't able to collect enough light to reveal most of them, so it is essential to stay within the optical system's limits to make the most of the Cosmos 90 GT.
Cosmos 90 GT: Verdict
A very good portable telescope for beginners and the whole family, the Celestron Cosmos 90 GT is an affordable instrument for those wanting to observe a variety of night sky targets with minimum fuss.
What’s more, it’s easy to set up and use — we are particularly impressed with the WiFi technology, given the telescope's moderate cost. The addition of eyepieces and red light function on the Cosmos Navigator app offers an excellent package and provides everything the skywatcher needs for a night of observing.
The Cosmos 90 GT’s optics boast a good performance when it comes to observing a selection of targets but a degree of color fringing is evident when focussing on bright objects such as Venus and the moon. Diffraction rings will appear around some targets if the telescope’s magnification is pushed beyond its capabilities.