The Optical Tube
The Celestron AstroMaster 90EQ is a 90mm (3.5”) f/11 achromatic refractor telescope with a focal length of 1000mm. Like most achromats, the 90EQ uses a Fraunhofer configuration with crown and flint glasses for the objective lens, and at this size and relatively slow focal ratio, chromatic aberration is kept at bay on all but the brightest targets. The Moon, Jupiter, Venus, and bright stars have slight purple halos around them, but the 90EQ has no trouble rendering otherwise sharp detail and pinpoint stars.
As with most refractors, the 90EQ has a built-in dew shield to control condensation as well as stray light, and baffles are placed within the tube to help reduce internal reflections. The lens cell is plastic and not able to be adjusted should the telescope’s collimation be seriously disturbed, but this is unlikely to ever be a problem. Like all of the AstroMaster telescopes, the 90EQ has a 1.25″ rack and pinion focuser made mostly of plastic, which works okay if you don’t use any heavy or expensive eyepieces, though it can sag and wobble somewhat.
One of many signs of Celestron’s cost-cutting measures is the short Vixen-style dovetail plate bolted to the bottom of the AstroMaster 90EQ’s optical tube. There is no way to rotate the tube in any rings to move the eyepiece angle, nor slide the whole thing along for balance, though the universally standard dovetail at least allows you to easily attach the telescope to a different mount if you wish.
The Celestron AstroMaster 90EQ refractor telescope is equipped with a 1.25” Amici prism diagonal that is designed for terrestrial viewing and is mostly made of plastic. It has a unique shape and grip that were designed to accommodate people grabbing and holding it like a handle. The body is plastic, and the Amici prism itself is not of high quality, causing internal reflections and glare. The Amici prism design also inherently produces a bright spike on bright objects.
What’s more is that Celestron’s cheap Amici prism is so undersized that it will actually vignette your view with low-power eyepieces like a 32mm Plossl, which explains Celestron’s unusual choice to provide a 20mm 3-element eyepiece for “low” power (50x). 50x is quite a lot for such a small instrument, and the 50-degree apparent field of view of both eyepieces means the true field the 20mm ocular provides with the 90EQ is only 1 degree across, or about twice the width of the full Moon. For high power, you get a 10 mm 3-element eyepiece for 100x. Both of these 1.25” eyepieces are of acceptable quality and provide sharp images, though their performance is bottlenecked by the low quality of the Amici prism.
To aim the telescope around the sky, the AstroMaster 90EQ comes with a standard, generic “StarPointer” red dot sight powered by a CR2032 battery. The provided red dot finder is great, and it’s actually an ad-hoc upgrade from the original low-quality plastic sights that were originally shipped with the AstroMaster line. Unfortunately, the retrofitted bracket used to attach the red dot sight to the AstroMaster 90EQ’s focuser is cheaply made and often misshapen enough to make it difficult to align the finder itself with the telescope, which can make trying to point the 90EQ at anything frustrating or even impossible. Making a DIY fix or buying a replacement finder/bracket is fairly easy to do, but it’s frustrating that Celestron created the problem in the first place.
The AstroMaster CG-3 Mount
The mount supplied with the Celestron AstroMaster EQ telescopes is known as the CG-3, though it is sometimes referred to as a CG-2 or EQ2. The CG-3 is a typical cheap German equatorial mount with small, decorative setting circles (advertised as a feature but essentially useless). It has 1.25” tubular steel legs, but much of the tripod and fittings, such as the leg locks and the accessory tray, are made of cheap plastic that is easily damaged.
The CG-3 has a Vixen-style dovetail saddle, allowing other optical tubes to be easily interchanged without any tools. It also has fine adjustments for polar alignment, which need to be done after you assemble the telescope and level the tripod. There are no provisions for a polar scope, so your polar alignment is going to be fairly inaccurate, but for a telescope that’s not for astrophotography, this isn’t a big deal.
The CG-3 also has to be balanced correctly with the telescope optical tube, as with any equatorial mount. Normally you’d have to slide the 90EQ’s tube forward-backward in some kind of tube rings or move the dovetail bar itself for balancing in declination, but the 90EQ’s Vixen-style dovetail bar is so short that you basically cannot adjust for balance on the declination axis, though the default position is just fine. Balancing in right ascension is done with the two provided counterweights. You can leave the counterweights installed or mark the counterweight shaft with a permanent marker or tape to avoid having to figure out balance every time you assemble your telescope.
After polar alignment and balancing are done, you aim the CG-3 around the sky by loosening the clutches and then locking them when you are close to your target. As with most manual mounts, it thankfully has flexible slow-motion cables for both axes; you turn the right ascension cable to track targets throughout the sky, and both slow-motion cables are used for finely pointing the telescope after the clutches are locked. You can add a cheap motor drive such as Celestron’s “Logic Drive” to obviate the need for manual tracking altogether, though neither the mount, telescope, nor drive are any good for long-exposure astrophotography.
While cheaply made, a little complex to use, and certainly not the steadiest, the CG-3 mount is alright for the AstroMaster 90EQ. It can be improved by avoiding extending the tripod legs all the way, filling the legs with foam or sand, and putting a heavy item such as weights or a water jug on the accessory tray to lower the telescope’s center of gravity.
Should I buy a Used Celestron AstroMaster 90EQ?
A used Celestron AstroMaster 90EQ is a decent scope if it is available at a low price. However, older units have a circular plastic sight that is essentially useless and gets in the way of installing a proper finder, which you may want to avoid. The plastic parts on the 90EQ are easily damaged, and you should make sure the lens is free of any chips or cracks, but there is little that can go wrong with a used unit that won’t be immediately obvious to even the most casual shopper.
The Celestron AstroMaster 90EQ provides poor value in its price range compared to many of the other high-quality reflectors, refractors, and catadioptric telescopes available.
- The Sky-Watcher Heritage 130P has significantly more light-gathering ability, a wider field of view, and a sturdier mount than the AstroMaster 90EQ, and is extremely portable thanks to its collapsible tube and tabletop Dobsonian mount.
- The Zhumell Z114/Orion StarBlast 4.5 Astro has larger aperture, a wider field of view, a steadier mount, and a much lower price tag than the AstroMaster 90EQ.
- The Zhumell Z100/Orion SkyScanner 100 are very inexpensive and either provides similar light-collecting power to the AstroMaster 90EQ but with a huge field of view, rock-solid mount, and a very low price tag.
- The Celestron StarSense Explorer LT 80AZ Refractor isn’t perfect and has slightly less aperture than the AstroMaster 90EQ but it is considerably easier to aim around the night sky.
- The Sky-Watcher Heritage 150P has significantly more aperture and is drastically easier to use than the AstroMaster 90EQ, as well as being extremely portable; it’s essentially an upsized Heritage 130P.
- The Sky-Watcher Virtuoso GTi 150P shares the optics, accessories, and design of the Heritage 150P but adds fully motorized GoTo and tracking, controlled by your smart device and still able to be aimed freely without ruining the accuracy of its computerized pointing abilities. The Virtuoso GTi 130P uses the same mount as the GTi 150P but with a Heritage 130P optical tube.
- The Popular Science Celestron StarSense Explorer DX 100AZ Refractor and Celestron StarSense Explorer DX 102AZ have more chromatic aberration than the AstroMaster 90EQ but include slightly better accessories and simpler mounts with the added bonus of a much wider field of view, slightly more aperture, and easy aiming with Celestron’s StarSense Explorer technology.
- The Orion SkyQuest XT6 has a rock-solid Dobsonian mount that requires neither a table nor tripod, a large 6” primary mirror, and a high-quality 2” Crayford focuser. The AstroMaster 90EQ is practically a toy by comparison.
- The Sky-Watcher Virtuoso 90 provides similar views to the AstroMaster 90EQ but in a much more compact package with motorized tracking, no chromatic aberration, and with better accessories provided by default, even including a solar filter.
Aftermarket Accessory Recommendations
The stock 1.25” Amici prism that comes with the AstroMaster 90EQ needs to go immediately. You should replace it with a quality 1.25” star diagonal, such as the Celestron 94115-A Prism Star Diagonal. Cheap mirror diagonals will provide a less-than-sharp image, and while a decent prism diagonal can seem expensive, it will drastically improve your views through the 90EQ’s otherwise-great optics.
A 32mm Plossl eyepiece coupled with a new star diagonal will provide 31x magnification and a wider true field of 1.7 degrees, ideal for finding and viewing deep-sky objects with the AstroMaster 90EQ. The stock 20mm and 10mm 3-element eyepieces are just fine for the rest of your eyepiece kit; a 6mm “goldline” or “redline” will provide 167x, which is about the limit of what a 90mm telescope can handle. Higher power than 100x may show you slightly more on the planets and split closer double stars than the 10mm Kellner but at the expense of providing a shakier and very dim image.
What can you see with Celestron Astromaster 90EQ?
The Celestron AstroMaster 90EQ is not well-suited for viewing deep-sky objects due to its small aperture and long focal ratio. Galaxies will appear as faint smudges with only 90mm of aperture, even under dark skies, with the exception of the brightest galaxies, such as M31 and M82, which may show hints of dust lanes and other detail. Globular star clusters and planetary nebulae are unresolved, dim, and faint fuzzies with the 90EQ, regardless of your light pollution conditions or lack thereof. Open star clusters like M35 and M11 are attractive in the AstroMaster 90EQ, but the telescope’s limited field of view due to its long focal length means that larger clusters like the Pleiades or Double Cluster are not nearly as pleasing as what a low-power, wide-field instrument of similar aperture might show them to be.
The brightest nebulae, such as Orion (M42) and the Lagoon (M8), reveal their brighter gas clouds and interior star clusters, but their fainter wispy regions are less obvious, and the view is far from as stunning as when viewed through a larger telescope, especially if you are stuck under light-polluted skies. Double stars are a good test of your observing skill and the steadiness of your current seeing conditions. Thousands of double stars are fairly easy to find with the AstroMaster 90EQ, appearing as colorful pinpoints at high magnification.
The AstroMaster 90EQ’s sharp optics make it a decent performer on Solar System objects (i.e., the Moon and planets). You can just barely resolve the phases of Mercury on a good night; Venus is far easier. The Moon always delights with craters, mountains, ridges, and other details on any given night. Mars’ polar ice cap is visible most of the time with the 90EQ. On the best nights when Mars is at its closest and seeing conditions permit, you can also resolve some dark markings on its surface.
Jupiter’s cloud banding, smaller atmospheric features like storms, and the Great Red Spot are usually able to be seen with the AstroMaster 90EQ, along with its four large Galilean moons. Io and Europa are hard to resolve as clear disks, as are their high-contrast shadows when they transit in front of Jupiter, but Ganymede and Callisto should be no trouble to see as tiny orbs when they transit, with their shadows following close behind. You can also resolve the rings and cloud belts of Saturn, the Cassini Division in the rings, and a few moons around Saturn as well. Uranus and Neptune appear as fuzzy dots, with disks too small to resolve and moons too faint to see; Pluto is similarly out of reach of a telescope this small.