The C6 XLT Optical Tube
The Astro Fi 6” SCT optical tube is merely a repainted Celestron C6 XLT, which is a 6” (150mm) f/10 Schmidt-Cassegrain telescope. Boasting a considerable enhancement in light-gathering capacity and resolution compared to the C5 XLT, as well as much more consistent quality control, the C6 has become a firm favourite amongst seasoned astronomers, who appreciate its “grab n’ go” convenience.
Similar to other catadioptric telescopes, the C6 XLT employs a knob located at the rear of the instrument to focus the telescope by shifting the primary mirror back and forth within the tube. This mechanism can occasionally cause “image shift” when focusing, which can be particularly troublesome at high magnifications or during astronomical imaging. However, the C6 XLT experiences minimal issues with this, partially due to the modest size and weight of the primary mirror.
Schmidt-Cassegrain telescopes, like the C6 XLT, necessitate regular collimation by adjusting the secondary mirror. Fortunately, collimating the C6 XLT is a relatively swift and straightforward procedure, and it won’t need to be done every time the telescope is set up, or really much at all once you’ve got it right. Contrary to what some individuals may suggest, this process is neither frightening nor overly complex and certainly does not necessitate bringing the telescope to the manufacturer or an experienced technician. To collimate the C6 XLT, one simply adjusts three small screws on the secondary mirror housing while aiming at a bright star – arguably, this is an easier task than collimating a Newtonian telescope. For more information, consult our collimation guide. However, it is advisable not to purchase the often-marketed thumbscrew collimation knobs for the Astro Fi 6” or other C6 XLT variants, as they are overpriced and, counterintuitively, cause the telescope to lose collimation more frequently due to their insufficient torque in securing the secondary mirror tightly.
The C6 XLT features a standard threaded port at its rear, which accommodates Schmidt-Cassegrain accessories and adaptors, such as an f/6.3 reducer. However, the nature of the internal baffling in the C6 XLT means that vignetting issues may arise if an f/6.3 reducer is utilised with a larger camera sensor or for visual purposes. The same issue occurs if one attempts to use a 2-inch diagonal. Consequently, the true field of view of the C6 XLT is restricted to just over 1 degree in the sky—a rather narrow range for a 6” instrument, especially when compared to the possibilities of a 6” f/5 reflector with a true 2” focuser.
Additionally, the C6 XLT possesses HyperStar compatibility. This feature allows the user to remove the secondary mirror, place Starizona’s Hyperstar corrector at the front of the telescope in its stead, and capture images with a color camera at a focal ratio of f/2. Nonetheless, the C6 only accommodates relatively small color sensors in its HyperStar configuration due to the physical limitations of the design and the fact that a large camera housing may begin to obstruct the majority of the telescope’s aperture. Furthermore, a HyperStar corrector comes with a price tag that exceeds the cost of the entire telescope and proves more beneficial when the C6 is mounted on a German equatorial mount, as opposed to an alt-azimuth mount like the Astro Fi.
To attach to the Astro Fi mount, the C6 XLT uses a standard Vixen-style dovetail bar enabling it to attach to almost any other astronomical telescope mount without tools.
The Celestron Astro Fi 6” SCT includes a 1.25” visual back which screws onto the rear of the tube to accept a 1.25” prism star diagonal. The provided prism diagonal is an excellent quality, multi-coated unit with no vignetting or significant glare issues, unlike cheaper Amici units.
For eyepieces, the Astro Fi 6” SCT includes two 1.25” Kellner oculars: a 25mm for 60x magnification and a 10mm for 150x. Both are decent eyepieces featuring all-metal housings, a 50-degree apparent field of view, and glass lenses; however, they lack eyeguards, and the 10mm has slightly limited eye relief. They are basically as sharp as a Plossl eyepiece at the Astro Fi 6” SCT’s relatively forgiving focal ratio of f/10, but you will probably want to supplement or replace them altogether with aftermarket high-quality eyepieces at a variety of different focal lengths.
For a finder, the Astro Fi 6” SCT comes with Celestron’s standard, generic “StarPointer” red dot finder, primarily used to align the Astro Fi telescope/mount with alignment stars during setup. It attaches with a standard interchangeable Synta/Vixen-style foot, allowing you to swap in a different finder if desired, although there is not much point in doing so on this GoTo telescope.
The Celestron Astro Fi Mount
The Astro Fi mount is an enhanced version of the older NexStar SLT mount. The Astro Fi mount is a straightforward GoTo alt-azimuth mount designed to support telescopes up to 5 inches in size. This means that the Astro Fi is obviously at least somewhat inadequate for supporting the C6 XLT optical tube, particularly thanks to its long 1500mm focal length and the rather high magnifications one usually uses it at as a result. The mount is not the best at supporting the C6 XLT when you extend the tripod legs all the way. However, if you can place a weight on the accessory tray, fill the legs with stand, or just do a good job at tightening all of the screws and nuts holding the tripod together, stability can be improved. The accessory tray is well-designed with slots for 1.25” eyepieces and either your phone or a dumbbell weight to hold the thing down. With a Vixen-style dovetail saddle, you can of course swap in other telescopes onto the Astro Fi mount as you please provided there aren’t any stability or clearance issues.
In comparison to the NexStar SLT mount, mechanically, the Astro Fi features superior gears and a more robust tripod, equipped with extruded aluminium legs as opposed to the SLT’s undersized tubular steel legs. This is why we don’t recommend the 6” NexStar SLT; it is completely unworkable with the C6 XLT optical tube.
Naturally, the Astro Fi mount is not suitable for deep-sky astrophotography, being a cheap alt-azimuth mount. However, it performs admirably for visual astronomy and planetary imaging purposes despite the weight of the C6 XLT. The mount is powered by a set of standard AA batteries and an external plug-in battery pack, or an alternative external power source can be used. It is highly recommended that you invest in a rechargeable power supply as soon as possible for cost savings and enhanced reliability.
Setting up the Astro Fi mount is a relatively straightforward process. Once you have switched on the telescope and levelled the tripod, simply connect to its Wi-Fi network and open either Celestron’s SkyPortal app or SkySafari Pro on your smartphone or tablet. The app will guide you through the process of centring and confirming a few alignment stars. After this, you are free to manoeuvre the telescope around the sky using the on-screen push-button controls or choose from a vast database of celestial objects that the telescope will automatically point at and track for you. There are some hidden advantages to this system, such as the easy interface of a touchscreen compared to a traditional keypad.The telescope can get precise location and time information from your device rather than relying on manual input, or expensive (and usually absent) internal GPS and clock units. If you walk away and your device disconnects from the Astro Fi’s WiFi network, the telescope mount will keep tracking and remember its alignment information once you reconnect.
Should I buy a Used Celestron Astro Fi 6″ SCT?
Purchasing a second-hand Astro Fi 6” SCT should be satisfactory; however, older models were known to experience reliability issues with maintaining Wi-Fi connections to devices, which could pose a problem. If you are considering a unit that was not purchased recently and the current owner has not used it successfully, or ever, this could be a warning sign. Price is another concern; as with any electronic device or telescope, avoid paying too close to the new price for a used unit, especially since it may lack a warranty or any guarantees regarding its condition or the possibility of a refund.
It is also extremely important to ensure that both of the mirrors in the Astro Fi’s C6 XLT optical tube are free of significant corrosion, as recoating is similarly too expensive to bother with. Also, ensure the front corrector plate in the telescope is in one piece as well as free of fungus, which can permanently damage the glass by chemically etching it. A broken front corrector plate cannot be repaired or replaced without also replacing the primary and secondary mirrors “matched” to it; this costs more than simply purchasing a new or used C6 XLT in working condition most of the time.
The Celestron Astro Fi 6″ SCT is acceptable, especially compared to many other options at its price range, but there are some cheaper and better options with superior capabilities available that you might want to consider having a look at instead:
- The StellaLyra 10″ f/5 Dobsonian offers over 50% more resolution and about triple the light-gathering capacity in comparison to the Celestron Astro Fi 6″ SCT, owing to its remarkable 10″ primary mirror. Supplied with the telescope are a plethora of accessories, including a 2″ low-power, 30mm focal length wide-angle eyepiece, a 9×50 finder scope, and a 9mm high-power Plossl eyepiece. The Dobsonian mount is easy to aim, quick to set up, and more stable than the Astro Fi computerised mount, too. The 8” StellaLyra is an equally excellent choice.
- The Celestron StarSense Explorer 8″ Dobsonian has some computerised pointing capabilities, albeit without motorised tracking, using Celestron’s StarSense Explorer technology. Additionally, it boasts superior light-gathering and resolving power compared to the Astro Fi 6″ SCT, thanks to its 8″ aperture. The telescope is user-friendly, simple to set up, transport, and operate, though the number of accessories provided is limited compared to other alternatives.
- The Sky-Watcher Virtuoso GTi 150P offers a wider field of view than the Astro Fi 6″ SCT thanks to having half the focal length. Featuring fully motorised tracking and GoTo technology, it also incorporates the FreedomFind encoder system, allowing for manual aiming by hand, regardless of whether the mount is powered on or aligned. The telescope is controlled via your smartphone or tablet, as with the Astro Fi, and its collapsible tube ensures easy transportation. The Heritage 150P, a more budget-friendly option, is identical to the Virtuoso GTi 150P optically and includes the same accessories but lacks electronics.
- The Sky-Watcher Skymax 127 AZ-GTi offers motorised tracking GoTo functionality, similar to the Astro Fi 6″ SCT, which can be operated through your smartphone or simply pushed around the sky by hand, thanks to its FreedomFind encoders. The Skymax 127 Virtuoso GTi boasts the same features but is mounted on the Virtuoso GTi tabletop Dobsonian mount, with the option to connect it to a third-party tripod. The Skymax 127 optical tube is comparable to the Astro Fi 6″ SCT’s C6 XLT optical tube for planetary views but falls short of it for deep-sky observation on account of its smaller aperture.
- The Celestron Astro Fi 130mm f/5 offers a wider field of view than the Astro Fi 6” SCT thanks to its 130mm f/5 optics and can technically use 2” eyepieces too, though it’s inferior to a larger 6” reflector in performance.
- The Celestron StarSense Explorer 10″ Dobsonian provides about triple the light-gathering power and half again as much resolution as the Astro Fi 6” SCT, with Celestron’s StarSense Explorer technology offering assistance in manually aiming this telescope around the night sky. The base is relatively lightweight for a 10″ Dobsonian, and transportation is made easier with cutouts in the base and handles on the tube. Although this telescope comes with minimal accessories besides the StarSense Explorer technology, it offers great value in terms of aperture and portability for its price.
- The Celestron NexStar 6SE uses the same C6 XLT telescope optical tube as the Astro Fi 6” SCT and features motorised tracking and GoTo, but lacks WiFi operability – though the mount is much sturdier and more well-suited for this telescope.
- The Sky-Watcher Skyliner 250P FlexTube GoTo Dobsonian is a massive 10” Dobsonian with plenty of light-gathering capability and resolution as well as fully motorized tracking and GoTo, with a collapsible tube to slightly improve portability. It also features Sky-Watcher’s FreedomFind technology to allow you to manually aim the telescope around the sky and can be controlled over WiFi too.
Aftermarket Accesssory Recommendations
The included diagonal and pair of eyepieces provided with the Astro Fi 6” SCT are decent, but you’ll want to purchase a few additional accessories and at least a couple of extra eyepieces to achieve the most enjoyable experience possible. A dew shield is indispensable for the Astro Fi 6” SCT, as it helps to minimise glare from nearby light sources entering the telescope, whilst also delaying frost or dew formation on the Astro Fi 6” SCT’s front corrector plate and preventing pollen, dirt, and dust accumulation. These factors can damage the delicate corrector plate and its anti-reflection coatings over time, as well as spoiling your view with blurriness and glare.
A 1.25” 32mm Plossl eyepiece for 47x with the Astro Fi 6” SCT provides a slightly wider field of view and lower magnification than the included Kellner, making it more appropriate for observing deep-sky objects. A 2” star diagonal will vignette with the Astro Fi 6” SCT, as will an f/6.3 reducer and low-power eyepieces, so there is little reason to acquire either. For medium-high power, we recommend a 1.25”, 16mm UWA (94x) or 15mm redline/goldline (100x) eyepiece. For higher magnification, while not strictly necessary, a 10mm UWA (150x) or a 9mm redline/goldline (167x) eyepiece is sharper, provides a wider field of view, and is more comfortable to use than the Astro Fi 6” SCT’s standard 10mm Kellner ocular.
A 7mm focal length eyepiece, such as a 7mm UWA or 7mm planetary (214x), provides magnification near the limit of what you can typically expect to find to be useful with the Astro Fi 6” SCT for the Moon, planets, and double stars.
And while perhaps a little bit expensive and limited in utility, a UHC nebula filter can be attached to any of your eyepieces and enhances views of nebulae with the Astro Fi 6” SCT, especially if you predominantly observe from light-polluted skies. While the scope’s limited field of view is not ideal for viewing enormous objects like the Veil Nebula, the filter aids in observing smaller objects such as, most importantly, the Orion Nebula, by increasing their contrast and dimming the surrounding sky background.
You will almost certainly need a power supply, such as the Celestron PowerTank Lithium or a TalentCell power bank, to use with the Astro Fi 6” SCT instead of expensive disposable batteries or cumbersome extension cords.
What can you see?
A 6″ telescope like the Astro Fi 6” SCT might not match up to a wide-field 6” f/5 reflector, nor a similarly-priced 8″, 10″, or even 12″ reflector when it comes to deep-sky viewing capabilities, particularly under light-polluted skies, which will constrain the types of objects and details that can be seen at any aperture. The limited field of view achievable with the Astro Fi 6” SCT, owing to its 1500mm focal length and 1.25″-only accessories, also precludes the observation of the largest open star clusters and nebulae.
Nonetheless, even under less-than-ideal conditions with the Astro Fi 6” SCT, you can still enjoy views of medium-sized clusters like M35 and M38, and begin to resolve the brightest star clusters, such as M13, into individual stars. Planetary nebulae like the Cat’s Eye and Blinking Planetary nebulae exhibit subtle details and greenish-blue hues, alongside the larger, colourless Ring (M57) and the massive Dumbbell (M27). A UHC nebula filter helps to highlight planetary nebulae at lower magnifications before examining them more closely at high power and enhances contrast on nebulae like Orion (M42). These nebulae shine with a filter and/or dark skies but can also be seen unfiltered even from fairly light polluted locales, too. Thousands of double stars can be split with the Astro Fi 6” SCT’s razor-sharp optics and are easily located in the Astro Fi mount’s database.
Dark skies are absolutely crucial for viewing galaxies, and the Astro Fi 6” SCT can display high-contrast dust lanes in galaxies such as M64, M82, and M104, and begins to reveal hints of not-quite-resolved spiral arms in galaxies like M51 at low magnification. You can also view numerous galaxy groups and clusters, with the Virgo Cluster of galaxies being a prime example, boasting dozens of members.
The Astro Fi 6” SCT’s high-quality optics and long focal length make it ideal for planetary observation, though an 8” telescope will of course probably do a better job. Proper collimation, allowing the telescope to cool to ambient nighttime temperatures for at least 15-30 minutes, and stable atmospheric conditions are also essential for obtaining sharp views of planets, lunar details, and other small targets like close-together double stars. You can easily discern the phases of Mercury and Venus, polar ice caps and dark surface features on Mars when it’s closest to us, and plenty of fine details such as craterlets, mountains, and ridges on the Moon.
The Astro Fi 6” SCT can also present you with the cloud belts, the Great Red Spot, and other atmospheric features of Jupiter, along with the disks and shadows of its four large moons, the Galilean moons, whenever they transit in front of the colossal planet. The rings of Saturn, the Cassini Division within them, Saturn’s own faint cloud belts, and a few moons can be observed as well. The Astro Fi 6” SCT lacks the light-gathering power to display the moons of Uranus and Neptune and will struggle to resolve the discs of either ice giant, while Pluto remains beyond the reach of a 6″ telescope due to its dimness.
The Astro Fi mount is neither accurate, steady, or well-made enough to support deep-sky astrophotography and it is also of course an alt-azimuth mount. The C6 XLT optical tube can be used with a reducer or HyperStar conversion atop a heavy-duty equatorial mount for deep-sky astrophotography, however, you can use the Astro Fi 6 as-is for lunar and planetary imaging easily – just a CMOS camera, laptop, and 2-3x Barlow lens are all you need to take decent shots, though a larger aperture instrument has more resolving power than the relatively small C6 XLT.