Celestron Astro Fi 102mm Maksutov-Cassegrain Telescope

102mm Maksutov-Cassegrain Optics

The Maksutov-Cassegrain is a catadioptric design with both a lens and mirrors: light passes through the corrector lens to the primary mirror and back to a secondary before reaching the eyepiece, and the images are bright, sharp and virtually free of chromatic aberration. Instead of a separate secondary mirror, the secondary is a spot of reflective coating applied to the inside of the corrector, so the telescope does not need the occasional collimation of a Schmidt-Cassegrain, and the small spot gives the Mak its high-contrast views.

The 102mm aperture and 1325mm focal length give 212x the light gathering power of the eye, a limiting stellar magnitude of 12.5 and a resolution of 1.37 arc seconds (Rayleigh) or 1.14 arc seconds (Dawes). The secondary obstruction is 34% by diameter and 12% by area, and the optics are fully coated. The aluminum tube is 381mm (15") long and 117mm (4.6") in diameter and weighs 6 lbs (2.7 kg). The larger, heavier Maksutov corrector takes longer to cool to the outside air temperature than a Schmidt-Cassegrain of the same aperture, so give it time outside before expecting the sharpest views.

WiFi and the SkyPortal App

The Astro Fi broadcasts its own WiFi network for Celestron's SkyPortal app, which replaces the traditional hand control for a fully wireless experience. Connect your phone or tablet to the telescope's "Celestron-xx" network, open SkyPortal and select Connect to Telescope: arrows move the telescope up and down (altitude) and left and right (azimuth), and a slider sets the motor speed. Because of the amount of information SkyPortal holds, download it before you go out to observe.

SkyPortal's interactive planetarium lets you hold your device up to the sky to identify bright stars, constellations, planets and deep-sky objects. Tap an object and select GoTo, or use the search icon to enter a name such as Orion Nebula, Jupiter or Pleiades. While you observe, SkyPortal shows information about the object, and it can generate a Sky Tour of the best objects for your exact time and location.

SkyAlign and Tracking

To align, set up the telescope with the 25mm eyepiece, level the tripod, plug in the battery pack and connect to the telescope's network. Confirm the time and location in SkyPortal, which normally update automatically from your device, then select the telescope icon and Connect and Align; the app walks you through each step. With SkyAlign you center any three bright objects in the eyepiece and the telescope calculates its position; 3-Star Align and Solar System Align are also available. Once aligned, the telescope finds any object you select and tracks it as the Earth rotates, keeping it centered, which makes it easy to share views with family and friends.

The Computerized Mount and Power

The computerized single fork arm alt-azimuth mount uses DC servo motors, slews at up to 3 degrees per second and tracks at sidereal, solar and lunar rates, in Alt-Az, EQ North and EQ South modes. Its instrument load capacity is 8 lbs (3.6 kg), and the tube attaches by a CG-5 dovetail bar. Two AUX ports take optional accessories; there is no autoguide port and no periodic error correction. A USB port comes only with the optional NexStar+ hand control, sold separately, which provides over 40,000 objects in its database. Do not push the telescope left and right by hand while it is off.

The telescope runs on 8 AA batteries (not included) in the supplied battery pack, on a 12V DC source with at least 1 amp (tip positive), or on a Celestron PowerTank, sold separately. Plug the battery pack into the power port on the base: the red WiFi light on the arm shows that it has power and is WiFi-ready. The small switch near the power socket must be in the right position, and the battery pack stays in the accessory tray.

Tripod and Assembly

No tools are needed. Open the tripod until the center brace is fully extended, thread the captive center bolt into the accessory tray, place the fork arm on the tripod and secure it with the captive knob under the tripod top, then seat the tube's dovetail flat in the fork arm's dovetail holder and firmly tighten the thumb bolt. Adjust the legs to a comfortable height and level the tripod with the lock knobs.

The adjustable aluminum tripod has 31.75mm (1.25") legs and adjusts from 863.6mm to 1371.6mm (34" to 54") with the mount. The mount head and the tripod each weigh 5 lbs (2.3 kg), and the accessory tray includes a rubberized holder for your phone or a small tablet.

Eyepieces, Star Diagonal and the StarPointer

Two 1.25" eyepieces are included: the 25mm gives 53x and the 10mm 132x (1325mm divided by the eyepiece focal length). The highest useful magnification is 241x and the lowest is 15x; always start with the 25mm to find a target before switching to the 10mm for a closer look. Insert the star diagonal into the focuser and tighten the two silver thumbscrews, then secure the eyepiece with the two thumbscrews on the diagonal; the diagonal puts the eyepiece at a comfortable 90 degree angle. With the mirror star diagonal in place the image is the right way up but reversed left to right, which is normal when using a star diagonal.

The StarPointer red dot finderscope slides into the dovetail base on the tube near the focuser and locks with a silver thumbscrew. Align it once, in daylight, on something at least a quarter mile away, such as a streetlight, a license plate or a tall tree: with the telescope off, move the whole tripod to point it, center the object in the 25mm eyepiece by moving the tube up and down, then turn the finder on (its power knob clicks and also sets the dot's brightness) and use its two adjustment knobs to put the red dot on the object. It should not need realigning unless it is bumped or dropped; turn it off after use to save its battery.

Magnification is the telescope's focal length divided by the eyepiece's. Celestron's Telescope Basics manual suggests finding objects with the longest focal length (lowest power) eyepiece, which gives the widest field, then switching to the higher power eyepiece for a closer view. If bright stars twinkle or change color, the air is not steady enough for high magnification, and a lower power will give a steadier view.

Smartphone Imaging with the Lens Cap Adapter

The lens cap doubles as a smartphone adapter. Pull the orange plug out of the camera opening by its lanyard, slide your phone under the bungee straps so the camera looks through the opening (eight anchor points let you angle the straps to suit where your camera sits), and center the camera lens in the opening. Then secure an eyepiece in the holder inside the cap and insert it into the telescope like any eyepiece, turn the phone's flash off, and focus on the phone's screen. It fits any smartphone and any eyepiece with an outside diameter of 45mm or smaller; most phones fit with the case on, but a very large or folio-style case may need to come off.

Practice in daylight first, then on the Moon, and start with the low-power eyepiece before trying the 10mm. Standard camera apps adjust focus and exposure automatically, which tends to overexpose a bright Moon or planet against a black sky, so an app with manual focus and exposure helps; a 2, 5 or 10 second shutter delay lets vibrations die down. This method can capture daytime land objects, the Moon and the bright planets (Venus, Mars, Jupiter and Saturn), but phone sensors are not made for very low light, so galaxies and nebulae are out of reach.

What You Can See and Observing Tips

The Astro Fi offers views of craters on the Moon, the rings of Saturn, the Great Red Spot on Jupiter, the Orion Nebula and the Hercules Globular Cluster at night. Nebulae do not look like the colourful long-exposure photographs in books: the eye is most sensitive to green, so most nebulae appear greenish-grey in the eyepiece.

Celestron's Telescope Basics manual suggests giving the telescope at least 10 minutes to cool to the outside air temperature, longer if the difference is large, and allowing about 30 minutes for your eyes to fully dark-adapt, using a red LED flashlight at low brightness if you need light. Because the Earth rotates, objects drift across the eyepiece, faster at higher magnification. Celestron's Telescope Basics manual suggests positioning the telescope just ahead of the target and letting it drift through the field.

If you wear glasses, Celestron's Telescope Basics manual suggests checking whether you see the same field with and without them; if not, observe without them and refocus. For additional eyepieces, it suggests designs with at least 18mm of eye relief if you want to keep your glasses on. Celestron also includes Starry Night Special Edition software for your computer, to learn about the night sky and plan observing sessions; you don't need it to use the telescope.