Celestron StarSense Explorer™ Smartphone App-Enabled Dobsonian Telescope - 8"

8" f/5.9 Newtonian Optics

The 203mm aperture and 1200mm focal length give 841x the light gathering power of the eye, a limiting stellar magnitude of 14.2 and a resolution of 0.68 arc seconds (Rayleigh) or 0.57 arc seconds (Dawes). The primary mirror is 25mm (0.98") thick, about a 1:8 thickness ratio, and the secondary 8.5mm thick with a 47mm (1.85") minor axis, a 23% central obstruction by diameter. The XLT reflective coatings have silicon dioxide and tantalum pentoxide protective overcoatings. All mirror surfaces have XLT coatings to visibly increase contrast and light throughput, and the mirrors are made of low-expansion optical glass to help ensure the best images under changing conditions.

The steel tube is 1117.6mm long and 241.3mm in diameter (44" x 9.5") and weighs 20.6 lbs (9.34 kg); the particle board base, with melamine surfaces and edge trim, measures 685.8mm x 482.6mm x 482.6mm (27" x 19" x 19") and weighs 22.8 lbs (10.3 kg). You can expect sharp, bright views of Jupiter's four Galilean moons, its cloud bands and Great Red Spot, the rings of Saturn, the gaseous glow of the Orion Nebula, dust lanes in the Lagoon Nebula and the Andromeda Galaxy.

StarSense Explorer: Your Phone Finds the Way

StarSense Explorer reinvents the Dobsonian telescope as the first Dobsonian that uses your smartphone to analyze the night sky and calculate its position in real time, and it is ideal for beginners thanks to the app's user-friendly interface and detailed tutorials. StarSense Explorer uses patented sky recognition technology and your smartphone to work out where the telescope is pointed: a Lost in Space Algorithm (LISA), like the ones satellites use to orient themselves in orbit, matches the star patterns the phone detects overhead to the app's internal database.

Place your phone in the StarSense dock and launch the app. After you align the phone to the telescope's optics, a quick and easy procedure, the app lists the celestial objects currently visible. Pick one from the planetarium view or the Tonight's Best list and arrows appear on screen to guide you; the bullseye turns yellow when the telescope has detected stars but is not yet on target, and green when the object is ready to view in the low-power eyepiece. As you get close, stop for a few seconds so the telescope can sense the stars in its new position. As you observe, you can listen to hundreds of audio descriptions and read detailed information about thousands of objects in the app's database; press the bar with the object's name at the bottom of the screen.

Setting Up the App and Phone

Download the StarSense Explorer app before your first night out; the app is large, so a Wi-Fi connection is recommended. The unlock code on the card in the box enables telescope control on up to 5 devices. The app works with most modern smartphones, and a compatibility list is on Celestron's site; on older phones the app can still be used, but it cannot be reinstalled once uninstalled.

To align the phone, which is best practiced during the day, insert the low-power eyepiece, place the phone in the dock and turn the knobs at the bottom of the bracket until the camera view is centered, unobstructed, in the dock's mirror. Point the telescope at a very distant object, at least a quarter mile away, such as a streetlight, a license plate, a stop sign or a treetop, center it in the eyepiece, then slide the camera view until the same object sits in the crosshairs. If you have removed the phone from the dock since the last alignment, the app suggests aligning again.

To attach the phone, remove the large cap over the dock's mirror, pull open the spring-loaded slider on top of the holder, set the phone flush with the bottom lip and release the slider slowly. Most phones fit with the case on, but a bulky or folio-style case may need to come off. The app keeps the phone's camera looking at the sky and its processor working, so it uses significant power; a fully charged phone should last several hours, but bring a charger to observe all night. The optional PowerTank Glow 5000, a USB power bank with a red flashlight, straps to the two posts on the StarSense dock with its included bands.

The Dobsonian Base and Pointing

The Dobsonian base is ultra-stable, with large diameter altitude bearings with variable tensioning, azimuth bearings and braces on the side panels for extra stability. The altitude tensioning knob, on the same side as the focuser, acts as a brake for the smoothest motion even with slight imbalances; the knob on the other side does not adjust tension. The telescope is balanced for normal use, but a heavy eyepiece or a larger smartphone may call for a little more tension.

To point the telescope, move the tube up and down and left to right, holding the panning knob at the front of the tube, which also keeps the heat of your hand out of the optical path. The Dob is not motorized, so objects drift out of the field within a minute or two depending on magnification, faster at high power; nudge the tube by the panning knob to keep them centered.

To transport the telescope, loosen the altitude tensioning knob and lift the tube off the base; the tube and base each have carry handles placed at the balance point. Dust covers are included for the front of the tube and the focuser; keep them on when the telescope is not in use.

2" Crayford Focuser, Eyepieces and Magnification

The 2" Crayford focuser, which moves precisely and smoothly without focus shifting, comes with a 2" extension tube and a 2"-to-1.25" adapter, so you can use virtually any 1.25" or 2" eyepiece. Keep the extension tube in place for most eyepieces, or they will not reach focus; remove it for eyepieces that need a lot of inward travel, and you may need to remove it for an imaging camera. The adapter's internal brass compression ring protects eyepiece barrels. For a 2" eyepiece, remove the adapter and secure the eyepiece in the extension tube with its thumbscrews. A focus lock thumbscrew holds the sharpest focus, and a set screw adjusts the focus knobs' tension.

The maximum magnification of any telescope is about 60 times per inch of aperture, but in many locations air turbulence makes sharp images much over 100x difficult; if stars twinkle heavily, stay at lower power. For the Moon and planets, try a higher-magnification eyepiece, such as a 10mm. The eyepiece rack on the front of the base holds three 1.25" eyepieces and one 2" eyepiece.

Like all Newtonian reflectors, the Dob shows images upside-down compared with the naked eye, which has little impact on astronomy but can confuse land viewing. You won't need the StarPointer red-dot finder when using the app, but it helps align the app with the telescope and is handy during the day, since the app works only under the stars. Align it on an object at least a quarter mile away, using its altitude and azimuth knobs to put the red dot where the eyepiece shows the object, and turn it off when not in use; its battery is a CR2032 3V lithium button cell.

Cooling, Collimation and Care

The best views come once the telescope reaches thermal equilibrium with the outside air. Allow around one hour when moving it from a heated house, store it in a dry but unheated place such as a garage, and take it out at sunset so it is acclimated by dark. The 8" Dob has no fan mount, so it is not compatible with the optional USB cooling fan.

Collimation aligns the parabolic primary and the flat secondary mirror, and it is worth checking periodically. With the eyepiece removed and the included collimation cap in the focuser, you should see the whole primary mirror centered in the secondary, and the reflection of the secondary centered in the primary, whose center is marked with an adhesive ring. Collimate during the day, with the tube horizontal and pointed at a blank wall, and adjust the secondary first with the three set screws in its holder, using the 2mm hex key. At night, a slightly defocused bright star at high magnification should show the secondary's shadow centered in the circle. On the 8" Dob, the primary mirror is tilted with three pairs of push-pull screws in the mirror cell, using the 2.5mm hex key and the Phillips head screwdriver: loosen one screw of a pair and tighten the other, one pair at a time.