Sky-Watcher Flextube 300P

Collapsible Dobsonian Design

The collapsible Dobsonian, complete with Crayford focuser, is Sky-Watcher's upgrade of the traditional design for large-aperture portability, and a good choice for first-time buyers. You swivel the tube in azimuth and tilt it in altitude by hand; because the Earth rotates, you nudge it in both axes to keep an object in the field.

Unlike other truss designs, it does not need to be disassembled between uses: it transports as two pieces that are ready to use in seconds, easy to collimate once set up, and hold collimation throughout the evening.

305mm f/4.9 Paraboloidal Optics

The 305mm paraboloidal primary mirror has a 1500mm focal length, giving f/4.9, and the secondary mirror is 70mm. skywatcher.com lists a faintest stellar magnitude of 14.9 and the highest practical power at 610x.

75x and 120x from Two Included Eyepieces

Magnification is the telescope's focal length divided by the eyepiece focal length. skywatcher.com lists the included 1.25" eyepieces as 70-degree 20mm and 12.5mm:

• 20mm eyepiece: 1500 / 20 = 75x; a 70-degree apparent field gives about 0.9 degrees of sky.
• 12.5mm eyepiece: 1500 / 12.5 = 120x, about 0.6 degrees.

The manual's rule of thumb for usable magnification is about 2x per mm of aperture in good conditions, because higher power also magnifies the turbulence in the air. It advises starting at a lower magnification with its wider field, then increasing it once you have found your target.

Dobsonian Base and Tension Control

Sky-Watcher redesigned the bearing system with smooth azimuth bearings, and even the twelve-inch model can be rotated with the push of a finger. The tube's side bearings rest on the base's four cylindrical saddle side bearings, with the tension control handle on the same side as the focuser. The base weighs 38.3 lb and measures 29" x 25".

The patented tension adjustment control on the altitude bearings removes the need for counterweights to balance the tube. Loosen or tighten the handle so the tube moves easily when nudged but stays in position when not, and re-adjust it when accessories are added or removed; leave the handle on the other side slightly loose.

Collapsible Tube: 1400mm to 920mm

skywatcher.com lists the tube at 450 mm in diameter, 920 to 1400 mm long and 21 kg.

To use the telescope, unlock the three locking screws on the bottom tube, hold the knob on the top tube and pull it up until it clicks in place, then retighten the screws. To transport it, unlock the three screws, hold the black handles on the upper tube, slide it down onto the bottom tube and tighten the screws. Remove the dust cap from the bottom tube before viewing.

Setting Up

The base needs assembly before first use. The manual's tube steps:

• Tube: place it between the base's side boards so its side bearings rest on the four cylindrical saddle side bearings.
• Handles: fit the handles, with the tension control handle on the same side as the focuser.
• Finder: remove the rubber O-ring before installation, slide the bracket into the mounting slot and position the finderscope with the two adjusting screws.
• Eyepiece: remove the black plastic end-cap, loosen the thumbscrews on the focus tube, insert an eyepiece and lightly tighten them.

Focuser, Finder and Eyepieces

An 8x50 right-angle correct-image (RACI) finderscope, 2" and 1.25" eyepiece adapters and a dust cap are included. Align the finder outdoors in daylight: center an object at least 500 yards away in the main eyepiece, then use the two small alignment screws, which work against a spring-loaded knob, to center the crosshairs on it.

The focuser is a 2" Crayford-style unit; Sky-Watcher ships its collapsible Dobsonians with smooth Crayford focusers. Turn the focus knobs slowly until the image is sharp. Refocusing over time is normal with short focal ratio telescopes, particularly before they reach outside temperature, and almost always needed after changing an eyepiece or adding or removing a Barlow lens.

Site, Seeing and Cooling

The manual's observing advice:

• Site: away from city lights, upwind from air pollution and as high as possible, with a dark, unobstructed horizon. Pavement and concrete radiate stored heat, and a window distorts the image.
• Seeing and transparency: in good seeing the stars appear steady, without twinkling, to the unassisted eye; ideal transparency is an inky black sky and unpolluted air. Do not observe immediately after sunset.
• Cooling: at least 10 to 30 minutes to reach outside air temperature, longer with a big temperature difference or larger optics, to minimize tube currents.
• Your eyes: nothing but red light for 30 minutes before observing; observe with both eyes open, and use averted vision on faint objects.

Collimation

Collimation aligns the mirrors so they deliver properly focused light to the eyepiece. Center a star and move the focuser so it is slightly out of focus: in good seeing, rings symmetrical about the Airy disc mean the optics are collimated. The manual's two steps:

1. Secondary: point the telescope at a lit wall, look through a collimating cap (or keep your eye against the back of the focus tube), and adjust the three small screws around the central bolt until all three primary mirror clips are visible.
2. Primary: loosen the locking screws, then use the adjusting screws at the back until the secondary's reflection is centered. A friend can turn the screws while you watch. Finish on a defocused star after dark.

Tube, Base and Care

The tube weighs 21 kg (skywatcher.com) and the base 38.3 lb (Sky-Watcher USA). They move as two pieces.

Tighten the three locking screws after collapsing the tube for transport. Replace the dust cap whenever the telescope is not in use, and do not clean the mirror unless you are familiar with optical surfaces.

Solar Safety

Never use the telescope to look directly at the Sun, as permanent eye damage will result. Use a proper solar filter for viewing the Sun.

When observing the Sun, place a dust cap over the finderscope. Never use an eyepiece-type solar filter, and never use the telescope to project sunlight onto another surface: the internal heat build-up will damage the optical elements.