Sky-Watcher Flextube 250P

Collapsible Dobsonian Design

The collapsible Dobsonian, complete with Crayford focuser, is Sky-Watcher's upgrade of the traditional Dobsonian design for large-aperture portability, and a good choice for first-time buyers. Unlike other truss tube designs, it does not need to be disassembled between uses: it transports as two pieces that are ready to use in seconds.

It is easy to collimate once set up and holds collimation throughout the evening.

254mm f/4.7 Paraboloidal Optics

The 254mm paraboloidal primary mirror has a 1200mm focal length, giving f/4.7. skywatcher.com lists a 58mm secondary mirror, a faintest stellar magnitude of 14.7 and the highest practical power at 508x.

60x and 96x with Included Eyepieces

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

• 20mm eyepiece: 1200 / 20 = 60x; a 70-degree apparent field gives about 1.2 degrees of sky.
• 12.5mm eyepiece: 1200 / 12.5 = 96x, about 0.7 degrees.

The true field is the eyepiece's apparent field divided by the magnification. The manual suggests starting at a lower magnification with its wider field, then increasing it once you have found your target. Its rule of thumb for usable magnification is about 2x per mm of aperture in good conditions.

Dobsonian Base and Tension Control

Sky-Watcher redesigned the bearing system with smooth azimuth bearings, and the patented tension adjustment control on the altitude bearings removes the need for counterweights to balance the tube.

Loosen or tighten the tension control handle to add just enough friction for the tube to move easily when nudged but stay in position when not, and re-adjust it when accessories are added or removed. Install it on the same side as the eyepiece and leave the handle on the other side slightly loose. Because the Earth rotates, you nudge the tube in both azimuth and altitude to keep an object in the field.

Collapsible Design Retracts from 1120mm to 800mm

skywatcher.com lists the tube at 380 mm in diameter, 800 to 1120 mm long and 15 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.

Assembly

The base comes flat-packed. The manual's parts list includes side panels A1 and A2, front panel B, base plates C and D, the screws, bolt and Teflon pad, an accessory tray, the handles, four cylindrical saddle side bearings, a Phillips screwdriver, two Allen wrenches and two wrenches.

Place the tube between boards A1 and A2 so its side bearings rest on the saddle side bearings, with the tension control handle on the same side as the focuser. Remove the rubber O-ring from the finder bracket, slide the bracket into its mounting slot, then remove the black plastic end-cap from the focuser before inserting an eyepiece.

Aligning the 8x50 Finder and Focusing

Align the 8x50 RACI finderscope 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 finder's crosshairs on it.

Sky-Watcher ships its collapsible Dobsonians with ultra smooth Crayford focusers; the 2" focuser comes with 2" and 1.25" eyepiece adapters. 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 Selection, Cooldown, and Dark Adaptation

Choose a site away from city lights, upwind from air pollution and as high as possible. Pavement and concrete radiate stored heat, and a window distorts the image. Do not observe immediately after sunset. Telescopes need at least 10 to 30 minutes to cool to outside air temperature, longer with a big temperature difference or larger optics.

Expose your eyes to nothing but red light for 30 minutes before observing, observe with both eyes open, and use averted vision on faint objects.

Collimation and Care

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 secondary mirror is aligned with the three small screws around its central bolt. On the 254mm/1200mm model, the flat-headed screws at the back are the primary's adjusting screws and the thumbscrews are the locking screws.

The manual's sequence: adjust the secondary until all three primary mirror clips are visible through the focuser, then adjust the primary, and confirm on a defocused star after dark. It suggests a collimating cap made from a 35mm film canister with a pinhole in the lid. Replace the dust cap whenever the telescope is not in use, and do not clean the mirror unless you are familiar with optical surfaces.

Tube and Base

It transports as two pieces: the tube, 15 kg (33 lb) and 380mm in diameter, retracted to 800mm, and the base.

The base weighs 26.5 lb and measures 29.5" x 20".

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.