Sky-Watcher Classic 250P

The Classic 250P: A 254mm Dobsonian for Visual Astronomy

John Dobson popularized this simple alt-azimuth mount for Newtonian reflectors. You swivel the tube in azimuth and tilt it in altitude; because the Earth rotates, you nudge it in both axes to keep an object in view.

254mm Optics at f/4.7

The Classic Dobsonian manual covers this 254mm/1200mm model. Sky-Watcher USA lists a 64mm secondary mirror, a 45" tube 11.375" in outer diameter and 28 lb with accessories.

Included Eyepieces: 60x and 96x Magnification Out of the Box

Magnification is the telescope's focal length divided by the eyepiece focal length. The two included 1.25" eyepieces give:

• 20mm eyepiece: 1200 / 20 = 60x.
• 12.5mm eyepiece: 1200 / 12.5 = 96x.

An optional Barlow lens goes between the focuser and the eyepiece to increase magnification. Sky-Watcher's rule of thumb for usable magnification is about 2x per mm of aperture in good conditions.

The Dobsonian Base: Pointing with a Tension Control Handle

The mount moves in altitude and azimuth. You point the tube by hand and nudge it in both axes to keep an object centered as the Earth rotates.

Sky-Watcher's patented tension control handle adds just enough friction for the tube to move easily when nudged but stay in position when not. Re-adjust it when you add or remove accessories. Install it on the same side as the focuser and leave the handle on the other side slightly loose. Do not over-tighten the handles.

Focuser, Finder and Adapters

The 2" Crayford focuser comes with 2" and 1.25" eyepiece adapters. Turn the focus knobs until the image is sharp. Refocus after changing an eyepiece or adding a Barlow lens, and as the temperature changes, which happens more often with short focal ratio telescopes before they reach outside temperature.

The 9x50 finderscope has a wider field than the main telescope. Once aligned, an object centered on its crosshairs is in the main eyepiece. Slide the finder bracket into its slot after moving the rubber O-ring to the groove on the finder tube.

Assembly

The flat-packed base is assembled with the screws, Teflon pad and four cylindrical saddle side bearings supplied; Sky-Watcher suggests assembling it in daylight in a large open area. Connect the side boards to the front board, then attach them to the two round boards with the central bolt and Teflon pad. Do not over-tighten the central bolt.

Then fit the accessory tray, the carrying handle and the four saddle side bearings. The tube's side bearings rest on the saddle bearings; fit the tension control handle on the focuser side.

Aligning the Finderscope

Align the finderscope in daylight:

• Aim the telescope: point it at a distant object at least 500 yards away and center it in the low-power eyepiece.
• Adjust the finder: use the two alignment screws on the bracket, which work against a spring-loaded knob, to center the crosshairs on the same object.

Cool-Down and Observing Site

Telescopes need at least 10 to 30 minutes to cool to outside air temperature, longer with a big temperature difference or larger optics. Cooling minimizes heat-wave distortion inside the tube (tube currents).

Choose a site away from city lights, as high as possible, with a dark, unobstructed horizon; the darkest sky is usually at the zenith. Expose your eyes to nothing but red light for 30 minutes before observing.

Collimation

Newtonians need occasional collimation. Check it on a slightly defocused star: symmetrical rings around the central disc mean the optics are aligned.

The process involves two adjustments:

• Secondary mirror: adjusted with the central bolt and three small screws behind it.
• Primary mirror: three adjusting screws on the back of the tube, with three locking screws.

The manual walks through both mirrors, and suggests a collimating cap made from a 35mm film canister with a pinhole in the lid.

Tube, Base and Care

The tube (45", 28 lb with accessories per Sky-Watcher USA) lifts off the Dobsonian base, so they move as two pieces.

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

Sky-Watcher's manual warns that looking directly at the Sun through the telescope will cause permanent eye damage. Use a proper solar filter for viewing the Sun.

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