Sky-Watcher Flextube 200P

Pointing a Dobsonian

With the mount level, you swivel the telescope in azimuth, parallel to the horizon, and tilt it up in altitude. Because the Earth rotates, you nudge the tube in both axes to keep an object in the field. Reference material lists altitude in degrees above or below your horizon and azimuth in degrees clockwise from North.

203mm Optics at f/5.9

The primary mirror is 203mm (8") with a 1200mm focal length, giving f/5.9. The secondary mirror is 48mm.

60x and 96x with Included Eyepieces

Magnification is the telescope's focal length divided by the eyepiece focal length. Sky-Watcher's rule of thumb for usable magnification is about 2x per mm of aperture in good conditions, and it advises starting at a lower magnification with its wider field before increasing it.

With the included 1.25-inch eyepieces:

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

Exit pupil is the aperture divided by the magnification: about 3.4mm at 60x and 2.1mm at 96x. The manual gives about 7mm as an average fully dilated pupil, less before full dark adaptation and with age.

Dobsonian Base and Tension Control Handle

The tube's side bearings rest on the base's four cylindrical saddle side bearings.

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. 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.

Collapsible Tube: 44" Extended, 33" Retracted

The tube is 44" long when extended and 33" retracted, with a 9.375" outer diameter. It weighs 24.2 lb with accessories.

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 is assembled from side panels A1 and A2, front panel B and base plates C and D, with the screws, bolt, Teflon pad, accessory tray, handles and four cylindrical saddle side bearings supplied. The kit includes a Phillips screwdriver, two Allen wrenches and two wrenches.

Then:

• Bearings and tray: fit the four cylindrical saddle side bearings and the accessory tray.
• Tube: place it between boards A1 and A2 so its side bearings rest on the saddle 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 loosen the two adjusting screws to position the finderscope.

Aligning the Finderscope and Focusing

Align the finderscope outdoors in daylight. Point the main telescope at an object at least 500 yards away and center it in the eyepiece.

Then use the two small alignment screws, which work against a spring-loaded knob, to center the finder's crosshairs on the same object.

Remove the black plastic end-cap, loosen the thumbscrews on the focus tube, insert an eyepiece and lightly tighten them. Turn the focus knobs slowly until the image is sharp. Refocusing over time is normal, due to temperature changes and flexure, especially with short focal ratio telescopes before they reach outside temperature, and almost always needed after changing an eyepiece or adding or removing a Barlow lens.

Site, Seeing 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; objects are best observed as they cross the meridian. Telescopes need at least 10 to 30 minutes to cool to outside air temperature, longer with a big temperature difference or larger optics.

In good seeing the stars appear steady, without twinkling, to the unassisted eye. 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 Cleaning Procedures

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 200mm/1200mm model, the Phillips-head screws at the back are the primary's adjusting screws and the hex bolts are the locking screws. The manual 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. Do not clean the mirror unless you are familiar with optical surfaces; clean the finderscope and eyepieces with special lens paper only.

Tube and Base

The tube weighs 24.2 lb with accessories and the base 26 lb, 29.5" x 20". They move as two pieces.

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.

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. When observing the Sun, place a dust cap over the finderscope.