Sky-Watcher Flextube 250P SynScan

GoTo on a Collapsible Dobsonian

The SynScan hand control can point the telescope at a chosen object or tour the sky at the touch of a button, and the motors track the object once found.

Even an inexperienced astronomer can master the SynScan features in a few observing sessions.

254mm f/4.7 Paraboloidal Optics

The 254mm paraboloidal primary mirror has a 1200mm focal length, giving f/4.7. The highest practical power is 508x.

Sky-Watcher fits its collapsible Dobsonians with a 2" Crayford focuser, which comes with a 2" to 1.25" eyepiece adapter. The manual notes that refocusing over time is normal with short focal ratio telescopes before they reach outside temperature, and after changing an eyepiece.

Collapsible Tube and Setup

The 15 kg tube is 380 mm in diameter and 800 to 1120 mm long (skywatcher.com).

Unlike other truss designs, Sky-Watcher's collapsible Dobsonian does not need to be disassembled between uses: it transports as two compact pieces, is ready to use in seconds, is easy to collimate once set up and holds collimation throughout the evening.

SynScan GoTo and Freedom Find Encoders

DC servo motors drive both axes with ten slew speeds from 1x to 1000x, and track at sidereal, lunar or solar rate in dual-axis tracking mode. GoTo pointing accuracy is up to 5 arc minutes.

Sky-Watcher's patented Freedom Find dual-encoder technology lets you move the telescope by hand or electronically in either axis without losing its alignment or position information. Each axis has one encoder on the axis shaft and one on the motor shaft. After you move to a new object, the telescope begins tracking it, with no re-setup in one observing session.

SynScan Database and Alignment

The menu system slews automatically to over 42,900 objects:

• Deep sky: the complete Messier (110) and Caldwell (109) lists, 7,840 NGC objects and 5,386 IC objects.
• Stars: 29,523 SAO stars, 212 named stars, 55 double stars and 20 variable stars.
• Solar system: the planets and the Moon.
• User: up to 25 user-defined coordinates.

Before GoTo, align the system on two stars. Brightest-Star alignment, recommended if you are new to the sky, asks for a region of the sky and lists the bright stars there; Two-Star alignment lets you choose two stars from a list, at least 60 degrees apart in azimuth. The TOUR key takes you on a preset tour of the sky you are under.

Power and Computer Control

An RS-232 cable connects the hand control's RJ-12 port to a computer at 9600 bits per second. Firmware version 3.00 and later work with the Celestron NexStar 5i/8i and NexStar GPS command protocol, which planetarium software can use to drive the mount.

From version 3.0 onward, the hand control firmware is user upgradable.

Power

The mount needs 10 to 15 V DC at 1 A through a 2.1 mm center-positive plug, into the Power jack on the altitude motor box. A 12V DC car-lighter power cable is included; the battery pack or power supply is sold separately.

To avoid tangling the power cable, the manual suggests placing the battery pack or power tank on the round base, behind the front panel.

Setting Up

The base is assembled once. For each session:

1. Level the round base with a bubble level; better leveling gives better tracking.
2. Loosen the three slider lock screws, pull the top tube out until it clicks, then tighten the screws without over-tightening.
3. Remove the dust cap and front cover, lift the tube with both hands at the positions shown, lower its side bearings onto the mounting platforms inside the side boards and tighten the locking screw.
4. Connect the power cable, the Az motor cable, and the hand control cable to their respective ports on the altitude motor box.

For Two-Star alignment, enter your location, date and time, choose the first star from the list, center it manually and confirm. The mount then slews to the second star; center it with the direction keys and confirm.

Focuser, Finder, Eyepieces and Tension Control

The 2" Crayford focuser comes with a 2" to 1.25" eyepiece adapter.

• Eyepieces: 20 mm (60x) and 12.5 mm (96x), 1.25-inch.
• Finderscope: 9x50 straight-through; align it in daylight before star alignment.
• Tension control handle: patented; adds or reduces friction between the tube and the side boards.

Tube and Base

It transports as two pieces: the 15 kg tube, retracted to 800mm, and the base, at 39 lb and 30" x 20". Retract the tube, loosen the locking screw and lift the tube off the base.

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, and 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.