Tessaro Ironwood 250 f/5 Dobsonian, tube only

Specifications

Description

The Ironwood 250 f/5 Dobsonian is a Newtonian reflector with a 250 mm parabolic primary mirror and a focal length of 1250 mm, which gives a focal ratio of f/5. The fast optics produce a wide true field at low power, so you can frame large nebulae and star clouds in the Milky Way with a 2 in eyepiece. The open-truss tube and rocker-box base keep the instrument manageable for its aperture.

The primary mirror sits in a cell with three collimation screws, and the secondary mirror is held by a four-vane spider. Check collimation before each session, because the optics shift slightly as the tube cools and is moved. The 2 in dual-speed rack-and-pinion focuser takes heavy eyepieces without slipping, and its fine-focus knob gives a reduction suitable for high-power focusing. The 22.0 kg tube is detached from the base for transport, so plan for a second person or a cart when you move it from a car to the observing site.

The rocker box rests on low-friction bearing pads and turns the tube smoothly in altitude and azimuth, which lets you follow objects by hand as they drift across the field. Set the base on a level, firm surface. Allow the tube about 45 minutes to reach outdoor temperature before observing planets at high power, since a large mirror holds heat longer than a small refractor and produces shimmering images while it cools.

Typical targets include the Andromeda Galaxy, the Orion Nebula, and the globular cluster M13 in Hercules, which resolves into individual stars along its edges at this aperture. Under dark skies the tube shows faint spiral structure in M51 and dust lanes in the Milky Way near Cygnus. Planetary views at high power are sensitive to seeing and thermal settling, so Jupiter and Saturn are best observed after the tube has settled.

About this configuration

The Tessaro Ironwood 250 f/5 Dobsonian is offered in several configurations, and the notes below describe what this particular combination adds or changes. Buying the tube alone makes sense if you already own a suitable mount or plan to choose one separately. Without a mount in the package, check the tube weight against the payload rating of your mount, with some margin to spare. The carbon fiber tube expands less as the temperature drops, so the focus point drifts less as the night cools.

With the starter kit, begin with the low-power eyepiece whenever you search for a new target, then switch up. The 2023 edition uses the same optical design as later years, with small differences in fittings and labels. Check the specifications table for the optical figures, since they stay the same across every configuration of the Ironwood line.

What's in the box

• Optical tube assembly with primary mirror cell
• Rocker box base with altitude bearing pads
• 2 in dual-speed rack-and-pinion focuser
• 9x50 right-angle finder
• Eyepiece holder and dust caps
• 25 mm and 10 mm eyepieces (1.25 in)
• 2x Barlow lens (1.25 in)

Compatibility notes

• Accepts 2 in eyepieces directly and 1.25 in eyepieces through a 1.25 in adapter.
• Eyepieces with field stops up to 46.5 mm are supported by the 2 in focuser.
• The rocker box accepts a standard accessory tray mounted on its side panel.