How to Use a Dobsonian Telescope

Before you go out

Choose a spot with a clear view of the sky, as far as possible from direct lights. Level, firm ground such as concrete, pavers or packed earth works best. A springy wooden deck transmits vibration and makes views shake.

Set up 20 to 30 minutes before you want to observe, longer for larger mirrors. The primary mirror needs to match the outdoor air temperature, otherwise warm air currents inside the tube blur the image. If you are new to the hobby, our Dobsonian telescope for beginners guide explains how these telescopes work.

Setting up

1. Place the rocker box on the ground and check that it sits steadily.
2. Lift the optical tube into the side bearings (the cradle). Many models have a loose fit that you tighten with knobs.
3. Remove the dust cap from the front of the tube.
4. Attach the finder to its bracket and insert an eyepiece in the focuser.
5. Adjust the tension knobs on the altitude bearings so the tube stays in position but still moves smoothly. Adding or removing a heavy eyepiece can change the balance.

Follow the instructions that came with your model, since details differ between solid-tube, truss and tabletop designs.

Aligning the finder

A finder lets you aim the telescope. It needs aligning once, and it is easiest in daylight.

1. Insert a low-power eyepiece (the one with the longest focal length).
2. Point the telescope at a distant, stationary object such as a chimney or treetop, ideally well over 100 metres away.
3. Focus until the object is sharp, and centre it in the eyepiece.
4. Without moving the tube, adjust the finder’s screws until the same object is centred in the red dot or crosshairs.
5. Check again at night using a bright star or the Moon.

Never point the telescope at the Sun without a proper solar filter designed for the front of the telescope. Even a brief glance can cause permanent eye damage.

Choosing eyepieces and magnification

Magnification equals the telescope’s focal length divided by the eyepiece focal length. For example, a 1200 mm telescope with a 25 mm eyepiece gives 48x, and with a 10 mm eyepiece gives 120x.

• Start with the lowest power. The field of view is widest, which makes objects easy to find.
• Use medium power for planets and the Moon, and higher power only when the air is steady.
• Higher magnification is not always better. If an image looks fuzzy at a higher power, switch back down.

Pointing and focusing

Look through the finder, centre your target, then look in the eyepiece. Use the focuser to sharpen the image. Stars should appear as small points. The Moon is an easy first target to confirm everything works.

Tracking by nudging

Because a Dobsonian has no motor, an object will drift across the field as the Earth rotates. Gently nudge the tube to re-centre it. A useful tip is to place the object at the edge of the field it is drifting toward, so you have time to enjoy the view before nudging again. At high power, drift is faster, so practice with low and medium power first.

Finding objects

• Star hopping: use bright stars as stepping stones and a star chart or app to move toward faint objects.
• Start with easy targets: the Moon, Jupiter, Saturn, the Pleiades, the Orion Nebula and the Andromeda Galaxy.
• Let your eyes adapt: give your eyes about 20 minutes in the dark and use a red light to read charts.
• Use averted vision: looking slightly to the side of a faint object can make it easier to see.

Collimation basics

Collimation means aligning the mirrors. Most Dobsonians hold alignment well, but moving the telescope can shift it. Signs of poor collimation include stars that look like comets or flares rather than tight points when slightly defocused.

Collimation tools include a Cheshire eyepiece, a sight tube or a laser collimator. The general steps are to align the secondary mirror under the focuser, then adjust the primary mirror’s screws until the reflections line up. Follow your telescope’s manual, since the screw layout varies, and make small adjustments. If you build your own scope, see making a Dobsonian telescope.

Care and storage

• Put the dust cap back on after observing.
• Store the telescope in a dry, dust-free place such as a garage or closet, and let it dry out if dew formed.
• Avoid cleaning the mirror unless it is truly dirty. Dust has little effect on the view, and cleaning can scratch the coating.
• Keep eyepieces capped and in a case.

Common problems

• Blurry images: let the telescope cool, check focus and try a lower power.
• Shaky views: move to firmer ground and avoid touching the tube while observing.
• Cannot find objects: re-align the finder, and begin at low power.
• Tube drifts down or up: adjust the altitude tension.

FAQ

How do you use a telescope?

Set it up on stable ground, align the finder, start with a low-power eyepiece, centre your target, then focus and increase magnification as conditions allow.

What telescope did Stephen Hawking use?

Hawking was a theoretical physicist and is not generally known as a telescope user, so there is no reliable link to Dobsonians that we can point to.

Which Dobsonian telescope is best for beginners?

An 8 inch solid-tube model is a popular choice. See our best Dobsonian telescopes guide.

Can you build your own Dobsonian telescope?

Yes. Many amateurs do. See our overview of making a Dobsonian telescope.