Choosing Your First Telescope

There is no universally correct first telescope, but there is a correct way to choose one: decide what you want to look at, decide how much weight you will actually carry, and then buy the most aperture that fits those two constraints. Everything else is secondary.

Aperture first, always

Aperture — the diameter of the primary mirror or objective lens — sets two things that cannot be improved by any accessory: how faint an object you can detect, and how fine a detail you can resolve. Doubling aperture quadruples light grasp. A 200 mm telescope collects about 4× the light of a 100 mm one and resolves detail twice as fine.

Magnification, by contrast, is trivially changed with a different eyepiece. A telescope sold on magnification is being sold on the one specification you can alter for £40.

The three optical designs

Refractor

A lens at the front. Sealed, permanently collimated, essentially maintenance-free, and sharp. The downsides are cost per millimetre of aperture and chromatic aberration on cheaper achromatic designs — a violet fringing on bright edges. An apochromat using ED glass removes the fringing and costs three to five times as much.

Choose a refractor if you value portability, sharpness and zero fuss, and if 80–120 mm of aperture is enough for what you want to see.

Reflector (Newtonian)

Mirrors instead of lenses, so no chromatic aberration and far more aperture per pound. The standard first serious telescope is a 6-inch or 8-inch Dobsonian — a Newtonian on a simple alt-azimuth base — and it is the standard for good reason.

The trade is collimation: the mirrors must be aligned, and they drift. Learn to collimate with a Cheshire or laser tool in about two minutes, and it stops being a problem. The open tube also collects dust, which matters far less than you would think.

Catadioptric (SCT or Maksutov)

A corrector plate and folded mirrors give a long focal length in a short tube. An 8-inch SCT with 2000 mm of focal length fits in a tube you can carry one-handed. Excellent for planets and for imaging; less good for wide fields, and it requires a focal reducer to become a competent deep-sky instrument.

Choose one if storage space or portability is your binding constraint, or if planetary imaging is where you are heading.

Focal ratio, briefly

Focal ratio is focal length divided by aperture. Fast (f/4–f/5) gives wide fields and short exposures but demands better eyepieces and more precise collimation. Slow (f/10) gives a narrow field and high magnification easily, which suits planets. Mid (f/6–f/8) is the flexible middle.

The mount is half the telescope

A superb optical tube on a shaky mount is a bad telescope. Two families: alt-azimuth, which moves up-down and left-right, requires no alignment and is the right choice for visual work; and equatorial, which requires polar alignment but tracks with one motor and does not rotate the field, making it necessary for long-exposure imaging.

• Visual only, large aperture, limited budget → Dobsonian (alt-azimuth reflector).
• Visual only, portability first → small refractor on a lightweight alt-azimuth mount.
• Planets and eventual imaging → Schmidt-Cassegrain on a GoTo equatorial mount.
• Imaging from the start → small apo refractor on a mount rated at twice your payload.

What to avoid

• Anything advertising magnification as its headline number.
• Telescopes on lightweight photographic tripods. The tripod is usually where cost was cut.
• A 76 mm reflector on a flimsy equatorial mount — the classic department-store instrument that puts people off the hobby.
• Buying eyepieces in a bundle. Buy three good ones as you find you need them.
• Solar filters of unknown provenance. Solar equipment is the one place where "cheap" is the wrong question entirely.