Refractors bend light through lenses, reflectors bounce it off mirrors, and catadioptrics combine both to fold a long light path into a short tube. Every design does the same job: gather starlight and bring it to a focus, where an eyepiece magnifies the image for your eye. Watch the gold photons below to follow the light.
Light passes through a large front objective lens that bends (refracts) it to a focus near the back of the tube; the eyepiece then magnifies that image. Sealed tube, no central obstruction → crisp, high-contrast views of the Moon, planets and double stars. Drawback: big lenses are heavy and costly, so apertures stay modest.
At SCOB: the 15 cm apochromatic refractor (f = 1800 mm) piggybacked on the main telescope.
Light travels down the open tube to a concave primary mirror, reflects back up the tube and is turned 90° by a small flat secondary mirror out to an eyepiece on the side. Mirrors are cheap per centimetre of aperture — the best light-gathering for the money. Often carried on a simple Dobsonian mount. Needs occasional mirror alignment (collimation).
At SCOB: portable Dobsonian/Newtonian overflow stations on the viewing deck.
Light bounces off a concave primary, forward to a small convex secondary that stretches the focal length and sends it back through a hole in the primary to an eyepiece behind the tube. The folded path packs a very long focal length (= high magnification) into a manageable tube — ideal for planets, the lunar terminator and tight double stars.
At SCOB: the showpiece 40 cm (16″) main telescope — a Cassegrain with a 5200 mm focal length in the 5.5 m dome.
A Cassegrain with a thin Schmidt corrector plate sealing the front: the lens fixes the mirror's edge distortion (spherical aberration), giving sharp stars across the field in a stubby, sealed, travel-friendly tube. The workhorse of public astronomy — long focal length, compact body, easy to motorise for GoTo tracking.
At SCOB: the Celestron CPC 800/925/1100 and C6 on the viewing deck, plus the 11″ catadioptric in the second dome.
The SCT's cousin: a thick, deeply curved meniscus lens corrects the mirror, and the secondary is usually just an aluminised spot on the corrector's inner surface — nothing to align, ever. Very long focal ratios (f/12–f/15) give razor-sharp, high-contrast Moon and planet views in a tiny sealed tube. Trade-offs: the thick glass takes a while to cool to night air, and large apertures get heavy, so most Maks are 90–180 mm (the classic Sky-Watcher/Celestron "Mak" spotting and planetary scopes).
At SCOB: same catadioptric family as the deck SCTs and the 11″ in the second dome — a compact Mak makes an ideal grab-and-go planetary station.
Earth spins at 15° per hour, so every object drifts westward; at 200× it crosses the eyepiece in under a minute. A mount's job is to point the telescope and keep it pointed. There are two families: alt-azimuth mounts move up-down and left-right like a camera tripod, while equatorial mounts have one axis tilted parallel to Earth's axis so a single slow motor cancels the spin. Singapore twist: at latitude 1.3°N the celestial pole sits right on the northern horizon, so equatorial mounts here are tilted almost flat.
The computer slews to any of 40 000 objects, then drives both axes at constantly changing rates to hold it centred. Fast setup, no polar alignment, ideal for public queues. Weakness: the field slowly rotates in the eyepiece, so long-exposure DSO photos need an equatorial wedge or derotator.
At SCOB: the Celestron CPC 800/925/1100 and C6 on the viewing deck.
Alt-azimuth at its simplest: a plywood rocker box on a swivel base. No motors — you nudge the tube by hand every ~30 seconds at high power as the sky drifts past. What it gives up in tracking it repays in aperture: the cheapest way to carry a big Newtonian, and newcomers learn the sky fastest on one.
At SCOB: the portable Dobsonian overflow stations on the deck.
The polar axis is tilted to run parallel to Earth's axis; one motor turning at sidereal rate (15°/hr) exactly cancels Earth's spin, so the object and the field orientation stay locked — which is why every serious DSO-imaging rig sits on one. A counterweight balances the scope, and the mount must "meridian flip" as targets cross due south.
Usage: deep-sky astrophotography; the design behind most hobbyist imaging setups.
A rectangular yoke rides on a polar axis carried by two massive piers, with the telescope swinging on pivots inside the frame. Same one-motor sidereal tracking as a GEM but with no counterweights and rock-solid stability — the classic way to mount a big observatory reflector. Its one blind spot, the celestial pole itself, doesn't matter in Singapore where the pole hugs the horizon.
At SCOB: the 40 cm main Cassegrain rides an English yoke in the 5.5 m dome — steady enough for the astro-camera.
Both mount types can keep a deep-sky object centred: the equatorial does it with one axis turning smoothly at sidereal rate, while an alt-az GoTo computer recalculates altitude and azimuth every second. The difference shows up in the camera: on an alt-az mount the field slowly rotates around the target, smearing stars in long exposures. That's why SCOB's imaging happens on the yoke-mounted 40 cm, while the alt-az CPC forks on the deck serve eyeballs — for which field rotation is invisible.
| Common eyepiece | Magnification |
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Rule of thumb: beyond ~2× the aperture in millimetres the image just gets bigger and blurrier — Singapore's steamy seeing often caps sharp views around 150–250× even on the 40 cm. An exit pupil above ~7 mm wastes light (wider than a dark-adapted pupil).