Eyepiece Guide
| Key Specifications | |
|---|---|
| Focal Length | 12mm |
| Apparent Field of View | 60° |
| Eye Relief | 20mm |
| Lens Elements | 5 (ED glass, fully multi-coated) |
| Barrel Size | 1.25" |
| Weight | ~165g |
| Features | Threaded for 1.25" filters, rubber grip |
| Waterproof | No |
| Available From | First Light Optics, Amazon UK |
The BST StarGuider 12mm is the eyepiece that fills the gap between low-power exploration and high-power planetary work. It's perfect if you own the 8mm and 18mm but find yourself wishing for something in the middle — a true all-rounder that shows Moon craters with clarity, planetary cloud belts with definition, and open clusters without too much magnification.
f/5, 650mm focal length
54×Perfect mid-power. Jupiter's Great Red Spot is visible. Saturn's Cassini division shows clearly. Moon craters at this power are genuinely spectacular — the terminator looks like a mountain range.
f/5, 750mm focal length
62×The ideal "general observing" power for a 150mm. More detail than the 18mm, less fatigue than the 8mm. The sweet spot for a 5-inch scope.
f/10, 1500mm focal length
125×Excellent planetary power on a 6-inch SCT. Jupiter's clouds are crisp. Saturn's rings show fine division. Moon terminator reveals deep rilles and tiny craters.
Any scope
ExcellentThe 12mm is brilliant for clusters like M35, M37, and the Pleiades. Wide enough to show structure, high enough magnification that stars don't blend.
The 12mm pairs brilliantly with the 8mm and 18mm. Together they form the core three-eyepiece set that handles virtually all observing scenarios — from finding objects to seeing deep planetary detail.
The BST StarGuider range is legendary for value. The 8mm and 18mm are forum staples, and observers consistently report the 12mm fills a genuine gap between them. It's the natural third eyepiece once you own the pair.
"Perfect mid-power for the Moon." Many observers describe the 12mm as the go-to eyepiece for lunar work. At 54–125× depending on scope, it's magnified enough to reveal tiny craters without overwhelming atmospheric turbulence.
ED glass makes the difference. The extra-low dispersion element minimises chromatic aberration — Jupiter's edges look clean, not purple-fringed. That's noticeable compared to cheaper eyepieces at this magnification.
Exceptional value outperforming twice the price. For £55, this eyepiece competes with designs costing £120+. The ED glass and 60° field are usually found in premium eyepieces.
The high-power planetary companion. The 8mm and 12mm together cover planetary work and Moon detail perfectly.
~£45 View at FLO →The low-power wide-field partner. These three eyepieces (8mm/12mm/18mm) form the complete core set for any observing scenario.
~£45 View at FLO →Thread this directly onto the 12mm for observation of a brilliant Full Moon. Reduces glare and lets you see faint craters near the terminator.
~£15 View at FLO →Double the 12mm to get a 6mm (245× in an SCT, 108× in a Heritage 150P). Great for extreme planetary nights, though eye relief gets tighter.
~£27 View at FLO →This is the completion piece. Budget observers start with the BST 8mm and 18mm (£110 total). Once they confirm the hobby sticks, the 12mm is the obvious third purchase — it costs no more than the first two but transforms observing versatility.
Three eyepieces cover everything. The 8mm (high power), 12mm (mid power), and 18mm (low power) together form a complete observing system that needs nothing else for at least a year. You can explore deep-sky objects, observe planets, study the Moon, and examine doubles — all with consistent optics and the same ED glass quality across the set.
Buy the 8mm and 18mm first if starting fresh. Once you've used those for 2–3 months, add the 12mm and you'll wonder how you ever observed without it.