| Key Specifications | |
|---|---|
| Focal Length | 8mm |
| Apparent Field of View | 60° |
| True Field of View | Varies by scope (7.5° in f/5 scope) |
| Eye Relief | 20mm |
| Lens Elements | 4 (multi-element ED design) |
| Barrel Size | 1.25" |
| Weight | ~170g |
| Coatings | Fully multi-coated (ED glass) |
| Waterproof | No |
| Available From | First Light Optics, Amazon UK |
The BST StarGuider 8mm is the eyepiece for anyone who's just bought their first scope and realised the bundled 25mm and 10mm aren't quite cutting it. It's the single most recommended first upgrade on UK astronomy forums for one simple reason: it works. Brilliantly. At the price.
If you own a Heritage 130P, 150P, Evostar 90, or any comparable f/5–f/10 scope with a 1.25" focuser, this eyepiece belongs in your kit.
The magnification depends on your telescope's focal length. Here's what 8mm delivers in four common UK beginner scopes:
f/5, 650mm focal length
94×Jupiter shows belt detail and the Great Red Spot on calm nights. Saturn's rings resolve the Cassini Division. Moon is breathtaking.
f/5, 750mm focal length
94×Same magnification, more light. Jupiter's storms jump out. Planets look crisp and detailed. This is where the 8mm shines.
f/6, 1200mm focal length
150×High power, but not so high you're fighting atmospheric turbulence. Perfect for planetary observing on a steady night.
f/10, 900mm focal length
112×A refractor at high power is pure luxury. Moon and planets look like oil paintings. Pin-sharp contrast.
Note: All magnifications assume good atmospheric seeing. On a turbulent night, you'll get better views at slightly lower power (25mm eyepiece instead). On a perfectly calm night at a dark-sky site, push higher.
The reviews are unanimously positive. Stargazers Lounge, CloudyNights, and UK astronomy Facebook groups consistently recommend this eyepiece as the essential first upgrade. Owner feedback is almost always "I wish I'd bought this earlier."
"It transforms what you see." The jump from the bundled 10mm (basic Plössl) to the 8mm BST is striking. Planetary detail suddenly appears. Contrast improves. The view sharpens. For £45 it's absurdly good value.
Paired with the 18mm, it's a complete system. Many observers own nothing but the 8mm and 18mm for their entire first year. Together at £90, they cover low-power wide-field work and high-power planetary observing. That's enough.
Build quality punches above the price. The eyepiece feels solid. The focus grip is smooth. The ED glass delivers clean views. This is the gateway drug to better optics.
One caveat: very short focusers. A handful of users with extremely short focusers (some compact refractors) report that the 8mm's length prevents full focus travel. 99% of scopes are fine; just check your spec if you own something unusual.
The 8mm is most powerful when paired with complementary eyepieces. Here's what works alongside it:
The perfect companion. Low-power wide-field work with the same optical quality. Together they cost £90 and handle 95% of observing scenarios.
~£45Turns your 8mm into a 4mm (188× in a Heritage 150P). High-power planetary work without buying a separate eyepiece. Halves eye relief but unlocks new magnifications.
~£27If you upgrade from the 18mm, the 68° wide field is immersive. More expensive, but the view difference is real on a 200P or larger.
~£120A step up in build quality and planetary sharpness. If the BST 8mm works for you and you want premium optics at high power, this is the next rung.
~£80The BST 8mm is the foundation. It's entry-level in price (£45) but not in quality. Many experienced observers own nothing fancier — they stick with the 8mm and 18mm pair for years.
The next step is better build quality. The Celestron X-Cel LX 9mm (£80) or Explore Scientific 68° eyepieces (£100+) are the natural upgrades. They offer sharper optics, better edge correction, and more durable construction. But they're not necessary — the BST does the job.
The step after that is specialisation. A dedicated 4mm for extreme planetary power. A 32mm wide-field for open clusters. A 2" eyepiece for immersive views. You only buy these after confirming which targets you actually observe.
Start with the 8mm. If you love planetary work, upgrade to a Celestron or Explore Scientific. If you love deep-sky objects, upgrade to a wider-field eyepiece. The 8mm gets you to the crossroads; your observing habits tell you which fork to take.