eeyepieces - SteveJustin1963/Telescope-Tec1 GitHub Wiki
1.25" Plossl Eyepiece Guide
A comprehensive guide to understanding and purchasing 1.25" Plossl telescope eyepieces in common focal lengths: 32mm, 17mm, 13mm, 8mm, and 6mm.
Table of Contents
- What Are Plossl Eyepieces?
- Understanding Focal Length and Magnification
- Eyepiece Focal Lengths and Their Uses
- Where to Purchase
- Physical Size vs Focal Length
- FAQ
What Are Plossl Eyepieces?
Plossl eyepieces are a popular and versatile 4-element design widely used in amateur astronomy. They are known for providing:
- Crisp, high-contrast images across the field of view
- ~52ยฐ apparent field of view (a comfortable viewing angle)
- 4-element optical design (two doublet lens groups)
- Multi-coated optics to minimize light loss and maximize transmission
- Standard 1.25" barrel diameter that fits most amateur telescopes
Key Specifications
| Feature | Description |
|---|---|
| Barrel Size | 1.25 inches (31.75mm) - industry standard |
| Optical Design | 4-element Plossl (2 achromatic doublets) |
| Apparent FOV | ~50-52ยฐ |
| Coatings | Fully multi-coated for best light transmission |
โ ๏ธ Note: Shorter focal length Plossls (โค12mm) have noticeably short eye relief, meaning your eye must be very close to the lens to see the full field of view. This can be uncomfortable for some observers.
Understanding Focal Length and Magnification
The Magnification Formula
Magnification = Telescope Focal Length รท Eyepiece Focal Length
Example: Using a telescope with 1000mm focal length:
- 32mm eyepiece โ 1000 รท 32 = 31x magnification
- 13mm eyepiece โ 1000 รท 13 = 77x magnification
- 6mm eyepiece โ 1000 รท 6 = 167x magnification
Key Relationships
| Eyepiece Property | Effect on Viewing |
|---|---|
| Shorter focal length | Higher magnification, narrower field of view, dimmer image |
| Longer focal length | Lower magnification, wider field of view, brighter image |
Eyepiece Focal Lengths and Their Uses
32mm - Low Magnification
Best for: Wide-field viewing
- โจ Widest possible view in 1.25" format
- ๐ Perfect for locating objects and framing star fields
- ๐ญ Ideal for large deep-sky objects (Andromeda Galaxy, large nebulae)
- ๐ Best for open star clusters and Milky Way sweeping
- ๐ก Provides brightest, sharpest images
Typical Magnification: 25-40x (depending on telescope)
17mm - Medium-Low Magnification
Best for: Versatile general observing
- ๐ Good for broader views of the Moon
- โ๏ธ Excellent for medium-sized star clusters
- ๐ซ๏ธ Nice views of larger nebulae
- โ๏ธ Balanced between magnification and field of view
- ๐ Easy to track objects as they move across the field
Typical Magnification: 45-75x
13mm - Medium Magnification
Best for: Detailed extended objects
- ๐ More detail on extended objects (Orion Nebula, smaller galaxies)
- ๐ Darkens the background sky for improved contrast
- ๐ญ Good for globular clusters
- ๐ Decent lunar detail
- ๐ Good all-around focal length
Typical Magnification: 60-100x
8mm - Medium-High Magnification
Best for: Lunar and planetary observation
- ๐ช Useful for seeing detail on planets
- ๐ Excellent for lunar surface features
- โญ Resolves some double stars
- โ ๏ธ Eye relief starts becoming noticeably short
- ๐ More affected by atmospheric turbulence
Typical Magnification: 100-150x
6mm - High Magnification
Best for: Fine planetary and lunar detail
- ๐ช High magnification for detailed planetary views
- ๐ Resolves fine lunar surface details (craters, rilles)
- โ ๏ธ Very short eye relief (uncomfortable for some users)
- ๐ Often pushing limits of atmospheric steadiness
- ๐ฌ Best used on nights with excellent "seeing" conditions
Typical Magnification: 140-200x
Where to Purchase
Complete Kits
Celestron Eyepiece & Filter Kit 94303
The most convenient option - includes all five focal lengths plus accessories.
Price: ~$362-450 AUD (regular price ~$500-575)
What's Included:
- โ All five Plossl eyepieces (32mm, 17mm, 13mm, 8mm, 6mm)
- โ 2x Barlow lens (doubles your options to 10 focal lengths)
- โ Colored planetary/moon filters
- โ Protective metal carrying case
- โ Multi-coated optics on all eyepieces
Available From:
- BinoCentral - $449.95
- Optics Central
- Rubber Monkey
- Amazon AU - $362.51
Customer Rating: โญ 4.6/5 (687 reviews)
๐ก Pro Tip: The included 2x Barlow effectively gives you 10 focal lengths:
- 64mm, 34mm, 26mm, 16mm, and 12mm (when combined with Barlow)
- Plus your original 32mm, 17mm, 13mm, 8mm, and 6mm
Individual Eyepieces
If you prefer to build your collection gradually or only need specific focal lengths:
Celestron Omni Series
Known for value and performance with blackened lens edges to minimize internal reflections.
- 32mm: Available at Amazon AU | Testar Australia
- 17mm, 13mm, 8mm: Generally available in kits
- 6mm: Available at Optics Central
Complete Set: Available on eBay - $265.41 (includes 2x Barlow)
Saxon Plossl Series
Reliable budget option with 4-element design and fully multi-coated optics.
- 32mm: Bintel | OZScopes
- 40mm: Optics Central - $76.95
- 6.3mm (closest to 6mm): Optics Central
Astromania Plossl Series
Good value individual eyepieces available via Amazon.
Physical Size vs Focal Length
Do Longer Focal Lengths Mean Longer Eyepieces?
For Plossl eyepieces: Yes, generally!
In classic Plossl designs, physical length is proportional to focal length because the design is relatively simple and scales directly:
32mm Plossl = Longest and heaviest
17mm Plossl = Medium-long
13mm Plossl = Medium
8mm Plossl = Short
6mm Plossl = Shortest and lightest
Shorter focal length Plossls also have:
- Smaller lens diameters
- Shorter eye relief (less comfortable viewing)
- More compact overall size
What About Other Eyepiece Designs?
For more complex, modern eyepiece designs (ultra-wide, zoom eyepieces, or those with built-in Barlow elements), the relationship changes:
- Ultra-Wide Angle Eyepieces: May be bulkier despite shorter focal lengths to achieve wide apparent fields
- Zoom Eyepieces: Physical length stays constant while focal length changes internally
- Premium Designs: Often use additional lens groups to maintain comfortable eye relief regardless of focal length
FAQ
Q: Which eyepiece should I buy first?
A: Start with a medium focal length like 13mm or 17mm. These provide versatile magnification suitable for both lunar and deep-sky observing. Then add a low-power (32mm) for wide views and a high-power (6-8mm) for planets.
Q: Can I use these with any telescope?
A: These eyepieces fit any telescope with a standard 1.25" focuser, which includes most beginner and intermediate telescopes. Check your telescope's specifications to confirm.
Q: What's the advantage of buying a kit vs individual eyepieces?
A:
- Kit Pros: Cost savings, matched quality, includes Barlow and filters, carrying case
- Individual Pros: Buy only what you need, can mix premium eyepieces, spread out cost
Q: How do I calculate the right magnification for my telescope?
A: Use the formula: Magnification = Telescope Focal Length รท Eyepiece Focal Length
Find your telescope's focal length in its specifications (usually 400-2000mm for amateur scopes).
Example:
- Telescope: 1200mm focal length
- With 20mm eyepiece: 1200 รท 20 = 60x magnification
Q: What's the maximum useful magnification?
A: A general rule is 50x per inch of telescope aperture:
- 4" (102mm) telescope = 200x maximum useful
- 6" (152mm) telescope = 300x maximum useful
- 8" (203mm) telescope = 400x maximum useful
Beyond this, the image becomes dim and fuzzy due to atmospheric turbulence and diffraction limits.
Q: Why is eye relief important?
A: Eye relief is the distance your eye must be from the eyepiece lens to see the full field of view.
- Long eye relief (15-20mm): Comfortable, great for eyeglass wearers
- Short eye relief (<10mm): Your eye must be very close; can be uncomfortable
Plossl eyepieces at 12mm focal length and shorter have noticeably short eye relief.
Q: What's a Barlow lens and should I get one?
A: A Barlow lens is a negative lens that increases magnification, typically 2x or 3x.
Benefits:
- โ Doubles (or triples) your eyepiece collection
- โ Improves eye relief on short focal length eyepieces
- โ Cost-effective way to get more magnifications
Drawbacks:
- โ Adds extra glass elements (potential for reduced image quality)
- โ Increases tube length (may cause focusing issues on some telescopes)
The Celestron kit includes a quality 2x Barlow that works well with the included eyepieces.
Recommended Reading
- BBC Sky at Night Magazine - How to Choose Eyepieces
- Cloudy Nights - Eyepiece Forum
- Your telescope's manual for specific eyepiece recommendations
Conclusion
A set of 1.25" Plossl eyepieces in 32mm, 17mm, 13mm, 8mm, and 6mm focal lengths provides excellent versatility for astronomical observing. Whether you purchase a complete kit or build your collection individually, these focal lengths will serve you well for years of lunar, planetary, and deep-sky observation.
Clear skies! ๐๐ญ
Last updated: November 2025