Assessing Celestron Luminos Eyepieces: A Valuable Investment for Stargazing Enthusiasts
Chatty Astro Assistant
Celestron's Luminos eyepieces, an ultra-wide-angle (UWA) series, have been around for a while and hold their ground against competitors. While there's some stiff competition, the Luminos remains a decent option for UWA enthusiasts. Here's a glance at what this series has to offer in focal lengths of 2" barrel (31mm, 23mm, 19mm) and 1.25" (15mm, 10mm, 7mm).
Celestron offers a 2.5x, 2" Luminos-branded Barlow lens. While it's a good Barlow, we wouldn't fully recommend it due to its weight and questionable need for such a big Barlow.
Picking Your Luminos
The 31mm Luminos is hefty and shows edge-of-field brightening, but delivers almost the maximum true field achievable with a 2" eyepiece, making it the best pick among the 2" Luminos eyepieces. You might want to skip the 23mm and possibly the 15mm, whereas the 10mm and 7mm have fewer flaws, and the 19mm is acceptable. The 10mm Luminos easily takes the crown as the best of the bunch, making it an excellent medium to high-power eyepiece.
Luminos Eyepieces: Optical Design and Performance
The Celestron Luminos eyepieces are, more or less, clones of the Tele-Vue Nagler optical design. The apparent field of view, as with the Nagler, is specified at 82 degrees and is relatively accurate. The main difference between the Luminos design and most other UWAs is angular magnification distortion.
For instance, the field stop of the 23mm Luminos measures 37mm in diameter, which gives it a 92-degree apparent field. However, the magnification of the eyepiece drops as you move towards the edges of the field of view – a type of extreme field curvature. The result is twofold – the apparent field of view remains at 82 degrees, and the edges of the field of view are "compressed" gradually, by up to 26% in the case of the 23mm Luminos. This effect is known as "angular magnification distortion."
However, this field curvature is rarely noticeable, with up to 35% field curvature found in the 7mm Luminos. The more noticeable issue is edge-of-field brightening, which many users find objectionable.
Edge-of-field brightening is a problem for those observing under moderately light-polluted skies and focusing on deep-sky objects at low power without a filter. Darker skies will help reduce its effect, while heavier light pollution will create such a bright background that the subtle brightening isn't enough of an increase to notice. High powers will dim the background so much that a subtle increase in brightness doesn't matter. The edge-of-field brightening is caused by a mix of the actual sky background being brightened, combined with internal reflections in the eyepiece, which amplify the perceived background sky brightness.
You won't notice edge-of-field brightening in the 10mm Luminos, 7mm Luminos, or under dark skies, or if you're using a Luminos with a telescope that produces a magnification that results in an under 3mm exit pupil (or around 8x per inch of aperture), or if you're looking at a bright target, or if you're using a narrowband nebula filter.
The chart below shows the parameters of each eyepiece and the specifications for angular magnification distortion. As you can see, despite the 7mm Luminos' massive angular magnification distortion, the edge-of-field brightening is fairly low, and the 35% field curvature is only mildly perceptible. The 19mm is the least-bad offender of the Luminos low-power eyepieces, but it's the most criticized for some reason.
All of the Luminos eyepieces perform well across virtually any focal ratio, with the only aberrations being field curvature and minor lateral chromatic aberration. However, edge-of-field astigmatism can become visible in telescopes faster than around f/4.5 with all of the Luminos eyepieces, regardless of whether you're using a coma corrector.
Mechanics
The Celestron Luminos eyepieces feature a metal housing surrounding the eyepiece and a rubber grip ring. Twisting this ring adjusts a twist-up eyecup, similar to the old Meade Series 5000 UWAs. However, instead of twisting the whole eyecup, you just turn the ring. It takes some getting used to but works well once set correctly. But, this design adds width and weight.
One essential thing to note about the Celestron Luminos eyepieces is that their heavy, bulky bodies and twist-up eyepieces can be removed through a process known as "decloaking." This requires self-made eyeguards and top caps but is a great option for those seeking to minimize weight or volume, or wish to use the 1.25" 15mm, 10mm, or 7mm Luminos eyepieces in a binoviewer, otherwise impossible due to their width.
Alternative Recommendations
If the Luminos lineup doesn't meet your needs, other UWA eyepieces are available:
- Meade's PWA and Astro-Tech UWA eyepieces have better performance for their longer focal lengths, although their 10mm and 7mm units share the same optics.
- Baader Morpheus eyepieces offer a narrower 76-degree apparent field but eye-catching sharp, high-contrast views with plenty of eye relief.
- The 82-degree line from Explore Scientific features several good units and some so-so focal lengths, with edge-of-field brightening/AMD or too short eye relief, but most of the set is quite good and rivals the Tele-Vue Naglers in performance.
- The Tele-Vue Nagler series boasts decent eye relief, sharpness, few aberrations, and superior optical coatings compared to other UWA eyepieces, but the cost is predictably steep.
In conclusion, the 10mm Celestron Luminos eyepiece is a budget-friendly choice, providing good optical quality. However, it may not offer the same performance as higher-end UWA eyepieces.
The Celestron Luminos eyepieces, similar in design to the Tele-Vue Nagler, offer an ultra-wide-angle (UWA) field of view with a specified 82 degrees. While these eyepieces are bulkier and heavier than some competitors, they can be "decloaked" for weight reduction.
The 10mm Luminos eyepiece stands out as the best among the bunch, delivering excellent performance as a medium to high-power eyepiece. However, edge-of-field brightening may be an issue for some users under moderately light-polluted skies or at low power without a filter.
For those seeking a wider apparent field of view, the Explore Scientific 82-degree line and the Baader Morpheus eyepieces offer different choices, with the former providing a disparate set of qualities and the latter offering sharp, high-contrast views.
Lastly, for those willing to invest more in their hobby, the Tele-Vue Nagler series offers superior optical coatings and performance, making it the cream of the crop in UWA eyepieces. This range of options caters to various lifestyles, home-and-garden setups, and gadget lovers who appreciate technology and its applications in the field of astronomy.
