Ask any forum what to put in a new Dobsonian and somebody will say "goldline" within three replies. Ask what the difference is between the goldline and the redline and you will get a much less confident answer.
Here is the short version: there is no optical difference. The gold ring and the red ring are the same eyepiece.
The same design, sold under six names
These are derivatives of the classic König three-lens formula, made in one factory and sold by whoever puts their sticker on the barrel:
- SVBony Redline, the red-banded version, currently marketed as 68 degrees
- SVBony Gold Line, the gold-banded version, marketed as 66 degrees
- Agena Enhanced Wide Angle (EWA), the gold band with Agena's name on it
- Meoptex, a white bezel
- Yoosoo, another gold bezel
- Antares, the same eyepiece at a considerably higher price
You do not have to take the forums' word for it. Compare the manufacturers' own published specifications and the optical prescriptions line up exactly, focal length for focal length:
| Focal length | Elements / groups | Eye relief |
|---|---|---|
| 6 mm | 5 in 3 | 14.8 mm |
| 9 mm | 6 in 4 | 15 mm |
| 15 mm | 4 in 3 | 13 mm |
| 20 mm | 5 in 3 | 18 mm |
SVBony and Agena publish the same element counts for each focal length, under different badges and different claimed apparent fields. Reviewers who have owned both report no difference beyond the rubber grip moulded onto the red barrels.
So buy whichever is cheaper on the day. That is the entire decision.
The set is not uniform, and that is the real question
The interesting choice is not red versus gold. It is which of the four focal lengths is worth your money, because they are genuinely different eyepieces that happen to share a badge and a price.
Ranked by optical quality, best first:
1. The 9 mm is the one to buy. It carries six elements where its siblings carry four or five, and that extra glass does correction work at the edge of the field. It holds up in telescopes as fast as f/3.5, which is unusual at any price under a hundred dollars and close to unheard of under forty. If you buy one eyepiece from this family, buy this one.
2. The 20 mm is the wide-field end and the most comfortable of the four, with 18 mm of eye relief that lets you keep your glasses on. It is very good in a slow telescope. In anything faster than about f/6 the outer field grows soft, so how good this eyepiece is depends on which telescope you own.
3. The 6 mm gets you real planetary magnification for the price of a filter, and a built-in Barlow group keeps its eye relief near 15 mm where a 6 mm Plössl would give you six. The catch is a small, unforgiving exit pupil: move your eye off axis and one side of the field goes dark, an effect observers call kidney beaning. It also measures nearer 63 degrees than the 68 on the label.
4. The 15 mm is the one to skip. It is the only member of the family built on four elements rather than five or six, and it shows: astigmatism across the outer field, worse than any of its siblings, and clearly visible in a fast telescope. Every reviewer who has compared the set puts it last. Buy the 9 mm and a Barlow instead and you have covered this focal length with better glass for the same money.
Two things the spec sheets get wrong
The apparent field varies by focal length. The family is sold under a single number, either 66 or 68 degrees depending on the badge, but independent measurement puts the 6 mm nearer 63 degrees and the 9 mm nearer 70. If you buy two expecting matched fields, they will not quite match.
SVBony's eye relief figures are a copy-paste error. Their spec table lists 17 mm for all four focal lengths. A 6 mm eyepiece with 17 mm of eye relief would be a notable optical achievement rather than a budget commodity. The per-focal-length numbers in the table above, published for the identical Agena-badged version, are the ones to trust.
What they are not
They are not premium eyepieces, and the honest limits are worth knowing before you order. The coatings and internal blackening are basic, so a bright object just outside the field can throw a visible ghost across the view. The build is plastic-heavy and the eyecups are unremarkable. Nothing here will make you stop wanting an Explore Scientific 68 or a Tele Vue.
What they are is the cheapest genuine upgrade in the hobby. A 68 degree field for the price of the 52 degree Plössl that came in your telescope's box is the single most convincing thing you can do to a beginner's kit for under fifty dollars.
If you want two, take the 9 mm and the 20 mm. They cover most of a night between them, and neither is the weak one.