My svbony 9mm 66 degree long eye relief eyepiece (I believe these are colloquially known as "goldline"s because of their small gold band) arrived today and I wanted to do a bit of a comparison with my much nicer 8mm Delos.
First off, some clarifications and justification:
- I bought the goldline for outreach. I bought the Delos earlier for planetary work. I have zero affiliation or relation with either brand.
- Unsurprisingly, the $400 Delos performed better than the $25 goldline. The interesting question here is how and by how much did the Delos outperform.
- I feel that comparisons like these are useful for beginners asking the question "what am I missing if I don't buy premium eyepieces?"
- Today's tests were done using an AT72EDII and were far from exhaustive. I'm confident there are differences I missed.
- I've read reports of the 9mm goldline performing the best of its lineup, so I'd caution about trying to translate my results to other focal lengths in the series. I do not own any others for testing.
- I didn't get a chance to compare performance on Jupiter tonight, which would be good to test at some point due to the low contrast detail available in its cloud bands.
Let's get into the comparison. The main differences I observed were eye relief, clarity in the edge of the FOV, AFOV itself, sensitivity to eye placement, and internal reflections when viewed outside of the telescope. The main similarity I noticed was clarity (tested only at f/6) and brightness in the center 60% the FOV, which was a welcome surprise.
Eye Relief: I believe the advertising on this one. The goldline claims 16mm and the Delos claims 20mm. Both were achievable, but the Delos showed the full FOV without having to poke my glasses to the rubber where as the goldline required that I first push my glasses up my nose and second place the glass all the way against the eyepiece. The goldline has enough eye relief to be usable and not too frustrating, but the extra 4mm of the Delos is appreciated. Additionally, the flip up eyecup on the goldline feels a bit small. It seems to do the trick, but I worry it will attract a lot of eyelash smudges on the lens. The Delos solves this problem completely with their adjustable eyeguard (my favorite design is twist-up, which neither compared eyepiece has).
Clarity at edge of FOV: This was probably the most stark difference between the two eyepieces (aside from the obvious price, weight, and size). The outer 10-25% (estimating this stuff is hard) of the goldline has strong astigmatism and even some false color rainbow effect when a bright object like the moon is placed on the outer edge. It seems to me that this eyepiece may as well be 55 or 60 degrees AFOV instead of the listed 66 because those outer few degrees aren't very usable outside of recognizing that something bright exists. The Delos on the other hand is sharp right until a star literally touches the edge.
AFOV: I was pleasantly surprised to find that the AFOV was actually similar to the listed 66 degrees. I was expecting it to be smaller. By holding one eyepiece up to one eye and another up to the other eye, I was able to confirm that the AFOV of the Delos is the biggest (72 degrees), followed by the goldline (66 degrees), and then followed by my DeLite (62 degrees). That said, see the clarity comment regarding usable AFOV.
Sensitivity to eye placement: I actually consider my Delos to be one of the fussier eyepieces I regularly use regarding eye placement (this is compared to my DeLites and Plossls which are both really easy) but the goldline was worse here when using glasses. I think this was mostly due to difficulty positioning my eye with the shorter eye relief fighting against my glasses. I regularly got some kidney beaning using the goldline. The Delos blacks out when I mess up, but at this point I've gotten extremely consistent with using that eyepiece and my eyeball knows exactly where to go.
Internal reflections noticed outside of telescope: When I view through both eyepieces at a distance outside of any telescope with an off-axis light in the room, the goldline shows a bunch of internal structure illuminated by the light where as the Delos shows extremely little internal structure. I didn't notice this observing, but suspect that under the right conditions such as off-axis light hitting the eyepiece in a reflector this could cause issues in the goldline which are better mitigated in the Delos.
Clarity and Brightness in the center 60% of AFOV: This was pretty incredible for me. Now, perhaps a sharper eye would be able to see differences here, but I narrowly split castor into 2 and noticed no difference in the view between the eyepieces when it was centered. I was also able to find all the same stars in both eyepieces BUT this is from my bortle 9+ home so more testing may be required on brightness from a dark sky. I think this is an important lesson for beginners though, a decent budget eyepiece can perform practically on par with premium eyepieces for centered objects. Differences tend to come about off axis, under difficult conditions or difficult targets, or in the other areas I elaborated on above.
Hopefully this is helpful for somebody someday. I found myself looking for stuff like this when I was first considering premium eyepieces and hope to contribute to the next person wondering what improves and what stays similar. P.S. there are other kinds of premium eyepieces with different benefits such as extremely wide AFOV. I like this comparison in particular because both eyepieces have very similar claims of reasonably wide AFOV while accommodating eyeglasses.
I'm also curious to hear what other optical aberrations people notice in their goldines that I missed. I'll probably try to see if I can notice them in mine.