A Celestron FirstScope in need of eyepieces. Time to find a couple inexpensive eyepieces. With the telescope destined for an eight year old girl I really did not expect the eyepieces to live up to any demanding standards. But neither do I want the views to be truly horrible either, it would be nice if the telescope is properly enjoyable.
Thus I went to the source of cheap optics and found a set of Chinese eyepieces on eBay that might just do the job. For $22 they were the right price. The set includes three eyepieces, a 23mm, a 10mm and a 4mm for high power, a nice selection of focal lengths. The right price, but how about image quality?
As the eBay sale is from a US seller the eyepieces arrive in just a few days. Unpacking the eyepieces is the first pleasant surprise… The quality is not bad, metal barrels, decently molded rubber bits, decent looking glass. An ebay sale often a bit of a gamble, but this does not look bad at all.
There is no brand name on the eyepieces or on the packaging, simply “Made in China” at one end of the box. I will have to simply call these the Chinese.
No other real way of testing… I need to put them in a telescope and try then. How? Compare them against eyepieces of known quality. As comparison eyepieces I left my Naglers and Radians in the case, selecting instead some well known comparable alternatives. Looking around I came up with a 26mm Meade Super Plössl and a 12.5mm University Optics Plössl, very standard eyepieces.
The old University Optics Plössl is a bit of a special eyepiece to me. It is the first eyepiece I ever bought, purchased for the first telescope I built as a teenager. I have spent long hours looking through this eyepiece. I note with some surprise that University Optics is still selling eyepieces.
I performed the testing in three telescopes… First a nearly perfect 90mm f/13 APO refractor, for a faster option I used my 6″ f/5 newtonian, and also in the Celestron Firstscope that would be the ‘scope these eyepieces would be used with. The FirstScope is a 75mm f/4 telescope, small, but suitable for a kid.
A half hour in the driveway once darkness set in and I had my answers… The 23mm Chinese is not bad, quite comparable to the Meade Super Plössl. I would give the Meade a bit of an edge in image quality, but the difference is fairly minor. Perhaps with a planet the difference would be more notable, but only Venus and a quite distant Mars were available.
The 10mm suffers from substantial coma. In a long focal length telescope this is minimal, but can be seen towards the outer 20% of the field. In a short focal length telescope the problem becomes a real issue, as soon as you move off the center 20% of the field the coma begins to appear, at the edge stars are pretty smears of light.
In the 4mm the chromatic aberration is horrible. Anywhere off the center of the field a star becomes a rainbow. Again this becomes more of a problem with shorter focal length telescopes. In the 90mm it could be clearly seen, in the 6″ f/5 it was significant, enough to be a serious issue. In the FirstScope? It was simply horrible.
There are two other claims worth thinking about in the sales descriptions… The eyepieces claim to be aspheric. I have no way to evaluate this claim, but in this day of decent molded glass there is no reason to suspect this is incorrect. The performance of the eyepieces is poorer than the standard spherical Plössl design, but offers a wider field… Possibly.
The 62° field claim is not totally bogus, the field is distinctly larger than the 45 or 50° of the classic Plössl. Of course that field is limited by the coma and chromatic refraction issues noted above in the 10mm and 4mm eyepieces, so not much of a real gain.
Eye relief was similar to that of a Plössl. The 23mm had some, but the higher power eyepieces had very little, not enough for glasses.
In my final judgement I would rate the 23mm and 10mm eyepieces acceptable, particularly for the price. The 23mm is fine, the 10mm is usable in longer focal length telescopes, f/10 and longer. What about the 4mm? Keep the other two and call the 4mm a paperweight. Two eyepieces for $22? Still a deal.