My impressions:
First, I have to say how shocked I was on how good the $12.00 Celestron 15mm spotting scope eyepiece turned out to be. Frankly, for what it is, I think you'd be hard pressed to find something much better.
Being a spotting scope eyepiece, and described as a wide angle ocular, it's quite user-friendly with ample eye relief with the stated specs of 20mm and an AFOV of 52 degrees. However, I peg it at 18mm at around 60 degrees AFOV based upon my comparos with my pentax eyepieces of the same-specced eye relief but without any tendency of either blackouts or sensitivity to head placement. I compared it to a vintage Celestron Ultima 15mm wide angle ocular with an AFOV of 70 degrees and an Antares elite 15mm 5 element plossl with a 50 degree AFOV, both of which are Japanese manufactured pseudo-Masuyama eyepieces.
I ran comparos of the eyepieces, with and without a paracorr, in my 16" f4.5 astrosystems dob last night in front my my moderately light polluted home (SQM 19.13). I had good seeing conditions and reasonable transparency along with moderate dew formation.
My first target was Venus and my observations were essentially a bit limited because I chose to view it during twilight since its too bright for me to take under darker conditions. That said, you will likely see flare and a bit of light fogging in the eyepiece with some of it bouncing off your cornea. Still, the planet was reasonably viewed in all three eyepieces, with the best edge correction noted in the lesser AFOV Antares eyepiece. Without a paracorr, the spotting scope eyepiece shows chromatic aberration at about 75% out from the center and about 85% out from center with a paracorr. Similar findings were noted with the Ultima wide angle eyepiece.
My second target was Arcturus and I noted astigmatism in the spotting scope eyepiece, with a similar presentation out from the center of the field as noted above on Venus. The wide angle Ultima also exhibited some stig but not as pronounced as in the spotting scope eyepiece. I did not notice any SAEP or any brightening at the edge.
Where the spotting scope eyepiece hit the sweet spot for me was in the observations of DSOs. I suspect it matched my exit pupil nearly perfectly under my light polluted conditions. Viewing various springtime galaxies with different eyepieces, I was, again, shocked on how well this eyepiece performed. With a paracorr in place for all observations, the "pup" in the whale and the pup appeared the brightest in the spotting scope eyepiece, with or without a broadband UHC filter, as compared to the 12mm Angel Eyes wide angle eyepiece or my 10.5mm pentax ocular. Of these eyepiece, the hockey stick appeared the brightest with the spotting scope eyepiece. Additionally, the large galaxy near the hockey stick, NGC 4631, appeared to seemingly fill almost two thirds of the AFOV. I was stunned by the view. Lower contrast galaxies, like NGC 3628, the dimmer member of the Leo trio, and the Siamese twins in Virgo, were identified in short order, despite the level of light pollution. I don't attribute the successful observations to this ocular being some super-duper eyepiece but, again, it apparently matched my exit pupil and observing conditions splendidly for me.
Although I didn't do any day time observations with it, I'd expect it to perform well since it's designed for use in spotting scopes and these are typically used during the day.
When I ordered it, my guess was it probably would be a plossl design but after using it, I suspect it's a repurposed binocular eyepiece, and a relatively good one at that, and my estimation is it's probably a modified erfle of some sort.
Finally, the eyepiece, along with the others I'm describing, does not approach the spot image diameter, quality and contrast and resolution of my 10mm Ethos. I suspect the 15mm spotting scope eyepiece would not fare all that well in one of Ernest's bench tests. Still, based upon what I saw last night, it'll be a go to eyepiece for various DSOs under my observing conditions. I'll reserve judgment on how well it does under dark sky conditions later in the month. Speaking of the latter, I feel it's interesting to note that the moon, at only a few days past full and at more than 90% illumination, didn't rise until 11:30pm and I couldn't see it until nearly midnight and when it did rise, it was well over to the southeast. Contrast this with one of the months that the moon rides the highest in the sky but nobody seems to notice, probably because of the holidays or inclement weather, that being the month of December. So, if the weather is decent in May, I should have quite a few nights of dark sky observing available to me, unfettered by the moon...
PS: The Celestron spotting scope eyepiece currently sells for $12.00 via a vendor on Aliexpress or $42.00 from vendors on ebay.
Turning now to the Angel Eyes 70 degree AFOV 12mm eyepiece, I found its AFOV to be closer to Ernest's measurement of around 60 degrees AFOV with about 12mm or so of useable eye relief. It would likely be tight for an eyeglass wearer. It has a funke blue border around the edge of the field of view, probably because of the barrel, and I call Ernest saw a blue border in his sample as well. My experience with edge performance was similar to that of the 15mm spotting scope eyepiece described above but with less edge aberrations. Contrast was relatively good on m51and m53. It was easier to spot the black eye in m64 and the dark lane in the large edge on galaxy, NGC 4565, than in the Celestron spotting scope eyepiece, probably because the increased magnification darkened the background a bit more.
The eyepiece currently sells for around $40.00 via a vendor on Aliexpress. I did like it but I was more enamored by the Celestron spotting scope eyepiece. No SAEP or edge of field brightening and no serious faults to speak of. Comparing it to my venerable UO 12.5mm ortho, it falls a bit short on planetary performance on Venus but otherwise it performs well on DSOs. Like the Celestron spotting scope eyepiece, I'll reserve judgment of performance under dark sky conditions at a later time...
