I just acquired the 20mm, 14mm, 9mm, and 8mm Starbase Orthos, some of which are included with the relatively new Takahashi 80mm Achromat that is meant for beginners. Note that the 20mm Starbase comes in both the Plossl-type two doublet configuration labeled as "Orthoscopic" and also in a three element Kellner configuration. I decided to get them because I wanted an inexpensive classic eyepiece with a good build that was single coated as I like single coats for double star observing and it is getting hard to find many of the old classics that were only single coated.
Their build is very nice, although I would have liked no undercut on the barrels. I did not disassemble any but viewing the elements through the eyepiece it is apparent that it is indeed an asymmetrical doublet Plossl-type. So not the standard Symmetricals we see today marketed as Plossls. The spacer between the doublets looked rather large so the doublets appear to be spaced widely.
While examining the inside wall of the eyepiece I noticed that the lens edges do not appear to be blackened, and both the spacer between the doublets and the retaining tube that has the field stop in it are smooth gloss black. I was not so concerned about the spacer but that retaining tube that has the field stop at the end can be problematic on bright targets but had to wait for a field test before jumping to conclusions.
Tonight it was luckily clear out and the Moon high near the zenith so I took out my f/8 TSA-102 to test these eyepieces. Takahashi mates these eyepieces with their new beginner Achromat which is f/10, so while I was not expecting perfect off-axis performance I was expecting respectable for these 45 degree AFOV Plossls labeled as "Orthoscopic" (not an ABBE design).
Here's what I saw through these using the Moon, Sirius, Rigel, and M42 as test subjects:
20mm -- On the Moon the central 75-80% of the AFOV showed sharp lunar features. The Lunar features in the last 20-25% were blurry and could not be refocused to sharp. Overall the contrast and crispness of the Lunar features in the central 75% was quite outstanding. When moving to Sirius the issue with the last 20-25% was astigmatism. As far as astigmatism goes, it was not all that bad so the star points, while deformed, were only slightly so in my f/8 TSA. Not great, but not annoyingly bad either. As I suspected, those smooth black walls also took their toll when a bright star was very near the field stop as you could see reflections of them in the black space outside the field stop. Similarly for the Moon when it's bright surface was at the field stop if you moved your gaze to observe the black region outside the field stop it showed the bright Moon reflected. With proper eye position it was not an issue so only when my eye was to a side a little to better catch the region outside the field stop. Using a Tele Vue 2x Barlow cleaned up the off-axis performance almost to the very edge. Eye relief was comfortable.
14mm -- Not parfocal with the 20mm but very close. Similar to the 20mm, the central 75-80% of the 14mm was sharp and showed high perceived contrast. The amount of astigmatism present in the outer 20-25% was still there, but less pronounced than the 20mm. This one did NOT show any reflections outside the field stop though. Using a Tele Vue 2x Barlow cleaned up the off-axis performance to the very edge. Eye relief was comfortable.
9mm -- Not parfocal with the 14mm or 20mm and very far off from those. Slight astigmatism was still present in the last 15-20% of the AFOV but more very mild compared to the 14mm. The central 80% of the 9mm was sharp and showed high perceived contrast. Like the 20mm, if stars or the Moon were at the field stop then you could see their reflection outside the field stop on that smooth black retaining tube wall if you looked for it. This eyepiece also showed a small dim circular green hued ghost in the center of the AFOV if the limb of the Moon was in the FOV but not near the center, or a bright star like Sirius (no Rigel). The spacing between the doublets for the 9mm must be such that the single coatings are not sufficient for backscatter between the doublets is what I conjecture is probably the culprit. It was very noticeable with the Moon and Sirius, but not there with dimmer stars. Using a Tele Vue 2x Barlow cleaned up the off-axis performance to the very edge. Eye relief was tight but not overly difficult.
6mm -- Not parfocal with the 20mm, 14mm or 9mm and very far off from those. This eyepiece showed a crisp high perceived contrast view and it did NOT show any reflections outside the field stop though. Optically the best performer of the bunch having no issues in the off-axis and not light artifacts of any kind. Eye relief however was the tightest and while not difficult, not comfortable but expected. It was not what I personally consider overly difficult or uncomfortable. I felt it was just a little easier than the 5mm XO in comparison. Barlowed very well with the Tele Vue 2x.
Summary
Overall a well build and handsome set of eyepieces. The 20-14-9-6mm of the series is a nice set of focal lengths as they are all close to a 1.5x jump in magnification from each other. They were designed for an f/10 Achromat that Takahashi markets so while their performance in my f/8 scope was acceptable, I would expect their off-axis to be a bigger issue in faster focal ratios. The smooth black reflective walls of the internal spacer and field lens retaining tube with field stop showed light artifacts in my test with the 4" TSA Apo, as well as a central ghost with the 9mm. While these issues were not show stoppers for me with my 4" aperture scope since my primary goal for them is for general double star observing instead of bright objects like the Moon, I would expect these light artifact issues might be more annoying as the main objective of the telescope gets much larger than 4". Finally, given that the lenses are also just single coated, there was also a tendency for eyeball glint to occur infrequently (i.e., a bright star point reflects off your eye and back onto the surface of the eye lens causing a rapid moving transient point of light in the FOV).
