Meade 26mm 5000
Well, I picked up another pair of eyepieces recently, looking for a better pair of low powered eyepieces for my BT100. new model Meade 5000, 26mm plossls with a 60° AFOV. These sell for $99.
On axis the Meade 5000 was pretty good, but off axis it deteriorated with coma (and astigmatism I think) very rapidly. By 50% out, it started to show. At 70-75% out coma was responsible for deterioration that prevented seeing a 36" double separated. That's poor. By 80-85% out, you couldn't tell it was a double. The TV and Meade SP LP can see doubles of 22" or closer all the way to the edge.
A Televue 26mm and the Meade 26mm SPLP allow seeing closer doubles completely separated all the way out to the edges. So both of them have the full use of the entire 50° AFOV without much noticable deterioration of the image quality. The TV26 has a slight edge in throughput over the Meade SPLP in the outer 25% ring.
Just so you can put this in perspective, in this same instrument, the TV26 can see a 14" double still clearly separated within 5% of the edge. A TV20mm plossl can see a 7 arcsec double at 95% out and the 14mm Radian can see a 5 arcsec double still clearly separated as it passes off the edge of the field stop. These are apparent separations of 330 to 220 arcseconds undistorted (without much coma, SA, curvature or astigmatism) at the edge of the FOV. The Meade 5000 26mm point image is distorted to an undesirable 450 arcseconds at 60% out and to an unrecognizable 850 arcseconds at 80-85% out. (Smaller apparent values are better).
This new Meade 5000 26mm has use of maybe at best 65-75% of the AFOV and in part even that is compromised. Effectively, that's no more than about 42-45° undistorted AFOV. At f/6.2, Coma in the outer 20% of the FOV was deplorable.
The 26mm 5000 did perform better with a wider corrected fov in the f/11 C5, that was as expected. But how much better? Are they any better than a standard plossl?
I measured the TFOV and calc that the Meade 5000 measures about 61-62° AFOV. Previously, I measured both the TV26 and SPLP26 and both of those have a 50° Afov. At 80% out in the Meade 500, the Tfov (not the image) is equal to the entire filed of the TV and Meade plossls. For the Meade 5000 to equal the plossls, I should at least be able to see the same image quality at a point 80% out from center.
Using Gamma Delphinus and Struve 2725 as my targets, I checked for furthest point out where I could still see the pairs resolved, even with some aberration present. When the aberrations take over and I can no longer see that the pair is a double star, that's the limit just beyond the point I record here.
Also, I previously measured my C5 and have found that it operates much closer to f/11. The standard figure of f/10 for an SCT is generally given for the plane of the visual back. I use a diagonal. I'm using F=1350. All three of these eyepieces give 52x in this scope. The Meade 5000 gets a 1.2° TFOV while both the others get almost 1°.
In the Meade 5000, Gamma Del is NOT seen as two stars when Stf 2725 is on the edge of the field stop. That places Y Del 55% out in the FOV. So, at 55% out the Meade 5000 looses a 9.8 arcsec double from view. At that magnification, 52x, that represents an apparent separation of 509 arcseconds. At 85-90% out in the view this eyepiece exhibits significant astigmatism. While the view is comatic, I could turn inside and outside of focus and easily watch the elongated shape of the out-of-focus image flip orientation 90°.
In both the TV26 and the Meade SPLP26 the views were nearly identical. This time, with Gamma Del on the edge of the field stop, Stf 2725 is still seen as separated. In fact I was able to move Y Del out past the field stop to reduce the edge distance by nearly 50% of the space between these two doubles. So that places Stf 2725 just beyond 70% out from center. At 70% out, both the TV26 and SPLP26 reach the limit, and that is the limit where they can both see this 5.8 arcsec double as separated. That means both these can see an apparent separation of 52x5.8 = 302 arcsec at 70% out.
Meade 5000 = 509 arcsec at 55%. AFOV 62° x 55% = 34° Afov produces 509 arcsec correction.
TV26 and SPLP26 = 302 arcsec at 70%. AFOV 50° x 70% = 35° Afov produces 302 arcsec correction.
Samller apparent separation is better.
All three of these eyepieces allowed some ability to refocus the image, at least to a small degree, with the image placed 50-70% out. That would indicate some Curvature present that could be focused out. In no case was that the predominant aberration. None seemed to be plagued by Spherical Aberration, at least not to any great degree. In the Meade 5000, the dominant aberration seemed to be coma, overwhelmed by astigmatism out beyond 80%. In the TV26 and SPLP26, it seemed to be mostly coma with little astigmatism.
Although the Meade 5000 is a very nice mechanical eyepiece, and I like the twist-up eyeguard even better that that of the Radians, it's correction for aberrations in the outer field does not match even that of standard plossls. The plossls provide a wider corrected view.