This Astro-Tech 15.5mm PF series eyepiece has excellent eyerelief and a fantastic flat field to further enhance your viewing pleasure.
This Astro-Tech 15.5mm PF eyepiece is an excellent medium power eyepiece for all types of observing in all types of seeing conditions. As long as you aren't out observing in a blizzard or downpour, you should be good. A focal length like the 15.5 fits a natural step left behind by most telescopes standard 25mm or so eyepiece.
With a short focal length refractor it is a low power eyepiece. With the 430mm focal length Astro-Tech AT72EDII, for example, its five lens/three group multicoated glass optics provide 27.8x and a field over four times as wide as the full Moon. This is well suited for observing globular clusters such as M13 and M22. Both fit nicely into its 1.67° field with the AT72EDII, with a framework of black sky around the clusters to set them off. Its sharpness and lack of astigmatism across the field make globular clusters sharp, vivid, and nearly three-dimensional. This series of eyepiece is designed to have an inherently flat field, so it will excell at wide field observation.
It functions so well for medium high magnification observing with long focal length refractors, reflectors, or catadioptric scopes. On an 8" f/10 catadioptric, for example, it yields 129x. This is an excellent magnification for planetary and close-up lunar observing. Its overall performance makes it a much better choice as a second eyepiece than merely doubling the power of most scopes' so-so standard 25-26mm eyepiece with a Barlow.
The five lens/three group design of the 15.5mm Astro-Tech PF provides all of the optical virtues described at the top of this catalog page - high contrast, a sharp and flat field from edge to edge, excellent color fidelity, and exceptionally low astigmatism and barrel distortion. It has a usable eye relief of 16mm. The eyepiece some with a traditional rubber fold down eye cup to assist that glasses wearers of the world find a comfortable eye position. This particular eyepiece has a nice 65° apparent field of view, which is a tad wider than the 5.5 and 10.5 PF.
Enjoy the night skies with this very reasonably priced premium eyepiece.
