This Astro-Tech 15mm Titan is a very wide 70° field medium power 1.25"
eyepiece at a very economical price. It is ideal for lunar observing,
resolving globular clusters, picking up detail in emission and
reflection nebulas, etc. Its 15mm focal length nicely fills the gap
between the 25/26mm eyepiece that typically comes as standard equipment
with most telescopes and an 8/10mm high power planetary/binary star
splitting eyepiece.
With short focal length refractors and fast focal
ratio reflectors, it gives an extraordinary “picture window on space"
image. A f/4 Meade 8" Schmidt-Newtonian reflector, for example, yields a
3.75mm exit pupil, 54x, and a generous 1.3° field of view that’s good
for observing open clusters such as the Pleiades; large nebulas such as
the Lagoon, Veil, and Orion; and large globulars such as M-13 and Omega
Centauri. All will fit nicely into the field of view, with a framework
of black velvet sky around them to set off their subtleties. Globular
clusters and nebulas are vivid and nearly three-dimensional.
The
66mm f/6 Astro-Tech AT-66 refractor, on the other hand, has a
magnification of 27x with this eyepiece, with a 2.5mm exit pupil and an
immense 2.63° field (over five times as wide as the full Moon).
Even
medium and long focal length telescopes provide impressive views. Use
this 15mm Titan on an 8" f/10 catadioptric scope and you still get a
0.52° field at 133x, perfect for the Moon and globular clusters like M9,
M13, and M22.
The 7mm eye relief is somewhat short for those who
must wear eyeglasses while observing due to astigmatism, and will
somewhat vignette the field. This is of minor concern, as even those who
don’t have to wear glasses while observing will find they have to move
their head from side to side while observing to see the entire huge
field of view.
