Art. –Nr. PEN-XO-2.5
Pentax XO 2,58mm
At the center perfect up to the diffraction limit of our measurement equipment, but half the way out a little bit, but measurable, worse.
x-axis: distance from optical axis in °, y-axis: resolution in arc minutes
Apparent Field of View:
The measured apparent field of view is approx. 42°, somewhat less than the 44° given by Pentax.
At 2,58mm focal length you can not expect a really famous contrast. Also Pentax has to respect physics, but for this short focal length the contrast is fine.
Very slight cushion distortion.
Not the right one if you have to wear glasses. The eye relief of 3.9mm is really tight.
That’s nothing we have to report on earnestly for Pentax eyepieces. Small, but perfect.
An eyepiec for highest magnification on fast telescopes. Sharpness outside of about 15° from the optical axis is not perfect at all. I strongly recommend to test an eyepiece of this short focal length on your own telescope, as magnification is really at the limit where more magnification will not lead to more visible details.