These days the dominant eyepiece design is the humble Plossl due to it’s overall good quality but there are still a few manufacturers releasing other formats of eyepiece such as Pentax and their 5mm orthoscopic.
The 5mm has a total of 5 lens elements spaced into 3 groups. Each lens element is fully multi coated and Pentax claim they have achieved 98% light transmission.
The 5mm provides a high magnification in any scope and is therefore predominantly a planetary/lunar eyepiece. It is then surprising how small the field of view is, a mere 44 degrees. Without a driven mount this means an object takes no time at all to move through the field of view necessitating a constant vigilance whilst moving the telescope. You have barely settled on an object and started observing before it’s time to readjust the telescope. This becomes frustrating and actually prohibits detailed viewing of an object as you’re unable to give it 100% of your attention. Luckily I have a clock drive but I tried using the Pentax without for the sake of a balanced review and the small field was a major issue.
Field of view aside the optical quality of the 5mm is impressive. Contrast is high, images are sharp, crisp and there are no aberrations to speak of. There is no light scatter and colour rendition is superb. The subtle differences in Jupiter’s atmosphere stood out clearly as did the great red spot.
Given the high performance of the 5mm it’s a great shame that it is quite uncomfortable to look through. There is only around 3mm of eye relief for example (eye relief is the distance ones head must be held at to see the entire field of view). This means the XO is not suitable for anyone wearing glasses. It also means one can develop eye strain or a headache by trying to peer through this eyepiece. The objective lens (the one you look through) is 6mm and the field stop is 4mm.
To summise it’s a real shame Pentax couldn’t have made this eyepiece more comfortable to look through, a longer eye relief would have gone a long way in doing that. Also 44 degrees is a very small field for what is a planetary/lunar eyepiece. With a bigger field and a more comfortable eye relief this would have been something special, optically it’s absolutely superb.
At £270 you might expect to be buying the perfect eyepiece but the XO is just too much of a chore to use, so close by Pentax yet so far.
