This is the Circle-T (identical to University Optics) Japan-made orthoscopic 5mm eyepiece. 1.25" barrel.
Best as a specialist planetary eyepiece or for splitting close doubles, these eyepieces are world-renowned for quality and their sharp view - sharper, more detailed and less distorted than widefield eyepieces or other "general purpose" eyepieces. The "volcano top" design makes viewing easier compared to flat-topped orthos or monocentric eyepieces. They're not made anymore and are something of a collector's item now.
Only 6 months old and in
brand new condition -
never used. Comes in original box with both end caps.
I have most of the rest of the Circle-T ortho series - I bought this one when the plan was to buy a refractor, hence the shorter focal length, but that plan has been scrapped. It's too short in F/L for my SCT, which is 2500mm, so I'll never use it. It's well suited (IMHO) to 'scopes with F/L in roughly the 900 - 1800mm range.
Asking
$55 including postage Australia-wide.
See photos.
Note: I also have the 4mm version for sale
here. You can have both for
$95 including postage Australia-wide.
