Pick this eyepiece up used based on a generally good reputation. Tried it out in a 660mm fl 102mm apo refractor on Jupiter over several nights, yields 166x in this instrument. The comparison eyepiece was a barlowed 7mm Pentax XL (AP 2x barlow), 189x in this instrument. Normally we would expect the barlowed eyepiece to be at a disadvantage a comparison like this. Higher magnification tends to be fuzzier, and the barlow adds an element of complexity that could in theory reduce image sharpness and contrast. In other words, this comparison is biased towards the Radian. Of the four people who critically examined the views, three found the Pentax/Barlow combination to offer superior clarity and detail. Since this is a high power eyepiece whose main purpose is zooming in on planets (or small planetaries) on axis we did not make extensive comparisons off axis. Both eyepiece arrangements offered comfortable eye relief. Nonetheless, to this reviewer the Pentax/barlow was unambiguously clearer and more detailed, and the Radian got sold not long after these tests. I will comment however that when I received the apo it had only a 1.25" diagonal, and for the first firew months I didn't have short focal length eyepieces to test in it. All I had was a Celestron plossl kit for beginners. If you are moving up from eyepieces in that category, the Radian is a huge relief and a vast improvement in color correction, performance, eye relief, and overall usability. Radians can be picked up used for $150 to $175 at this writing. New prices are $190 to $240 (the low price was a pre-Xmas promotion). So the Radian is a reaonable upgrade. It has the fov for high-power dso use and reasonable performance. Nonetheless, it leaves a lot of room at the top. Pentax did not offer a 3.5mm XL (hence the need for the barlow) but there is a 3.5mm in the XW series which retails for $300 and is available used for $250 or so. The XLs sell for $200 to $220 which is almost their new price, and indicative of the price premium the market attaches to their peformance. But to use the 7mm XL or 5mm XL at high powers in fast refractors will require a top notch barlow such as AP's. (Be very careful about this as inexpensive barlows are like achromat refractors and will introduce significant performance issues). The 5.2XL is another alternative but will offer less power. The other "way up" from the Radian is through a TMB Supermonocentric or AP Superplanetary, or other specialized eyepiece such as the UO HD orthos. Here we must note that these eyepieces have very short eye relief in comparison to the Radian or Pentax. Your eye is always close to the glass in a planetary eyepiece, and eye moisture will quickly fog the small glass of a planetary ortho or monocentric in humid or very cold conditions. Even if planetary eyepieces can outperform the Radian, they can't do so when they are fogged up, and the Radian is a practical choice unders such conditions. But the Pentax 3.5XW also offers the relief to keep eye moisture away from the eyepiece, and a big wide piece of glass that holds heat and resists fogging. It also offers superior contrast on both planets and dsos. It will return value for the extra outlay.

Pros

High end of the middle end.   Widely accepted by observing community as a "serious" eyepiece.
Definite step up from introductory eyepieces.
Readily available used, easy to re-sell.
Good eye relief and practical in cold and humid conditions.

Cons

Significant image softness and lack of contrast compared to alternatives.
Not that much cheaper, given its performance, than the Pentax 3.5XW.
Consider the XW3.5 as a wide field alternative and outstanding planetary performer with equivalent or superior eye relief and resistance to fogging in adverse conditions.
Consider specialized planetary eyepieces such as UO ortho to save money and exceed performance, with limited fov and eye relief.
Consider higher priced planetary eyepieces (SPL, TMB) for superior performance at similar prices.

Rating

-- End of review