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Pentax XF Zoom

Discussion in 'Eyepieces, Barlows, and Filters' started by Mak the Night, Oct 19, 2019.

Pentax XF Zoom

Started by Mak the Night on Oct 19, 2019 at 12:12 PM

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  1. Mak the Night

    Mak the Night Well-Known Member

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    The Pentax XF 6.5mm-19.5mm Zoom eyepiece at 240 grams is about half the size and weight of its larger sibling the XL 8mm-24mm Zoom. The focal lengths range from 19.5mm at the longest to 6.5mm at the shortest, there are no click stops. The apparent field of view ranges from 42° to 60° with a claimed eye relief of 15mm to 11mm. It has multi coated high-refraction, low dispersion lanthanum glass consisting of six lenses in four groups. The XF stands 90mm tall with a diameter at its largest point of 50mm. Like the XL it is also waterproof and has a twist up rubber eyeguard. The 31.7mm barrel features a shallow undercut.

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    The larger Pentax XL zoom had performed superbly with my 102mm Altair Starwave ED doublet. So I was looking forward to testing the smaller Pentax with my Sky-Watcher Evostar ED80. Especially as it has a similar focal ratio to the Starwave but is on a lighter mount/tripod combination. The magnifications with the XF and the ED80 are comparably similar to the XL and the Starwave. Using the ED80 the lowest magnification is 30.7x for 1 arc degree, 22 minutes and 5 arc seconds of true field of view with a decent sized exit pupil of 2.6mm. The highest magnification is 92.3x for 39 arc minutes of TFOV with an exit pupil of 0.86mm. This range of about a two and a half to a one millimetre exit pupil is a nice sweep of what could be described as a medium magnification series.

    z2.jpg

    The XF effectively replaces several eyepieces. Ideally and potentially you could venture out with this zoom, a Barlow and a large rich field widefield eyepiece and have a complete magnification range for an entire night’s viewing. My original plan was to use the 31.7mm XF in combination with a 51mm Baader dielectric diagonal but ended up with placing it in the 31.7mm Baader (Zeiss prism) Amici as I’d been looking at the Moon earlier with another eyepiece and was just too lazy to change diagonals. Predictably the first light target with the XF was the lunar surface. At 30.7x the Moon was clear and bright although I inevitably zoomed up to 92.3x which is the highest I could go without using a Barlow. The detail in the Atlas and Hercules craters was stunningly sharp, as was Posidonius and in the general Lacus Bonitatus area.

    z3.jpg

    When my eyes had adjusted to the dark a bit after the lunar observing I split some doubles, including Gamma 1 and 2 Andromedae (Almach). The bright Moon wasn’t helping much and I was also losing the transparency. As I waited for M42 to rise high enough for a decent view I looked around a Cassiopeia that was close to the zenith. The Double Cluster and the Owl Cluster were pretty impressive considering the overbearing moonlight. I could detect no light scatter. Stars were sharp virtually to the edge and overall colour separation and contrast were superb. Although this is not too surprising with an f/7.5 ED scope. The XF field stop seems slightly smaller and more restrictive than the XL stop, however I got the feeling that this was a bigger eyepiece than it actually is. The 42° of field on the lowest magnification tends to feel a little restricted. The only real remedy for this is increasing the magnification slightly to open the field up more. Although this is more or less de rigueur with most zooms. Eventually the Orion Nebula had ascended just enough for me to get a relatively decent view. The Trapezium stars were very defined and the nebula could be seen faintly. Even with the obscuring and slightly hazy conditions I was impressed with the Pentax XF.
     
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