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Finally! A barlow that (sort of) works with PF65ED

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Old Sunday 22nd January 2006, 19:05   #1
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Join Date: Feb 2005
Location: New York
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Finally! A barlow that (sort of) works with PF65ED

A number of people, including myself, have tried in the past to find a barlow that would work with the Pentax spotting scopes. The problem has always been that even shorty barlows (such as the Celstron Ultima Barlow and (identical) Orion Shorty Plus) were too long to seat fully into the scope, and it was not possible to get focus at infinity. Since the Ultima Barlow has a lens element right at the very bottom of its barrel, cutting down the barlow (or cutting a notch to clear the Pentax locating pin) was not an option.

Celestron has recently come out with a new barlow, the X-Cel ED (available from B&H and Adorama for about $65-70). The barrel of this barlow is significantly shorter than that of the Ultima, and a week ago I obtained one to see if it would work on my Pentax 65ED scope with Vixen 8-24 zoom eyepiece. The good news is that the barlow seated fully into the scope. The bad news is that, even properly seated, the barlow only allowed focus out to a maximum of about 40 feet! (By comparison, the Ultima barlow -- even though not fully seated -- allowed focus out to a maximum of about 300 feet).

The construction of the barlow is a bit unusual -- all of the optical elements are contained in a very compact capsule which is mounted in the middle of the barlow, at the top of the barrel. I wondered what would happen if I could place that optical capsule closer into the scope. The barrel (with the optical capsule) unscrews from the body of the barlow, and the optical capsule then unscrews from the barrel. Since the barrel is threaded for its entire length, it's then possible to screw the optical capsule into the bottom of the barrel, and the barrel is so short that it's still possible to seat the resulting assembly all the way into the scope. The only problem is that the optical capsule is only threaded on one side, so in order to mount it on the bottom of the barrel it's necessary to reverse the capsule (ie, what had been the bottom surface is now facing up).

To my considerable surprise, this hack seems to work! In this configuration, the scope is just able to reach focus at infinity. The magnification of the barlow is a bit more than 2x (say, 2.3x), and optical quality seems to be fine.

Comparing the Vixen zoom against the Vixen zoom/hacked barlow (and adjusting the length of the zoom to roughly equalize image size), the images look damn close. So far as I can tell sharpness (both center and edge to edge) and contrast are about the same.

However, with the zoom/barlow combination once you move beyond the marked 12mm position on the zoom the image does start to fall apart. Contrast is reduced, and some significant chromatic aberration develops. I don't know whether this is due to ill effects of the hacked barlow, or if it simply means that the PF65ED/Vixen zoom combination is meeting its limits at about 65x.

FWIW, even without the barlow, my PF65ED seems to show a slight bit of CA, and I'm planning to send it back to Pentax (while it's still under warranty) to make sure that it's functioning within spec.

Hope this is helpful.

zack2 is offline  
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