televue 5x powermate
this is one of the
best investments i have made when it comes to lens. it far exceeds
my expectations and it has redefined my imaging goals. i was able
to see more with less effort and added a lot of detail to my images even
under poor conditions. i am now putting my loose change in a jar toward
a televue 2x powermate. do not confuse this with a standard barlow
manufacture by = televue
cost = @ $ 190 us
vendor/supplier = www.telescopes.com
( below is the description available at the time of my purchase 2/2005 which
outlines the product)
televue 5x powermate 1-1/4 inch
the 5x powermate™ is ideal for long focus ccd and film photography, magnifying
about 7.7x. it’s an attractive, high performance alternative to eyepiece
projection. the 5x is also perfect for slowing down your scope for hydrogen-alpha
excellent with long eye-relief, 2" eyepieces. using the 4x model, imagine
the quality of a 35mm panoptic at 8.8mm with 25mm of eye relief! the powermate™
is not a barlow so the panoptic-barlow interface is not needed! the 2" powermate™
fits all the way into 2" star diagonals, requiring minimal refocus compared
to a 2" barlow.
to understand the powermate, we must first understand the barlow lens. barlows
amplify the power of a telescope. they can be considered “focal reducers”
for eyepieces, or “focal extenders” for objectives. terence dickenson, in
his barlow test report in sky and telescope, july 1997, says: “technology
has erased the old objections. a modern barlow will not degrade your telescope’s
optics. anyone telling you otherwise is using outdated information. moreover,
the highly regarded nagler eyepieces and their clones have built-in barlows,
ample evidence that the lens is not some detrimental intruder.” thanks,
terence, for laying the myth of the degrading barlow to rest.
a high quality barlow must be properly designed and manufactured in order
to avoid compromising a telescope’s color and spherical aberration corrections.
the “invisibility” of tele vue’s 2-element barlows has been noted in test
reports in the astronomy magazines. however, by the nature of its negative
power lens, a barlow will do more than just increase magnification, regardless
of the number of elements.
the diverging rays leaving the barlow result in moving the exit pupil further
out, thereby extending the eye relief. in short to medium focal length eyepieces
the change is not noticeable. however, in the case of long focal length
eyepieces, the increase can be significant and not without performance consequences.
vignetting can occur due to the altered ray path, when the eyepiece’s lenses
are not large enough in diameter to allow all the rays to make it through.
shorter barlows, or ones with too much magnification, only exacerbate the
problem because the ray path entering the eyepiece is steeper.
we had to go beyond the barlow concept to achieve the goal of a compact,
high power, fully corrected image amplifier. the powermate™ consists of
a negative doublet plus a positive “pupil-correcting” doublet. this 4-element
system provides the magnifying function of a barlow without its limitations
by restoring the field rays back to their original direction, as if the
powermate™ were not there. the result is a pure magnification increase.
unique benefits and applications for powermates
vignetting, edge field aberrations and pupil movement - all introduced when
using barlows with long focal length eyepieces, are minimized. telecentric
operation (field rays leave parallel to optical axis, unlike barlow lenses
which diverge rays). this is ideal for hydrogen-alpha filter use such as
with daystar models. high performance for image amplification, ccd or film
(some of the finest earth-based planetary imaging is being done with powermates
flexible for visual and imaging with all types of telescopes and eyepieces.
essentially parfocal, and with nearly constant magnification regardless
of image distance behind top surface (except 5x model which increases 1x
for every 35mm of image distance increase). higher magnifications are possible
with high optical performance compared to “doubling up” barlows. all will
reach parfocal point in star diagonals, except for the 2”, 2x which comes
in addition to full multi-coating, powermates™ include other tele vue special
touches such as safety undercuts where practical on the chrome barrel and
captive lock screws that can’t fall out, and filter threads. our 100% qc
program at f/4 insures that powermates™ will add dramatic power (and nothing
else) to your favorite eyepieces.
if, for example, you own a 24mm panoptic, you’ll have the power of a 12mm
focal length eyepiece with the 2.0x powermate, 9.6mm with the 2.5x powermate,
a 6mm with the 4x powermate, or a 4.8mm with the 5x powermate!
telescopes.com pages 3-2005