Review of
the DGM Optics™ OA-4.0™ Off-Axis Newtonian Telescope
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The following
specifications are a result from testing the DGM Optics OA-4.0 (98.5mm clear
aperture, F/10) Off-Axis Newtonian telescope as a system:
1] Wave front-
0.125 wave (1/8)
2] Strehl ratio- >99%
3] Resolution- 1.1 arc sec
4] Transmission- >90%
5] Central obstruction- none
No 4" design telescope
that I have ever tested, or been witness to testing, performed as well as the
DGM Optics, OA-4.0, Off-Axis Newtonian reflector.
I have many telescopes
available to me for laboratory and field- testing, and side by side (in the
field) visual comparison tests. Here are the telescopes that were used for such
tests:
1]
4" F/10 Maksutov
2]
4" F/10 Newtonian, 10%
central obstruction
3]
4" F/10 achromatic doublet
4]
4" F/8 Triplet/Fluorite
5]
4" F/15 achromat doublet
6]
4" F/10 Fluorite
7]
4" F/8 ED
The above telescopes have been
previously laboratory tested and optimized for excellent performance. Since
most of the telescopes have the same focal ratio, the use of the same eyepiece
(no diagonal
introduced), and
preservation of the exit pupil are
important factors. The range in magnification used was 32X to 255X on stellar
objects, and up to 408X for lunar features. **See Note below.
At 32X, across a 2 degree
field of view, the OA-4.0 exhibited no image defects across its field. The
images were sharp to the edge of the field, and the contrast was clearly better
than any telescope along side it. The high contrast yielded fainter stars than
any of the 4" comparison
telescopes. Along the earthshine (lunar edge) were several faint stars, 12+
magnitude, that were not seen in any of the other telescopes at 32X to 64X; but
were seen in the OA-4.0. The dark sky views and the limiting magnitude suggest
that this is a 5.5 to 6 inch aperture telescope, not 4"!
Double stars of similar and
different colors were viewed at 85X to 160X. The most striking difference
between the telescopes was that the color provided by the OA-4.0 was more
apparent than in the other telescopes. For other double stars, the absolute
"black" shown by the OA-4.0 allowed close pairs to be discerned; where
the other telescopes favored higher magnification use to accomplish the same
task.
The lunar observations were
perhaps the most revealing of all. Since this object provides an outstanding
source for features of known resolution, and the terrain, shadows and phase
angle provide a variety of
contrast differences. The
OA-4.0 and others showed an enormous wealth of details at low magnification. It
became apparent that the OA-4.0 provided more "dimension" and
"texture" of the lunar surface at 160X upwards. The shadows and
shades of gray were best discerned by the OA-4.0. While the other telescopes
showed fine discrete lunar features at 200X, the OA-4.0 revealed an additional
"texture" to the surrounding areas studied.
While I am a skilled
observer and do not require high magnifications to see all that a telescope can
provide, I did "push" all the telescopes to 100X per inch on the
moon. At these magnifications, all the telescopes (except the DGM Optics,
OA-4.0) showed a softer image, reduced contrast, and more focus travel to see
differences in the sharpness of a crater or peak. Since the OA-4.0 failed to
breakdown, the OA-4.0 was set to the highest magnifications to find its limit.
**NOTE: Using a 2.8X Klee Barlow, a 4mm orthoscopic eyepiece, a magnification
of 714X was obtained. Another combination used a 3mm Clave with the Klee
Barlow, yielding 952X.
This extreme magnification demonstrated several
issues:
1] The
OA-4.0 still showed details that would readily go through focus, even at 714X.
At 952X, the image finally became lost in the very low contrast and brightness
factors at an effective F/238!
2] The
best (premium) refractors did show color, contrast, and image breakdown after 75X per inch.
3] The
obstructed systems showed contrast and brightness loss after 60X per inch.
Conclusion:
This sample 4" F/10
Off-Axis Newtonian, by DGM Optics, stands alone as the finest optical system at
4 inches of aperture, I have seen worldwide.
Mike Palermiti,
Optical Consultant
Jupiter, FL
June 11 2000