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FIELD TESTING THE TMB-92L SIGNATURE SERIES F/5.5
Image 2 - The Andromeda galaxy imaged with a Hutech-modified Canon 50D and the
TMB-92L f/5.5 equipped with an AstroTech field flattener.
Image 3 - Close-up of the four corners of Image 2
before his untimely death in 2007. His
outstanding TMB-130 f/7 has been available
for several years and is a fine performer
in its own right. For details, see
my review in ATT Volume 2, #5, 2008. I
now use both telescopes mounted piggy
back, alternately for imaging and as guide
scopes (see Image 1).
Visual Performance
Due to its short f-ratio and 506-mm
focal length, the TMB-92L is an ideal
“richest-field” telescope. It provides an
eye-popping 4-degree actual field of view
with a 26-mm Nagler, 3.7 degrees with a
23-mm Axiom and around 3 degrees
with a 22-mm Panoptic. In short, it fully
frames extended objects like the
Andromeda galaxy, the Pleiades, the
Rosette nebula and the North America
complex, to name a few. Moreover,
thanks to this little scope’s incredible
high contrast, under really dark, transparent
skies, many of these objects stand
out in sharp contrast against the
background giving an almost 3-D like
appearance.
While I expected good deep-sky
performance from the TMB-92L, I was
astounded by how well it also performs
on the moon and planets. Even a bright
gibbous moon showed no detectable
chromatic aberration. Under low power,
the moon too provided stunning views of
a world suspended in space. With higher
magnification, numerous craterlets
popped into view inside major basins like
Ptolemaeus and Clavius.
The biggest surprise, though, was the
remarkable high-power views the
TMB-92L provided of Jupiter. Using an
older, 2.5-mm Orion Lanthanum-series
eyepiece at 200x on a night of very good
seeing, Jupiter’s cloud bands and the
currently rather faint Red Spot stood out
in remarkably sharp contrast and subtle
colors. This is truly impressive performance
by a pocket-size telescope of less
than 4 inches in aperture.
Imaging Performance
My main reason for obtaining the
TMB-92L was for imaging with my
Hutech-modified Canon DSLR. I
wanted something in the 500-mm focallength
range, with “fast” optics and widefield
capabilities to supplement my
longer-focal-length telescopes and to
serve as potential guide scope for an
Orion StarShoot auto guider. I have not
been disappointed.
Like all fast apochromatic refractors
42 Astronomy TECHNOLOGY TODAY