Image
brightness in a telescope is crucially dependent on
the reflectivity of the telescope's mirrors and on the
transmission of its lenses. Neither of these
processes, mirror-reflectivity or lens-transmission,
is, however, perfect; light loss occurs in each
instance where light is reflected or transmitted.
Uncoated glass, for example, reflects about 4% of the
light impacting it; in the case of an uncoated lens 4%
of the light is lost at entrance to and at exit from
the lens, for a total light loss of about 8%.
Early
reflecting telescopes of the 1700's and 1800's
suffered greatly from mirrors of poor reflectivity?
reflection losses of 50% or more were not uncommon.
Later, silvered mirrors improved reflectivity, but at
high cost and with poor durability. Modern optical
coatings have succeeded in reducing mirror-reflection
and lens-transmission losses to acceptable levels at
reasonable cost.
Each time light encounters a mirror (top
drawing) or lens (bottom drawing) surface, some light
is lost. In the case of a lens, light is lost both at
entrance to and at exit from the lens.
Meade Standard Coatings
The
optical surfaces of all Meade telescopes include
high-grade optical coatings fully consistent in
quality with the precision of the optical surfaces
themselves. These standard-equipment coatings include
mirror surfaces of highly purified aluminum,
vacuum-deposited at high temperature and over coated
with silicon monoxide (SiO), and correcting lenses
coated on both sides for high light transmission with
magnesium fluoride (MgF2). Meade standard mirror and
lens coatings equal or exceed the reflectivity and
transmission, respectively, of virtually any optical
coatings currently offered in the commercial telescope
industry.
The Meade UHTC Group
Technologies recently developed at the Meade Irvine
coatings facility, however, including installation of
some of the largest and most advanced vacuum coating
instrumentation currently available, have permitted
the vacuum-deposition of a series of exotic optical
coatings precisely tuned to optimize the visual,
photographic, and CCD imaging performance of Meade
telescopes. These specialized, and extremely
advantageous, coatings are offered here as the Meade
Ultra-High Transmission Coatings (UHTC) group, a
coatings group available optionally on many Meade
telescope models.
In
Meade catadioptric, or mirror-lens, telescopes
(including the ETX-90EC, ETX-105EC, and ETX-125EC;
LX10, LX90, and LX200GPS Schmidt-Cassegrains; and
LXD55-Series Schmidt-Newtonians) before incoming light
is brought to a focus, it passes through, or is
reflected by, four optical surfaces: the front surface
of the correcting lens, the rear surface of the
correcting lens, the primary mirror, and the secondary
mirror. Each of these four surfaces results in some
loss of light, with the level of loss being dependent
on the chemistry of each surface's optical coatings
and on the wavelength of light. (Standard aluminum
mirror coatings, for example, typically have their
highest reflectivity in the yellow region of the
visual spectrum, at a wavelength of about 580nm.)
Mirror Coatings
Meade ETX, Schmidt-Cassegrain, and
Schmidt-Newtonian telescopes equipped with the
Ultra-High Transmission Coatings group include primary
and secondary mirrors coated with aluminum enhanced
with a complex stack of multi-layer coatings of
titanium dioxide (TiO2) and silicon dioxide (SiO2).
The thickness of each coating layer precisely
controlled to within +/-1% of optimal thickness. The
result is a dramatic increase in mirror reflectivity
across the entire visible spectrum; at the important
hydrogen-alpha wavelength of 656nm. ? the predominant
wavelength of emission nebulae ? reflectivity is
increased from 89% to over 97%.
UHTC Stamped
A special UHTC label is affixed to the optical tube of
the each Meade telescope equipped with these advanced
coatings.
Total Telescope Light Transmission By Wavelength Of
Light
click image for enlarged view.
These graphs show the total amount of light
transmitted to the telescope focus by the four optical
surfaces (primary mirror, secondary mirror, and two
lens surfaces) of Meade ETX, Schmidt-Cassegrain, and
Schmidt-Newtonian telescopes. The wavelengths of the
brightest nebular emission lines are indicated in red.
Meade standard coatings equal or exceed the total
light transmission of virtually all other optical
coatings currently offered in the commercial telescope
industry; notwithstanding this fact, Meade UHTC
coatings enable a dramatic increase in lunar,
planetary, and deep-space image brightness. Note that
the graphs presented here are spectrophotometric
results of actual Meade mirrors and correcting lenses,
not theoretical abstractions.
Correcting Lens Coatings
Meade telescopes ordered with the UHTC group include,
in addition, an exotic and tightly-controlled series
of coatings on both sides of the correcting lens or
correcting plate, coatings which include multiple
layers of aluminum oxide (Al2O3), titanium dioxide
(TiO2), and magnesium fluoride (MgF2). Per-surface
light transmission of the correcting lens is thereby
increased at the yellow wavelength of 580nm., for
example, to 99.8%, versus a per-surface transmission
of 98.7% for the standard coating.
The importance of the UHTC group
becomes apparent when comparing total telescope light
transmission, or throughput, caused by the multiplier,
or compounding, effect of the four optical surfaces.
With each optical surface contributing significantly
to telescope light throughput, the effect of all four
surfaces combined is indeed dramatic, as demonstrated
by the graphs on the facing page, as well as by the
table of the brightest nebular emission lines. At the
H-alpha wavelength of 656nm., total transmission
increases from 77% to 93%, an increase of 93/77 or 21%
at all three nitrogen-III and sulfur-II wavelengths of
655nm. and 673nm.? prominent lines in certain galactic
nuclei and in supernova remnanats such as the Crab
Nebula? transmission increases by 21%; ; at the helium
wavelengths of 588nm. and 469nm. ? strong emission
lines in hot planetary nebulae ? total telescope
transmission increases by 18% and 19%, respectively;
at the two nitrogen II lines of 655nm. and 658nm. and
at the sulfur II line of 673nm., transmission is
increased by 21%. Averaged over the entire
visible spectrum (450nm. to 700nm.), total light
transmission to the telescope focus increases by about
20%.
Observing with the UHTC
Meade ETX, Schmidt-Cassegrain, and Schmidt-Newtonian
telescopes equipped with the UHTC present dramatically
enhanced detail on the full range of celestial objects
? from emission and planetary nebulae such as M8, M20,
and M57 to star clusters and galaxies such as M3, M13,
and M101. Observations of the Moon and planets, since
they are observed in reflected (white) sunlight,
benefit in image brightness from the full spectrum of
increased transmission. The overall effect of the UHTC
is, as it relates to image brightness, to increase the
telescope's effective aperture. Image brightness
(i.e., the ability to see faint detail) of the Meade
10" LX200GPS is, for example, effectively increased by
about one full inch of aperture.
* The % increase is obtained by dividing the UHTC-transmission
(column 4) by the standard coatings transmission
(column 3).
Effects on CCD Imaging
While the human eye loses sensitivity to light beyond
wavelengths of about 700nm., CCD imaging chips remain
sensitive to about 750nm. and longer, wavelengths at
which the reflectivity of an aluminum coating is near
its lowpoint. Importantly, however, the UHTC's total
light transmission at 750nm. is 83%, vs. 72% for
standard coatings, an increase of 83/72, or 15%.
Ordering the UHTC
The Meade Ultra-High Transmission Coatings group, if
desired, must be specified at the time of telescope
purchase; the UHTC can not be retrofitted. Purchase
orders must be paid for in full before order is
processed. NO RETURNS, CANCELLATIONS, EXCHANGES OR
REFUNDS.
The UHTC option is available for any of the following
Meade telescopes:
-
ETX
Telescopes:
-
ETX-90EC
-
ETX-105EC
-
ETX-125EC
Schmidt-Cassegrains
-
8"
LX10
-
8"
LX90
-
7"
LX200GPS
-
8"
LX200GPS
-
10" LX200GPS
-
12" LX200GPS
-
16" LX200GPS
LXD55 Schmidt-Newtonians
-
6"
Model SN-6
-
8"
Model SN-8
-
10" Model SN-10
MEADE
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