An
important optional feature to optimize the performance of your
Meade telescope.
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. |
|
|
|
| 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 from
Meade include mirror surfaces of highly
purified aluminum, vacuum-deposited at high temperature and overcoated 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
Meade ETX-90AT, ETX-105AT, and ETX-125AT; 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 is 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%.
Correcting Lens Coatings:
Meade telescopes ordered
with the Meade 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
Meade 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-a wavelength of 656nm., total
transmission increases from 76.9% to 93.1%, an increase of 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
Meade UHTC present
dramatically brighter images 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. |
|
|
Emission Line |
Wavelength
(nm.) |
Transmission: Standard
Coating (%) |
Transmission: UHTC
Group (%) |
Increase* |
|
Hydrogen-alpha (Ha) |
656 |
76.9 |
93.1 |
21% |
|
Hydrogen-beta (Hb) |
486 |
75.3 |
85.8http://www.opticsplanet.net/meade-brand.html |
14% |
|
Oxygen
III |
496 |
76.5 |
85.4 |
12% |
|
Oxygen
III |
501 |
77.0 |
85.4 |
11% |
|
Helium II |
469 |
72.5 |
86.1 |
19% |
|
Helium I |
588 |
79.5 |
93.5 |
18% |
|
Nitrogen
II |
655 |
77.0 |
93.2 |
21% |
|
Nitrogen
II |
658 |
76.7 |
92.8 |
21% |
|
Sulfur II |
673 |
75.7 |
91.8 |
21% |
|
| * The % increase is
obtained by dividing the UHTC-transmission (column 4) by the
standard coatings transmission (column 3). |
|
| The
overall effect of the UHTC is, as it relates to image
brightness, to increase the telescope's effective aperture.
Image brightness of the Meade 10" LX200GPS is, for example,
effectively increased by about one full inch of aperture.
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 about 83%, vs. about 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.
The UHTC is available for any of these Meade telescopes:
ETX telescopes: ETX-90AT, ETX-105AT, ETX-125AT
Meade Spotting scopes |
|
|
|
Schmidt-Cassegrains (pp. 28 - 57): 8" LX10, 8" LX90, 7"
LX200GPS, 8" LX200GPS, 10" LX200GPS, 12" LX200GPS, 14" LX200GPS,
16" LX200GPS, LXD55 Model SC-8 |
|
|
|
LXD55
Schmidt-Newtonians: 6" Models SN-6 and SN-6EC, 8" Models SN-8
and SN-8-EC, 10" Model SN-10 |
|
|
