Knowledge Base

Most SBIG CCD cameras have a remote guide head port built into the camera's body,
even those cameras that already contain a separate self-guiding CCD chip. This remote
guide head port lets you use an optional external guider, such as the SBIG
#RGH remote guide head, instead of the camera's built-in guiding chip.

Internal guiding using the built-in guiding detector eliminates the need for a guide
scope and separate autoguider. It also eliminates the problem of differential flexure
of the guide scope or differential motion of the primary mirror relative to the
guide scope. However, some users prefer to use an external guider for imaging in
regions of the sky with sparse guide stars, as the remote guide head can be positioned
to find a guide star far off the camera's imaging axis. The optional remote guiding
head can be used in an off-axis guider body or attached to a separate guide scope
in such a case and plugged into the main camera body to make all the guiding corrections.

Also, using an optional external guider instead of the built-in guiding detector
is sometimes helpful during color imaging through a color filter wheel. In some
areas of the sky that have few reasonably bright stars to use for guiding, the color
filters will dim the only available guide stars to the point where guiding with
the camera's internal guiding chip may become erratic as seeing conditions fluctuate.
The use of the optional separate external guiding head will eliminate any such issues,
as it will be guiding using a star undimmed by any filtration.

The remote guide head port also lets you add an #RGH guider to an SBIG "industrial"
or "I" version of an SBIG camera (those imaging-only cameras that are supplied without
a built-in guiding chip).

Some events on the Sun - notably active flares and coronal mass ejections (CMEs) - are extremely fast moving. If such features occur on the disc of the Sun, rather than on the limb, their high velocity towards the observer results in their wavelength being Doppler-shifted to a shorter wavelength than the 6562.8 Ångstrom H-alpha line to which the filter is tuned. If the velocity of the event is high enough, its Doppler-shifted wavelength could be outside the normal passband of the filter and the feature would not be visible. The T-Max bandpass tuner allows the observer to de-tune the solar filter's passband slightly towards the blue wing of the H-alpha line to make such Doppler-shifted phenomena more visible.

All optical interference filters shift their passband towards shorter wavelengths when tilted. The T-Max bandpass tuner fits between the SolarMax etalon and objective lens at the front of the scope. Turning a rotary wheel on the T-Max body mechanically tilts the SolarMax etalon filter element by up to 0.7°, shifting the passband toward the shorter-wavelength blue wing of the H-alpha line. Any more of a tilt will shift the filter completely off the H-alpha line. As the filter tilts, Doppler-shifter filaments that were formerly invisible or barely visible will often start to appear or darken. Filaments that appear darker and prominences that appear brighter are moving towards your line of sight.

With a passive filter such as the SolarMax, its passband will broaden with increasing amounts of tilt, resulting in somewhat lower resolution and contrast on non-Doppler-shifted features when compared to their on-band images. There is also an increasing loss of transmission efficiency as the passband shifts away from its nominal center frequency on the H-alpha line. However, these minor shortcomings are more than offset by the increased number of features that become visible as you tilt the T-Max to scan back and forth across Doppler-shifted wavelengths.

Features of this Telescope's Optical System . . .

• Advanced Coma-Free catadioptric designed to emulate the optical performance of a Ritchey-Chrétien telescope: The traditional two-mirror Ritchey-Chrétien (RC) design uses approximately hyperbolic primary and secondary mirrors to produce images that are free from coma over a wide field. Because of this wide coma-free field and a relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography. The RC is the design of choice for most of the major professional observatory telescopes built in the last half-century. For example, the Hubble Space Telescope and the twin 10-meter Keck telescopes in Hawaii are Ritchey-Chrétiens.
      However, because of the complexity of fabricating and testing a large aperture hyperbolic mirror (just ask the people who built the initially-flawed, but not discovered until it was in space, Hubble Space Telescope), traditional two-mirror Ritchey-Chrétiens are very expensive to manufacture and purchase, too expensive for many amateur astronomers.
      To emulate the coma-free performance of a true RC telescope, while keeping the cost within reason, the LX200-ACF Advanced Coma-Free (ACF) catadioptric optical system uses a full aperture aspheric corrector lens in conjunction with a simple spherical primary mirror. This creates a two-element primary mirror system that performs like an RC's single hyperbolic primary mirror from the optical point of view of the LX200-ACF secondary mirror. The hyperbolic secondary mirror itself is mounted directly on the rear of the corrector lens, rather than in the traditional RC's conventional spider vane assembly. This eliminates the image-degrading diffraction spikes of the secondary mirror support structure visible in commercial RC scope images. The result is RC-class coma-free wide-field performance in the LX200-ACF, at about a fourth the cost of most true RC systems.
      The corrector-modified design would itself be expensive to fabricate were it not for Meade's more than a quarter-century of experience making Schmidt-Cassegrain correctors, which are in the same optical family as the corrector needed for the coma-free design of the LX200-ACF. An additional benefit of the full aperture corrector in the ACF design is slightly better correction for astigmatism than the traditional RC design.
      In addition, the LX200-ACF, due to its front corrector plate, is a closed tube design. This keeps the primary optical components protected from dust, moisture and other contaminants that might fall on the optical surfaces of the primary and secondary mirrors as can happen with the traditional open-tube RC design.
      While the LX200-ACF may not be a traditional RC design, its performance is RC-like in all important characteristics. A review in Sky & Telescope magazine of the predecessor of the Meade ACF optics said the bottom line is that the optics do "indeed perform like a Ritchey-Chrétien." Another such review, in Astronomy magazine said, "This scope delivers Ritchey-Chrétien-like performance at a fraction of the cost."

• Oversized primary mirror: The diameter of the primary mirror of each LX200-ACF is larger than the diameter of the corrector lens at the front of its optical tube that admits the light. The primary mirror of the 8" scope is actually 8.25" in diameter, compared to the 8" diameter of the corrector lens. The 10" primary is 10.375" in diameter; the 12" is 12.375"; the 14" is 14.57"; and the 16" primary is 16.375" in diameter. Oversizing the primary mirror in this way gives you a wider fully-illuminated field than a conventional catadioptric scope whose corrector and primary mirror are the same size. The result is a gain of 5% to 8% more off-axis light available to your eye or camera, depending on the telescope model.

• Fully multicoated UHTC (Ultra High Transmission Coatings) optics: The primary and secondary mirrors are vacuum-coated with aluminum, enhanced with multiple layers of titanium dioxide and silicon dioxide for increased reflectivity. A overcoating layer of durable silicon monoxide (quartz) assures long life.
      A series of anti-reflective coatings of aluminum oxide, titanium dioxide, and magnesium fluoride are vacuum-deposited on both sides of the full aperture corrector plate. These antireflection multicoatings provide a high 99.8% light transmission per surface, versus a per-surface transmission of 98.7% for standard single-layer coatings. Overall light throughput (the amount of light collected by the objective lens that actually reaches your eye or camera) is approximately 89% at the focal plane.
      UHTC multicoatings provide a 15% increase in light throughput compared with standard single-layer coatings. They effectively add the equivalent of 15% extra light-gathering area to the performance of a scope with standard coatings. It's the equivalent of three-quarters of an inch of extra aperture in the case of a 10" scope, for example, but with no increase in actual size or weight. UHTC coatings also improve contrast, for lunar and planetary images that appear sharper and more crisply defined.

• Fully baffled optics: A cylindrical baffle around the secondary mirror, in combination with the cylindrical baffle tube projecting from the center of the primary mirror, prevents stray off-axis light from reaching the image plane. In addition, a series of field stops machined into the inner surface of the central baffle tube effectively eliminates undesirable light which might reflect from the inside surface of the tube. The result of these baffle systems is improved contrast in lunar, planetary, and deep space observing alike.

• Mirror lock: A progressive tension lock knob on the rear cell locks the telescope's primary mirror rigidly in place once an approximate manual focus has been achieved. The standard equipment electric focuser, described below, is then used for fine focusing. Locking the mirror eliminates the possibility of mirror shift (the image moving from side to side while focusing, caused by the primary mirror tilting on the central baffle tube as the mirror moves fore and aft along the tube). Mirror shift, once the bane of CCD astrophotographers because it could easily move the image off a small CCD chip, is non-existent with the Meade system.

• Electric focuser: The supplied zero image-shift electric microfocuser is controlled by the Autostar II computer hand control. It moves an externally-mounted eyepiece or camera to focus, rather than moving the primary mirror. This eliminates mirror shift during precise image centering and focusing for CCD applications. The microfocuser has four different operating speeds, from very fast down to an extremely slow creep, giving you focusing accuracy to a truly microscopic level during critical visual and astrophotographic applications.
      The focuser is designed to hold 2" star diagonals and eyepieces. A supplied 1.25" adapter allows the use of 1.25" diagonals and eyepieces in the 2" focuser. Another supplied adapter duplicates the 2" rear cell thread used on Schmidt-Cassegrain telescopes to allow the use of off-axis guiders, T-adapters, etc. A 1.25" visual back is not supplied with the scope. If you want to do high magnification eyepiece projection photography of the Moon and planets, you will have to add an optional 1.25" visual back #9135 and a tele-extender to the focuser's supplied 2" rear cell thread adapter.

Features of this Telescope's Optical System . . .

• Advanced Coma-Free catadioptric designed to emulate the optical performance of a Ritchey-Chrétien telescope: The traditional two-mirror Ritchey-Chrétien (RC) design uses approximately hyperbolic primary and secondary mirrors to produce images that are free from coma over a wide field. Because of this wide coma-free field and a relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography. The RC is the design of choice for most of the major professional observatory telescopes built in the last half-century. For example, the Hubble Space Telescope and the twin 10-meter Keck telescopes in Hawaii are Ritchey-Chrétiens.
      However, because of the complexity of fabricating and testing a large aperture hyperbolic mirror (just ask the people who built the initially-flawed, but not discovered until it was in space, Hubble Space Telescope), traditional two-mirror Ritchey-Chrétiens are very expensive to manufacture and purchase, too expensive for many amateur astronomers.
      To emulate the coma-free performance of a true RC telescope, while keeping the cost very much within reason, the LX90-ACF Advanced Coma-Free (ACF) catadioptric optical system uses a full aperture aspheric corrector lens in conjunction with a simple spherical primary mirror. This creates a two-element primary mirror system that performs like an RC's single hyperbolic primary mirror from the optical point of view of the LX90-ACF secondary mirror. The hyperbolic secondary mirror itself is mounted directly on the rear of the corrector lens, rather than in the traditional RC's conventional spider vane assembly. This eliminates the image-degrading diffraction spikes of the secondary mirror support structure visible in commercial RC scope images. The result is RC-class coma-free wide-field performance in the LX90-ACF, at about a fifth the cost of most true RC systems.
      The corrector-modified design would itself be expensive to fabricate were it not for Meade's more than a quarter-century of experience making Schmidt-Cassegrain correctors, which are in the same optical family as the corrector needed for the coma-free design of the LX90-ACF. An additional benefit of the full aperture corrector in the ACF design is slightly better correction for astigmatism than the traditional RC design.
      In addition, the LX90-ACF, due to its front corrector plate, is a closed tube design. This keeps the primary optical components protected from dust, moisture and other contaminants that might fall on the optical surfaces of the primary and secondary mirrors as can happen with the traditional open-tube RC design.
      While the LX90-ACF may not be a traditional RC design, its performance is RC-like in all important characteristics. A review in Sky & Telescope magazine of the ground-breaking predecessor of the Meade ACF optics said the bottom line is that the optics do "indeed perform like a Ritchey-Chrétien." Another such review, in Astronomy magazine said, "This scope delivers Ritchey-Chrétien-like performance at a fraction of the cost."

• Oversized primary mirror: The diameter of the primary mirror of each LX90-ACF is larger than the diameter of the corrector lens at the front of its optical tube that admits the light. The primary mirror of the 8" scope is actually 8.25" in diameter, compared to the 8" diameter of the corrector lens. The 10" primary is 10.375" in diameter, and the 12" is 12.375" in diameter. Oversizing the primary mirror in this way gives you a wider fully-illuminated field than a conventional catadioptric scope whose corrector and primary mirror are the same size. The result is a gain of 5% to 8% more off-axis light available to your eye or camera, depending on the telescope model.

• Fully multicoated UHTC (Ultra High Transmission Coatings) optics: The primary and secondary mirrors are vacuum-coated with aluminum, enhanced with multiple layers of titanium dioxide and silicon dioxide for increased reflectivity. A overcoating layer of durable silicon monoxide (quartz) assures long life.
      A series of anti-reflective coatings of aluminum oxide, titanium dioxide, and magnesium fluoride are vacuum-deposited on both sides of the full aperture corrector plate. These antireflection multicoatings provide a high 99.8% light transmission per surface, versus a per-surface transmission of 98.7% for standard single-layer coatings. Overall light throughput (the amount of light collected by the objective lens that actually reaches your eye or camera) is approximately 89% at the focal plane.
      UHTC multicoatings provide a 15% increase in light throughput compared with standard single-layer coatings. They effectively add the equivalent of 15% extra light-gathering area to the performance of a scope with standard coatings. It's the equivalent of three-quarters of an inch of extra aperture in the case of a 10" scope, for example, but with no increase in actual size or weight. UHTC coatings also improve contrast, for lunar and planetary images that appear sharper and more crisply defined.

• Fully baffled optics: A cylindrical baffle around the secondary mirror, in combination with the cylindrical baffle tube projecting from the center of the primary mirror, prevents stray off-axis light from reaching the image plane. In addition, a series of field stops machined into the inner surface of the central baffle tube effectively eliminates undesirable light which might reflect from the inside surface of the tube. The result of these baffle systems is improved contrast in lunar, planetary, and deep space observing alike.

Optical features of this Telescope's Optical System . . .

• Advanced Coma-Free catadioptric designed to emulate the optical performance of a Ritchey-Chrétien telescope: The traditional two-mirror Ritchey-Chrétien (RC) design uses approximately hyperbolic primary and secondary mirrors to produce images that are free from coma over a wide field. Because of this wide field and a relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography. The RC is the design of choice for most of the major professional observatory telescopes built in the last half-century. For example, the Hubble Space Telescope and the twin 10-meter Keck telescopes in Hawaii are Ritchey-Chrétiens. However, because of the complexity of fabricating and testing a large aperture hyperbolic mirror (just ask the people who built the Hubble Space Telescope), traditional two-mirror Ritchey-Chrétiens are very expensive to manufacture and purchase.
  
      To emulate the coma-free performance of a true RC telescope, while keeping the cost within reason, the LX400-ACF Advanced Coma-Free (ACF) catadioptric optical system uses a full aperture aspheric corrector lens in conjunction with a simple spherical primary mirror. This creates a two-element primary mirror system that performs like an RC's single hyperbolic primary mirror from the optical point of view of the LX400-ACF secondary mirror. The hyperbolic secondary mirror itself is mounted directly on the rear of the corrector lens, rather than in the traditional RC's conventional spider vane assembly. This eliminates the image-degrading diffraction spikes of the secondary mirror support structure visible in commercial RC scope images. The result is RC-class coma-free wide-field performance in the LX400-ACF, at about a fourth the cost of most true RC systems.
  
      The corrector-modified design would itself be expensive to fabricate were it not for Meade's quarter-century of experience making Schmidt-Cassegrain correctors, which are in the same optical family as the corrector needed for the modified RC design. An additional benefit of the full aperture corrector in the modified RC design is slightly better correction for astigmatism than a traditional RC scope. In addition, the RCX, due to the front corrector plate, is essentially a closed tube design (however, due to the front cell focusing method discussed below, there is an small opening between the tube wall and the periphery of the corrector, which prevents the tube from being totally sealed). This essentially closed tube design reduces the amount of image-degrading dust, moisture and other contaminates that would otherwise fall on the optical surfaces of the primary and secondary mirrors as is the case with traditional open-tube RC designs.
  
      While the LX400-ACF may not be a traditional RC design, its performance is RC-like in all important characteristics. A review in Sky & Telescope magazine of the predecessor of the Meade ACF optics said the bottom line is that the optics do "indeed perform like a Ritchey-Chrétien . . . Some hobbyists mistakenly believe that a product review without equal doses of praise and criticism is biased or unbalanced. Truth is, I can't find many negative things to say." Another review, in Astronomy magazine said, "All things considered, this scope more than lived up to my hopes. It is well-made, easy to use, and has high-quality optics . . . This scope delivers Ritchey-Chrétien-like performance at a fraction of the cost."
  

• Fully multicoated UHTC (Ultra High Transmission Coatings) optics: The primary and secondary mirrors are vacuum-coated with aluminum, enhanced with multiple layers of titanium dioxide and silicon dioxide for increased reflectivity. A overcoating layer of silicon monoxide (quartz) assures long life.
      A series of anti-reflective coatings of aluminum oxide, titanium dioxide, and magnesium fluoride are vacuum-deposited on both sides of the full aperture corrector plate. These antireflection multicoatings provide a high 99.8% light transmission per surface, versus a per-surface transmission of 98.7% for standard single-layer coatings. Overall light throughput (the amount of light collected by the objective lens that actually reaches your eye or camera) is approximately 89% at the Cassegrain focus.
      UHTC multicoatings provide a 15% increase in light throughput compared with standard coatings. They effectively add the equivalent of 15% extra light-gathering area to the performance of a scope with standard coatings. It's the equivalent of three-quarters of an inch of extra aperture in the case of a 10" scope, for example, but with no increase in actual size or weight. UHTC coatings also improve contrast, for lunar and planetary images that appear sharper and more crisply defined.

Mechanical features of this Telescope's Optical System . . .

• Fixed primary mirror with computer optimized primary and secondary baffling: Unlike traditional catadioptric designs that move the primary mirror fore and aft along the central baffle tube in order to achieve focus, the LX400-ACF primary mirror is fixed and independent of the baffle tube. The primary mirror is laser aligned to the true optical path, then float-bonded in place on the rear cell. Although fixed in position in the optical tube, it literally floats on a layer of adhesive instead of resting on the baffle tube. This results in zero stress to the glass and no distortion in the optics (unlike conventional mirror cells, which can cause pinched optics if not properly assembled). This fixed and independent mirror allows a no-compromise baffle design with full stray light cut-off, producing the maximum possible contrast. A series of field stops machined into the inner surface of the baffle tube effectively eliminates undesirable light which might reflect from the inside surface of the tube.
      The secondary mirror baffle, which is attached directly to the rear of the full aperture corrector plate, is machined of aluminum with a series of distinctive knife-edges around its outside. These prevent stray off-axis light from reaching the image plane. The result of these primary and secondary baffle systems is improved contrast in lunar, planetary, and deep space observing alike.

• Encoder-measured digital front cell focusing: The patented LX400-ACF electric front focusing system produces a razor sharp focus, with no image shift. With the laser-aligned primary mirror fixed in position, focusing is done by using three encoder-controlled motors to precisely move the entire corrector lens, with its attached secondary mirror, fore and aft in the front cell. It moves in increments as fine as 1/100 of a millimeter, at any of four different focusing speeds. There's a digital readout of the focus position on the telescope's AutoStar II hand control. Since focusing is accomplished without sliding the primary mirror along the baffle tube, image shift (once the bane of CCD astrophotographers because it could easily move the image off a small CCD chip) is virtually non-existent.

• Electronic focus presets: The LX400-ACF allows the observer to preset up to nine individual focus positions to customize the focus for observers with differing eyesight characteristics (similar to the custom settings in luxury cars that change the mirror, seat, and steering wheel settings from one driver to another). The feature is also very useful when switching between various eyepiece and Barlow combinations, or when switching from a visual setup to a camera setup.

• Electronic collimation: Precision collimation adjustments to the secondary mirror are made electronically by using the arrow keys of AutoStar II hand control while observing, rather than by using a screwdriver or hex-head wrench to adjust small collimating screws in the dark on a trial and error basis. The LX400-ACF observer sees the results of a collimating adjustment instantly as it is made, shortening the time needed to collimate the scope by as much as a factor of ten. Collimating a Cassegrain telescope has never been easier.
      In addition, Meade precisely collimates the optics at the factory and sets that position as the default setting in the AutoStar computer. So, if a newcomer to astronomy succeeds in accidentally decollimating the scope, rather than collimating it, he or she can always return to the correct factory default setting by simply pushing a button.

• Built-in dew heater: Aftermarket dew heaters wrap a heating element around the telescope's optical tube to send heat through the scope's metal front cell to warm the corrector. This prevents dew from forming on the corrector plate. Having to heat the entire metal cell in order to slightly warm the lens can drain a battery quickly. The LX400-ACF incorporates a standard equipment nickel-chromium wire heating element that is in direct contact with the glass of the corrector plate. This quickly, efficiently, and safely warms the lens using the lowest power drain possible.
      There are two onboard temperature sensors, one inside the fork arm to measure the ambient temperature, and one to measure the temperature of the corrector plate itself. By using the information from these two sensors, the built-in dew heater can be set to keep the corrector plate warmed to a user-defined setting just above ambient temperature. By automatically using the dew heater only when needed, battery drain is kept to a minimum. All functions to operate the dew heater are controlled by the AutoStar II hand control.

• Advanced front and rear cell architecture: The front and rear cells of the LX400-ACF are designed to allow the maximum amount of air-flow around the optics to achieve the quickest "cool down" times. To accelerate the cool-down, a built-in fan on the rear cell can be turned on and off through the AutoStar II hand control.
      The rear cell incorporates a panel with eight electronic ports - three USB 2.0 ports to connect auxiliary equipment, an autoguider input, the input for the AutoStar II hand control, an output to power an illuminated reticle eyepiece, an RS232 communications port, and a port for future "smart" accessories. Some equipment moves with the scope, such as the AutoStar II hand control and Deep Sky Imager. Attaching the cords and cables of this equipment directly to the tube, rather than stretching them to attach to the drive base, virtually eliminates cord wrap. This protects CCD cameras and other accessories from accidentally pulling out of the telescope as the scope slews. The rear cell is flatter than conventional scopes to maximize the clearance between the rear cell and the fork mount. Two tube positioning handles are built into the rear cell.

• Carbon fiber and Kevlar optical tube: The optical tube is fabricated from a woven carbon fiber and Kevlar composite that forms a unique, light-weight, and high strength material. It has ultra-low thermal expansion characteristics. This maintains the critical spacing between the optics so that the focus does not change due to tube expansion and contraction as the ambient temperature changes, a critical feature to astrophotographers.
      Instead of forcing the mechanics of the scope to fit into a perfectly round optical tube assembly, the tube is shaped to conform to the internal mechanisms and drive system of the front focus and collimation assembly. This "form follows function" philosophy gives the LX400-ACF optical tube its unique look and style.

Optical features of this Optical Tube . . .

• Ritchey-Chrétien optical design: This Astro-Tech optical tube is a true Ritchey-Chrétien (R-C) reflector optical system. Unlike a Maksutov-Cassegrain or Schmidt-Cassegrain catadioptric scope (that uses simple spherical mirrors and corrector lenses), or Newtonian reflectors (that use a coma-producing parabolic primary mirror), this Astro-Tech R-C is a Cassegrain-type two-mirror optical system that uses a concave hyperbolic primary and a convex hyperbolic secondary mirror to form its images. These sophisticated and difficult-to-make mirrors combine to produce images at the Cassegrain focus at the rear of this Astro-Tech scope that are free from coma and spherical aberration, with a smaller spot size, over a much wider field than conventional Newtonians or catadioptrics. The images are likewise free from the chromatic aberration found in refractors and some catadioptrics.
  
      Because of this wide coma-free field, small spot size, and relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography, rather than visual observing. For imaging, the R-C is the optical system of choice for most of the major professional observatory imaging telescopes built in the last half-century. For example, the Hubble Space Telescope, the twin 10-meter Keck telescopes in Hawaii, and the four 8.2 meter telescopes of the Very Large Telescope array in Chile are all Ritchey-Chrétiens. For serious amateur astronomers and astrophotographers without NASA's optical budget, an Astro-Tech R-C is likewise the imaging system of choice.
  

• Fully multicoated quartz and BK7 mirrors: The primary mirror of the 6" Astro-Tech is first-quality BK7 optical glass, while the 8" and larger Astro-Tech R-Cs use primary mirrors of low thermal expansion quartz for maximum focus stability during long exposure imaging sessions. Both 6" R-C mirrors are vacuum-coated with enhanced aluminum for high reflectivity and overcoated with a durable layer of silicon monoxide (quartz) for long life. The 8" and larger mirrors are dielectric multi-coated for long life and reflectivity approaching 99%+.
  

• Computer designed and fabricated optics: To keep the cost of each Astro-Tech R-C so reasonable when compared to competitive R-C scopes, the computer-optimized Astro-Tech hyperboloid mirrors are automatically ground and finished to very high tolerances using custom-made computerized mirror grinding machines. This precision computer control guarantees an exact repeatability of figure from mirror to mirror that is difficult to achieve using more costly conventional hand figuring. After grinding and polishing, each mirror is individually tested multiple times during fabrication using Zygo interferometers to assure that it meets or exceeds its designed performance standards.
  

• Frill-free design: To further keep its cost reasonable, an Astro-Tech R-C does away with most of the bells and whistles found on competitive scopes that add little to their performance (but much to their cost). For example, Astro-Tech front and rear cells are first die-cast, then CNC machine-finished, rather than completely CNC machined from raw stock at considerably greater expense but no significant improvement in performance as is the case with other R-Cs. Glare stops in many of the optical tubes are a molded insert, rather than machined aluminum, resulting in a significant savings in cost at no appreciable difference in performance. The Astro-Tech scopes use an external manual dual-speed Crayford focuser, rather than the considerably more complicated and much more costly motorized movable secondary mirror system that other manufacturers use for focusing. The result of the Astro-Tech no-frills approach is genuine Ritchey-Chrétien wide-field performance at a fraction the cost of other commercial R-C systems. While the mechanical bells and whistles may be limited in an Astro-Tech R-C, an Astro-Tech scope still has the high precision flat field/coma-free true Ritchey-Chrétien optics that are the most important reason for buying an R-C scope.

Mechanical features of this Telescope's Optical System . . .

• Fixed primary mirror with computer optimized primary and secondary baffling: Unlike traditional Cassegrain designs that move the primary mirror fore and aft along the central baffle tube in order to achieve focus (which can lead to image shift and focal length changes as the mirror position is adjusted) each Astro-Tech R-C primary mirror is fixed at the precise focal length required for optimum sharpness. The Astro-Tech is focused externally by means of a dual-speed 2" Crayford-style focuser on the rear cell, thereby eliminating a Cassegrain's moving mirror image shift and focal length change during focusing. Molded field stops are installed along the interior of the optical tube to effectively prevent stray off-axis light from reaching the image plane, resulting in improved contrast. In addition multiple glare-stop microbaffles on the inner surfaces of the primary mirror baffle tube and the secondary mirror light shield further prevent off-axis light from reaching the image plane, resulting in still further improved contrast.
  

• Collimatable secondary mirror: Since the primary mirror of an Astro-Tech R-C is fixed in position, only the secondary mirror can (or needs to) be collimated. This makes it easy to keep the Astro-Tech RC optics aligned for peak performance. Collimation adjustments to the secondary mirror are made by adjusting the three collimating screws in the back of the secondary mirror holder.
  

• Cooling fan: The open tube R-C design allows for fast cool-down of the primary and secondary mirrors. Built-in fans on the rear cell of the 10" and larger scopes increases the air-flow around the optics to achieve still quicker "cool down" times of the larger primary mirrors. The 6" and 8" scopes do not have primary mirror cooling fans, as their mirrors are small enough to cool down quickly without any external aid.

Achieving the correct color balance when doing tri-color CCD imaging has long been more of an art than a science. Due to the differing transmission characteristics of normal red, green, and blue CCD filters, exposures times vary for each of the colors. To complicate matters still further, different types of CCD detectors (full frame and back-thinned detectors versus interline detectors) have differing response characteristics to the incoming flux of light.

To get the right color balance with your particular camera, in the past you have had to take a separate G2V star measurement through your red, green, and blue filters individually (sometimes more than once) to arrive at the approximate individual filter exposure times needed to reach a white-point balance with your particular CCD/filter combination. The eventual resulting weighting of the exposure times (for example, a 1.2 to 1.0 to 1.7 ratio) produced reasonable star colors for galaxies and globular clusters.

However, these exposure weightings were not likely to produce the correct "teal" color of OIII emissions within planetary nebula. We have all seen images of the same planetary nebula (the Dumbbell, for example) where the central OIII color ranges from very green to very blue, depending on the astrophotographer. These colors are unrealistic, although often pretty. However, if you maintain the different exposure times required to achieve the correct G2V white-point balance, there is little you can do to achieve a true "teal" nebula color. Wouldn't it be simpler if you could achieve correct star and nebula colors without having to worry about weighting the exposure times? Tru-Balance filters do that for you.

Astrodon Generation 2 Tru-Balance filters simplify tri-color CCD imaging by equalizing the photon flux through each filter so exposure times are identical for each individual red, blue, green, and dark frame exposure. Rather than a perhaps a 1.2 to 1.0 to 1.7 exposure time ratio found by experimentation or trial and error, your RGB color combine weights will be approximately 1:1:1, within perhaps 10%. This can never be perfect, but it does allow you to take equal time exposures through each filter, simplifying your imaging in the cold and the dark. You can then combine the 1:1:1 color data to achieve virtually automatically a G2V white point balance for correct star colors and balanced OIII teal color in emission objects, with the least amount of color balancing needed in the computer later during image processing. Trial and error exposures, guesswork, and extensive color-balancing work at the computer become things of the past.

There are specific Astrodon Generation 2 Tru-Balance filter sets with passbands designed for either full frame and back-thinned detectors (E-Series) or interline (I-Series) detectors, depending on your camera type. Each set provides ease-of-use, high optical throughput, and great resulting color for galaxies, star clusters and nebulae.

Compared to previous filter designs, the Astrodon Generation 2 filters provide better color separation (spectra) and better color rendition for galaxies based upon color theory. They yield significant reflection and star halo reduction and enhanced contrast for HII regions in galaxies. A spectral "gap" between the green and red passbands minimizes the effect of mercury and high and low pressure sodium city light pollution, as well as airglow emission lines due to ionized nitrogen in our atmosphere. The width of the gap depends on the type of camera for which the filter set has been optimized (whether it is an E-Series or I-Series filter set).

The Astrodon Generation 2 filters are made with striae-free 1/4-wave fused silica substrates, polished to 30 arc second parallelism. They use ultra-hard and durable sputtered coatings for long life. Each is parfocal with Astrodon's high-performance narrowband (H-alpha, Oxygen III, etc.) and near-IR filters. They have the highest efficiency blue filter available, with less UV transmittance.

Previous Astrodon filters have been known for being parfocal on most telescope systems. With a thickness tolerance of 3mm +/-0.025mm (25 microns), the new Generation 2 filters are a factor of 2 better than the original Astrodon filters, and should be parfocal for optical systems as fast as f/3.5, if your refractor or camera lens optical system is well color-corrected.

Astrodon has reduced the UV contribution in the luminance and blue filters to reduce star bloat in systems that are not particularly well corrected. The clear (no near-IR blocking) filter may produce bloated stars if your optical system has poor near-IR focus. You will need to use the near-IR blocked Astrodon luminance filter in this case. The clear (no near-IR blocking) filter is likely a better choice to use with reflectors, such as Ritchey-Chrétiens, rather than for use with refractors or camera lenses.

Features of this Telescope's Optical System . . .

• Advanced Coma-Free catadioptric designed to emulate the optical performance of a Ritchey-Chrétien telescope: The traditional two-mirror Ritchey-Chrétien (RC) design uses approximately hyperbolic primary and secondary mirrors to produce images that are free from coma over a wide field. Because of this wide coma-free field and a relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography. The RC is the design of choice for most of the major professional observatory telescopes built in the last half-century. For example, the Hubble Space Telescope and the twin 10-meter Keck telescopes in Hawaii are Ritchey-Chrétiens.
      However, because of the complexity of fabricating and testing a large aperture hyperbolic mirror (just ask the people who built the initially-flawed, but not discovered until it was in space, Hubble Space Telescope), traditional two-mirror Ritchey-Chrétiens are very expensive to manufacture and purchase, too expensive for many amateur astronomers.
      To emulate the coma-free performance of a true RC telescope, while keeping the cost within reason, the Meade Advanced Coma-Free (ACF) catadioptric optical system uses a full aperture aspheric corrector lens in conjunction with a simple spherical primary mirror. This creates a two-element primary mirror system that performs like an RC's single hyperbolic primary mirror from the optical point of view of the ACF system's secondary mirror. The hyperbolic secondary mirror itself is mounted directly on the rear of the corrector lens, rather than in the traditional RC's conventional spider vane assembly. This eliminates the image-degrading diffraction spikes of the secondary mirror support structure visible in commercial RC scope images. The result of the Meade Advanced Coma-Free optical design is RC-class coma-free wide-field performance, at about a fourth the cost of a true RC system.
      The corrector-modified design would itself be expensive to fabricate were it not for Meade's more than a quarter-century of experience making Schmidt-Cassegrain correctors, which are in the same optical family as the corrector needed for the coma-free design of the ACF system. An additional benefit of the full aperture corrector in the ACF design is slightly better correction for astigmatism than the traditional RC design.
      In addition, the ACF optical system, due to its front corrector plate, is a closed tube design. This keeps the primary optical components protected from dust, moisture and other contaminants that might fall on the optical surfaces of the primary and secondary mirrors as can happen with the traditional open-tube RC design.
      While the Meade ACF system may not be a traditional RC design, its performance is RC-like in all important characteristics. A review in Sky & Telescope magazine of the predecessor of this Meade ACF optical system said the bottom line is that the optics do "indeed perform like a Ritchey-Chrétien." Another review, in Astronomy magazine said, "This scope delivers Ritchey-Chrétien-like performance at a fraction of the cost."
  

• Premium optical glasses: The primary and secondary mirrors are crafted of low thermal expansion Pyrex optical glass. The corrector lens is made of water white glass, for up to 10% higher light transmission than conventional soda-lime (plate) glass.

• Oversized primary mirror: The diameter of the primary mirror of the LXD-75 8" ACF system is larger than the diameter of the corrector lens at the front of its optical tube that admits the light. The primary mirror is 8.25" in diameter, 3% larger in diameter (6% larger in area) than the 8" aperture of the Schmidt corrector lens. This gives you a wider fully-illuminated field than a conventional 8" SCT - with about 6% more off-axis light available to your eye or camera.

• Fully multicoated UHTC (Ultra High Transmission Coatings) optics: The primary and secondary mirrors are vacuum-coated with aluminum, enhanced with multiple layers of titanium dioxide and silicon dioxide for increased reflectivity. A overcoating layer of durable silicon monoxide (quartz) assures long life.
      A series of anti-reflective coatings of aluminum oxide, titanium dioxide, and magnesium fluoride are vacuum-deposited on both sides of the full aperture corrector plate. These antireflection multicoatings provide a high 99.8% light transmission per surface, versus a per-surface transmission of 98.7% for standard single-layer coatings. Overall light throughput (the amount of light collected by the objective lens that actually reaches your eye or camera) is approximately 89% at the focal plane.
      UHTC multicoatings provide a 15% increase in light throughput compared with standard single-layer coatings. They effectively add the equivalent of 15% extra light-gathering area to the performance of a scope with standard coatings. It's the equivalent of more than half an inch of extra aperture in the case of this 8" scope, for example, but with no increase in actual size or weight. UHTC coatings also improve contrast, for lunar and planetary images that appear sharper and more crisply defined.

• Fully baffled optics: A cylindrical baffle around the secondary mirror, in combination with the cylindrical baffle tube projecting from the center of the primary mirror, prevents stray off-axis light from reaching the image plane. In addition, a series of field stops machined into the inner surface of the central baffle tube effectively eliminates undesirable light which might reflect from the inside surface of the tube. The result of these baffle systems is improved contrast in lunar, planetary, and deep space observing alike.