Advanced Telescope Supplies

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RC Optical Systems (RCOS) Telescopes Our RC Optical Image gallery is here and you can obtain even more information with RC Optical systems

The RCOS telescope range is a pure Ritchie Chrétien design and is offered in 10", 12.5", 14", 16", 20" , 24" and 32" apertures in user specified F-Ratios.

Pictured above, the RCOS 24" on the RCOS professional mount.

Unlike mass produced telescopes, each instrument is interferometrically tested and certified to a *Strehl ratio of around 93% (or **better) and is manufactured from high stiffness, ultra low thermal expansion materials to ensure this optical perfection is delivered over extremely wide range of temperatures. The result is a telescope that you do not have to battle just to stay in focus, but can consistently deliver research quality images and data.

Amateurs seeking the ultimate in large aperture telescopes should look no further. The super smooth polish and superb correction of an RCOS telescope delivers star images that have a sharp, almost "searing" quality.  The optical tube assembly can be easily mounted onto a range of computer controlled German equatorial mounts which can deliver sub arc minute pointing accuracy.

Coupled with an SBIG CCD camera hunting down 20th magnitude asteroids, comets and supernovae are easily within the reach of these superb instruments.

   *the Strehl ratio for a perfect optic is 100%. Mass produced SCT's  typically deliver about 80%

  ** RCOS optics can be optionally ordered with Ion-beam figured optics, which are virtually perfect to a molecular level of precision.

 

 

 

 

 

 

 

 

 

 

Some important Notes on Ritchie Chrétien Optics

Meade Instruments produce an "Advanced Coma Free" telescope.  They are clearly not Ritchie Chrétien's. The Ritchie Chrétien design ( inverted in 1910s by George Willis Ritchey and Henri Chrétien,  their design is used in many, if not most of the largest professional astronomical telescopes ) uses two hyperbolic mirrors only. The Meade ACF design is a clear step ahead of the standard schmidt cassegrain telescope.

However the correcting plate in their design has off-axis chromatic aberrations which are shown "spot diagrams" below. The polychromatic spot size matrix shown here clearly shows the chromatic error introduced by the addition of a corrector plate. While the spot sizes are significantly larger than a classical Ritchie Chretien, the off-axis performance is superior to a standard Schmidt Cassegrain.

 

These diagrams (click on either for a larger image) show the difference between Meade RCX (left) and Classical RC (right) telescope designs. The Curved field RMS and geometric (in brackets)  spot sizes for both are tabulated below 0.4 degrees off axis is similar to the field position at the edge of a 35mm CCD. Degrees off axis RCX Spot Size  Rms (Geo)  RC Spot Size  Rms (Geo) 0 17.1 (74) 0.01 (.38) 0.15 17.1(75) 1.2 (2.1) 0.25 17.1(76) 2.2 (3.4) 0.35 17.4(77) 6.9 (9.7) 0.40 17.6(79) 9.0 (12.5)

 

Optical Testing

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Optics can be tested in a number of ways, however the only way to obtain an objective and quantifiable measure of a telescopes figure is to use a laser interferometer. Some examples are show below. It should be pointed out that a less than "perfect" optic is not necessarily a bad telescope. As a balance between price an performance an 84% strehl ratio is a good compromise.

 

Pictured above an inteferogram of a optically certified (inteferometrically tested) mirror.  This optic is for most purposes as perfect as current technology allows.  Click on the image for a larger picture.	Pictured above. According to the manufacturer this is a  "finest available" or "diffraction limited" optic, but it has not been  inteferometrically certified. The  interference fringes should be straight and evenly spaced.  This is not an excellent optic and shows nearly a 1/2 wave of spherical error.  Click on the image for a larger picture.

 

Still not convinced? Roll your mouse over the image below to compare an RCOS 12.5" to a Celestron 11" SCT. The C-11 image had more than 50% extra exposure with an SBIG AO7 adaptive optics unit. The RC still shows better correction and fainter stars across the entire image field.

Professional astronomers often choose the Ritchie Chreiten (RC) telescope design (as opposed to Newtonians, Schmidt Cassegrains and Refractors) as this compact design transmits all wavelengths of light equally and exhibits no coma over a wide field.  Some more famous Ritchie Chretien telescopes include: The Hubble Space Telescope Anglo Australian Telescope Keck Telescope ESO 4 metre
	Pictured above, A 16" RCOS Ritchie Chretien telescope (5" refractor mounted on top) Pictured right,  comparative spot diagrams between RC and SCT designs

Simply put, most large professional telescopes are Ritchie Chretiens!

The excellent off-axis performance of the RC design allows extremely accurate aperture photometry to be performed over a very wide field. The focal plane is also extremely flat compared to popular Schmidt Cassegrain (SCT) designs, and shows pin-point edge of field stars even with large CCD arrays (as in the RCOS 12.5" picture of M42 at right taken with a 24mm square CCD))

As there is no corrector plate, the optical system does not introduce any higher order spherical aberration and has a much wider spectral transmission, allowing studies from the deep ultra violet to deep infrared ends of the spectrum. (Gold based coatings can also be specified for specialized very deep infrared observing). 

Each RCOS optical tube assembly is fitted with 3 variable speed cooling fans. These gently draw ambient air into the optical tube and inhibit the formation of thermal air currents which can ruin the image formed by optics of an otherwise excellent  telescope.

They are also fitted with a 2.7" (or 4" depending on model) Astro-Physics focuser. Their precise and silky smooth action is revered amongst telescope makers as being the industry's best.  

Unique to RCOS telescopes is an optional servo driven digital focuser. The system can accurately and repeatedly place the secondary mirror to within 1/10,000th of an inch. The system is also programmable, allowing up to 10 focus positions to be stored, and returned to with the press of a button.

Above: Actual Off-axis performance of an SCT and RC compared at edge of SBIG model ST8 CCD field. Note the RC exposure time was 1/4 that of the SCT.

RC12.5" and C-11 comparative sizes

 Advanced Telescope Supplies can deliver on a timely basis (around 90-180 days, depending upon the aperture) diffraction limited RC Optical Systems Ritchie Chretien telescopes. Compared to other premium telecopes, their feature list is impressive.

 

• Carbon Fibre = Low thermal mass, low expansion, and stable focus
• Coefficient of Expansion... (x / unit length / Deg. C x 10 -6 power)
• Aluminum = 18.35
• Steel = 11.0
• Titanium = 8.5I
• Invar 1.1
• Carbon Fiber (High Modulus with Epoxy) = 0.9

Feature	RC optical	Meade ACF SCT	Celestron  SCT
Zero Expansion Ceramic Astrositall or ZeroDur optics	Yes	No	No
Interfererometrically tested certified diffraction limited optics	Yes	No	No
User Specified reflective coatings	Yes	No	No
Precision Astro-Physics 2.7" focusser	Yes	No	No
Zero Image shift focuser	Yes	Yes	No
Variable speed optical tube cooling fans	Yes	Yes	No
Low mass, Pure carbon fibre tube assemblies	Yes	No* (heavy front corrector)	No
Low tube thermal expansion/contraction co-efficient	Yes	Yes	No
Optional programmable remote secondary focusser	Yes	Yes	No
User specified F-Ratio's.	Yes	No	No
All refelective optics	Yes	No	No
Tube Assembly mountable on choice of mounts	Yes	No	Yes

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If you have an Advanced Amateur, School, College or University robotic observatory application in mind, please be wary of the following shortcomings of, albeit more affordable, SCT systems. RCOS telescopes while not inexpensive, are superior in every respect to mass produced SCT optics.

• The off axis SCT performance is very average when compared to a RCOS telescope and will degrade the quality Photometric and Astrometric measurements over more than approximately  8 mm sensor radius. 

• Focus will shift with a SCT as the telescope's altitude and temperature change during an observing run.

• Mirror flop (seen in practically all SCT's) will degrade computer pointing models, and even cause "tracking" errors due to the primary mirror settling on its central baffle after a slew.

Q: What advantage are Astrositall or Zerodur optics?
A: The tolerance for precision telescope optics is within a fraction of a wavelength of light, and while there are many telescope manufacturers that claim this "1/4 wave" precision, it is impossible to maintain over a range of temperatures using common glass or even temperature resistant pyrex. While considerably more expensive, Astrositall and Zerodur ceramic glasses have over 10,000x less thermal expansion than pyrex as a result, telescope optics made from this material will not suffer from temperature induced distortions.

Astrositall ( Russian equivalent to Zerodur) is a glass-ceramic has the following properties Coefficient of linear thermal expansion, 1/Kfrom 213K to 333K 0.0(±0.6)×10-7 Density, g/cm3 at 293K 2.46 Young's Modulus, GPa 90.2 Poisson's Ratio 0.28 Specific Heat, kJ/(kg K) 0.92 Thermal Conductivity, W/(m K) 1.18 Heat Resistance, K 823

Q: How can I be sure RCOS optics are excellent?
A: Each RCOS telescope is supplied with an interferogram and wavefront analysis which objectively describe the precision of each optical set.  The optical sets are made by Star Instruments (USA) who also supply professional observatory optics to manufacturers such as DFM Engineering and Optical Guidence Systems Inc. We certify each and every optical set. Many mass production manufacturers simply do not supply ANY data, and simply rely on their legal and advertising departments.

Q: What about advantages of the RC design from a visual, CCD and photographic perspective?
A: The Ritchey-Chretien was originally designed specifically as a photographic instrument. Its wide coma free field is advantageous for any application. Especially with medium format film and high-resolution large array CCD cameras. Visually the image is rather like a refractor, with pin point stars to the edge of the field. Despite the large secondary obstruction, planetary images are very crisp, due to the excellent figure and polish of all RCOS optics.

Q: Why use a secondary focuser?
A: Most SCT's achieve focus by moving their primary mirrors. While this does allow a massive range of focus, it also imparts mirror flop and field shift as the telescope is moved or focused. The secondary focuser in all RCOS telescopes uses extremely high precision bearings to impart zero image shift over a similarly wide focus range. As the primary is rigidly held in place there is no mirror flop or field shift.

Q: Can I use an F/6.3 or a F/3.3 reducer?
A: We recommend the Astro Physics .75X telecompressor. Other brands of focal reducer (e.g. Celestron and Meade) will cause modest vignetting which can be removed by an accurate CCD flat frame, but would be difficult to remove from a photographic image.

Q: Does it use baffles?
A: The primary and secondary mirrors are both baffled. The primary baffle has internal light stops.

Q: Are the optics coated?
A: Since coatings and applications can be varied, its your choice on what type of coatings and which company to use. If requested, we'll be happy to make suggestions.

Q: Why are cooling fans necessary?
A: Heat build up in any set of optics will cause thermal air currents which can severely distort the image seen through the telescope. RCOS telescopes have cooling fans which gently draw ambient air through the telescope to negate any thermal air current effects.

Q: Why use a Carbon Fibre tube assembly?
A: Carbon fibre has excellent thermal and strength for weight properties that make it an ideal material for telescope tube assemblies. The carbon tube has significantly less thermal expansion and contraction than an aluminium tube, yet weighs a fraction of a similarly rigid metal or sonotube (i.e. cardboard) assembly. As a result the telescope will not sag or lose alignment under different orientations or shift focus as the temperature changes during a night's observing.

 Note: some SCT manufacturers are now using carbon fibre outer tubes, but still fully mount and move their mirrors along an aluminium baffle tube, totally negating any benefits of a carbon composite tube assembly.

Q: With these significant advantages, why aren't Ritchie Chretien telescopes commonly seen in amateur hands?
A: Ritchie Chretien telescopes require extremely tight optical and mechanical specifications, hence take considerable time, effort and not suprisingly, expense to make. They are intolerant of any mechanical mis-alignment for example, as caused by tube flexure or thermal expansion. Additionally their complex hyperbolic primary and secondary mirrors figures are unique to the optical set being figured, and must be kept within exacting spacing tolerances when mounted in the optical tube assembly. It is for these reasons such telescopes are rarely made by amateurs, and are certainly not mass produced, as they require considerable skill and labour by mechanical and optical craftsmen to build well.


Pricing:
The smallest telescope in the RCOS range (250mm) costs about double that of a 14" Schmidt Cassegrainian (about $A25,000) A 12.5" RCOS telescope complete with a robotic German equatorial mount is priced similarly to competing 16" SCT's ($A43,000), yet offers vastly superior optics, instrument tracking, pointing, and stability. 

Exact prices however, are quoted on a per instrument basis, and will vary with the $A to $US at the time of ordering.

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