Celestron - C11-A XLT - 11" SCT OTA for CGE/CGX - 91036-XLT

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Cutting edge optics with the Celestron 11" SCT design, an astrophotographer must have!

Celestron Starbright XLT Coatings renders excellent quality views and images

The Fastar application turns an f/10 system to an f/2 for very short exposure times

Lightweight and portable enough for relatively easy transportation

Celestron - C11-A XLT - 11" SCT OTA for CGE/CGX - 91036-XLT

The Celestron C11 11" SCT Optical Tube Assembly is an excellent gem for serious astrophotgraphers who enjoy crisp images with shorter exposure times. Being lightweight with its aluminum design, it is only 24" in length with a weight of 27.5 pounds, thus still making is reasonably portable to be taken to locations for viewing or imaging with one person.

Using the Celestron Starbright XLT Coatings, high contrast views at high magnification are a walk in the park! The coatings increase contrast and light transmission; significantly improving image quality to very superior standards which is a major plus for shorter exposure astrophotography. When taking into account the durability of the Fastar system as well as the f/6.3 focal reducer, making shorter exposure times even better at 25 times shorter at f/2 than its typical f/10 optical focal ratio.


  • Cutting edge optics with the 11" SCT design, an astrophotographer must have!
  • Starbright XLT Coatings renders excellent quality views and images
  • The Fastar application turns an f/10 system to an f/2 for very short exposure times
  • Lightweight and portable enough for relatively easy transportation

Starbright XLT Coatings

  • Maximizing light transmission through a perfect optical path
  • Starbright XLT improves color retention, contrast, sharpness, resolution, and brightness
  • Starbright XLT system provides flat wavelength transmission across the entire visual spectrum. Therefore you will minimize your wavelength curvature
  • Multi-layer mirror coatings with layers of aluminum, TiO2 (titanium dioxide) and SiO2 (quartz),
  • Starbright XLT Multi-layer anti-reflective corrector plate coatings with layers of MgF2 (magnesium fluoride) and HfO2 (hafnium dioxide); hafnium has a wider band-pass than the commonly used titanium in lesser quality optics
  • Starbright Xtra High-Transmission water white glass corrector plate limits loss of light due to absorption

Optical Coatings

Since Schmidt-Cassegrain designs use both mirrors and lenses, obtaining maximum light transmission without sacrificing quality is the key. This is where the StarBright XLT coatings come into play. 

Using a technique known as "vacuum deposition", optical coatings are applied in a thin manner on the optical glass components and positioned properly relative to each other to maximize light transmission. The mirror is coated with metallic coatings that effectively increases light transmission from 4% for mirrors without coatings and 86-88% with standard coatings to a whopping 95%. This is the first step to achieving the necessary light transmission.

The second step is applying the coatings to the lens which is more sophisticated and complex process due to the fact that light is lost in both the reflection and absorption, thus resulting in further light loss. 4% is reflected back intially while more light is lost in the absorption through the glass and second lens surface. To solve this problem, Celestron has implemented dielectric materials known as A/R (anti-reflection) coatings to obtain a very low and flat reflection across the visible spectrum.

Combining these factors results in a remarkable peak transmission of 89% at 520nm and an overall average transmission of 83.5% from 400 to 750nm. The graph below demonstrates this.

How Starbright XLT is Different

The charts below show the improvement of transmission, reflectivity and corrector transmission obtained through Starbright XLT coatings verses previous coatings. It has a peak transmission of 89% vs the previous Starbright peak of 80%. In terms of reflectivity, the XLT coatings have an average peak of 93% vs previous coatings at 91%.

The Starbright XLT corrector transmission is 97.4% compared to other coatings with a lower peak.

Coating Testing

Celestron has implemented two methods to test the effectiveness of the StarBright XLT coatings.

The first method is by measuring the optical tube performanec in its entirety. This is done by comparing a beam of light passing through the optical components of the optical tube against a beam of light in air. By dividing the light ratio of the beam through the optical tube verses the beam through air, Celestron can measure the effectiveness and performance. However there are challenges to this method including but not limited to making sure the beam is constant and that light capture is effective, steady and clean and making sure the optical components are properly lined up so they don't skew the results in one way or another.

The second method is easier to execute and simply measures each component's spectrographic performance to make sure all are working properly. This method in addition to not having the setbacks of the first method, also provide greater analysis of each individual component of the optical tube, something not possible in the first method. This method also helps determine the upper limit of the telescope's upper throughput using the calculation below:

%TT = %TC x %RP x %RS


%TT is Total Telescope Throughput

%TC is Corrector Plate Transmission

%RP is Primary Mirror Reflectance

%RS is Secondary Mirror Reflectance

Corrector Plate Transmission - %TC

Using a Shimadzu UV1601 spectrophotometer, Celestron has implemented an effective way of analyzing the corrector plate transmission by using a double beam instrument with a 190 to 1100 nm spectral range with transmission usually collected within 400 to 750 nm. To avoid scratching and damaging the correct plate in the process, "witness plates" which are small samples of the corrector material are used as an alternative. This method is effective for correctors up to 8" diameter.

Primary and Secondary Reflectance - %RP, %RS

The protocol to measuring the reflectance of the primary and secondary mirrors also includes the use of "witness plates". Using 1-2" diameter polished glass substrates coated along with the primary and secondary mirrors that serve as a "stand in" for the mirrors during the testing to avoid unnecessary scratching and damage and to serve as an easy reference to measure reflectance on flat surfaces.

Celestron chose to use enhanced aluminum-coated quartz flat scaled and measured against a NIST (National Institute of Standards and Technology) specular reflectance standard. The formula to measure the reflectance is calculated by comparing it against the baseline formula below:


%RS = %RSR x %RR


%RS is Sample Reflectance Factor

%RSR is Reference Standard

%RR is Reference Standard Known Reflectance


Flat mirrors are very easy to measure using the formula above, but curved primary and or secondary mirrors on the other hand, that is a totally different story, particularly if there are no witness plates available for use. Measuring a curved mirror against a flat reflectance standard would not render accurate results. As an alternative, another method is used known as an integrating sphere. This method essentially ignores the curvature and simply measures light intensity. The downside however is this is a pricey and long process to setup. To bypass this time and cost restraint, Celestron has developed another method by creating their own reference standard for secondary and primary mirrors. By using secondary and primary mirrors that have existing coating removed and replacing it with flat witness plates scaled against the NIST standard, the data can then be applied to the curved samples. The instrument used in this process are an Ocean Optics USB2000 Spectrometer with an LS-1 Tungsten Halogen Light Source. This single beam instrument has a 0.3nm resolution with a 340-1024 nm scanning range and is equipped with a fiber optic curved surface reflectance measuring probe.

More Information
UPC 050234103695
Brand Celestron
Free Shipping Free Shipping Available
Aperture 279.4mm (11")
Case Included No
Focal Length 2800mm (110")
Focal Ratio f/10
Focal Length of Eyepiece 1 40mm
Magnification of Eyepiece 1 70x
Focal Length of Eyepiece 2 9x50
Finderscope Included 9x50 Optical Finder
Diagonal Included 1.25" Star Diagonal
Included Dovetail CGE D Series Dovetail
Optical Tube Material Aluminum
Highest Useful Magnification 660x
Lowest Useful Magnification 40x
Limiting Stellar Magnitude 14.7
Resolution - Rayleigh 0.5 arc seconds
Resolution - Dawes 0.42 arc seconds
LIght Gathering Power (Compared to the Human Eye) 1593x
Size of Secondary Obstruction 95mm (3.75")
Secondary Mirror Obstruction By Diameter 34%
Secondary Mirror Obstruction By Area 17%
Optical Tube Length 610mm (24")
Optical Tube Diameter 312.42mm (12.4"
Optical Tube Weight 27.5 lbs (12.4 kg)
Warranty 2 Years
In The Box
  • Celestron 11" SCT Optical Tube Assembly
  • 40mm eyepiece (70x)
  • 9x50 finderscope
  • 1.25" star diagonal
  • 1.25" visual back
  • CGEM style dovetail bar
  • Celestron 11" SCT Optical Tube Assembly
  • 40mm eyepiece (70x)
  • 9x50 finderscope
  • 1.25" star diagonal
  • 1.25" visual back
  • CGEM style dovetail bar
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