C14-A XLT (CGE) Optical Tube Assembly
Item #91037-XLT

Celestron is proud to introduce a revolutionary coating system that outperforms any other coating in the commercial telescope market. Our most popular Schmidt-Cassegrain telescopes are now available with this high quality optical coating at an incredible value.

Navigate through this section to get detailed information on how XLT is designed, and how XLT measures up against our current StarBright coatings. We have also used this site as a resource to provide information on our testing methods and we offer a section detailing which models this new coating will be available with and the cost breakdown for each model.

OVERVIEW
COATINGS OVERVIEW
STARBRIGHT XLT SYSTEM
CURRENT STARBRIGHT
OUR MEASUREMENTS

To find a Celestron Dealer near you who carries telescopes with StarBright XLT, please click here for our Dealer Locater.

StarBright XLT & Our Commitment to Quality   TOP

We strive to design and engineer products with quality components using a state of the art manufacturing process that is followed up with uncompromising quality assurance. You can see it in the design and quality of our entire product line. And our new StarBright XLT coating system is no exception.

Design - We design and test our optical coatings with the aid of thin film design software in wide use throughout the optical, semiconductor, aerospace, and telecommunications industries. Using this software we have improved on our multi-layer enhanced mirror coatings, shifting the peak reflectance to the center of the visible spectrum. We have designed a completely new multi-layer anti-reflective coating and have introduced a new low absorption, high transmission glass for our corrector lens. This unequaled combination is standard with every StarBright XLT system.

Quality Components and Process - Our coating process uses state-of-the-art thin film vacuum deposition technology. To ensure consistent optical coatings of the highest quality the process is tightly monitored and controlled by highly trained coating technicians. Prior to coating, each optical element is thoroughly cleaned and inspected to ensure proper adhesion of the films during the coating process. The materials used in our reflective and anti-reflective coatings including Aluminum, Hafnium Oxide, Titanium Dioxide, Silicon Dioxide, and Magnesium Fluoride are the purest available, exceeding 99.99%.

Quality Assurance - Our QA process is designed to prevent any optical element from passing if it does not meet our strict standards of optical quality. Witness plates are included in each coating run, and are subjected to spectrophotometric analysis to determine if the minimum acceptable transmission or reflectance has been achieved.

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Optical Elements of the Schmidt-Cassegrain Telescope    TOP

A telescope is a group of optical elements that collects light and focuses it for observation by an eyepiece or some other imaging device. There are two types of optical elements: mirrors and lenses. Mirrors reflect light and lenses refract, or bend light. The Schmidt-Cassegrain telescope uses both mirrors and lenses. The diagram below shows a cross-section of a Schmidt-Cassegrain. In this telescope, light first passes through the corrector lens, and then reflects off the primary mirror. Finally, it reflects off the secondary mirror and comes to a focus at the focal plane.

Optical Coatings

The purpose of a telescope is to collect as much light as possible. The amount of light collected affects the brightness of the resulting image. Unfortunately, there are sources of light loss at each optical surface, and within each lens. Fortunately, we can design optical coatings and choose lens materials that minimize the amount of light lost to these sources.

Optical coatings are very thin layers of material that are applied to the glass in a process called 'vacuum deposition '. The physical properties and thickness of each layer in the coating, as well as their orientation with each other and the glass to which they are applied, determine how well they will do their job.

Since the function of a mirror is to collect light by way of reflection, we use highly reflective metallic coatings on these optical elements. A mirror without coatings reflects about 4% of the light that hits its surface. A mirror coated with standard Aluminum coatings reflects about 86 - 88%, and a mirror coated with StarBright XLT reflects 95%.

Light traveling through a lens is a little more complicated. In this case, light is lost to both reflection and absorption. When light first strikes an uncoated lens, about 4% is reflected back and never has the chance to make it through. Some of the remaining 96% will be absorbed on its way through the glass, and then the second lens surface reflects another 4%. To minimize unwanted reflection, dielectric materials are used in pairs of alternating high and low refractive index. A good anti-reflection (A/R) coating for telescope lenses is one that will deliver very low, very 'flat ' reflectance across the entire visible spectrum.

Although A/R coatings can dramatically reduce the amount of light lost to reflection, no optical coating can reduce the amount of light lost to absorption within the glass. To reduce this source of light loss, it is important to choose a glass that absorbs as little light as possible.

For many A/R coating applications, it is standard to measure the reflection of the coated surface and to ignore the amount of light that is being absorbed by the glass. But for a telescope lens, stating how well an A/R coating suppresses reflection without also revealing how much light is lost to absorption within the glass can be quite misleading. For this application, actual transmission, which accounts for light lost to both sources, should be measured directly. You can learn more about how we did these measurements in the section titled Our Measurements.

Telescope System Transmission

System transmission is the percentage of light that arrives at the focal plane compared to the light that enters the telescope, and is calculated by taking the product of the corrector lens transmission, the primary mirror reflectance, and the secondary mirror reflectance. Here is an example; if the corrector lens transmits 92% of the light, and the primary and secondary each reflect 89% of the light, then:

Total System Transmission = .92 * .89 * .89 = .73 (73%)

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StarBright XLT - An Optical System Breakthrough!    TOP

Celestron has brought its renowned StarBright technology to an even higher level of light transmission with the introduction of our new optional StarBright XLT High Performance Optical Coating System.

StarBright XLT Optical System Design - You'll See The Light.

One of the most important factors in the evaluation of a Schmidt-Cassegrain telescope's optical system performance is its transmission - the percentage of incoming light that reaches the focal plane. The design of the XLT System accomplishes two crucial objectives: Develop a coating system that is optimized for visual use and for CCD/Photographic imaging.

The StarBright XLT System - What Makes It Different Makes It Better

There are three major components that make up our StarBright XLT high transmission optical system design:

1. Unique enhanced multi-layer mirror coatings
Our mirror coatings are made from precise layers of Aluminum (Al), SiO2 (quartz), TiO2 (Titanium Dioxide), and Si02. Reflectivity is fairly flat across the spectrum, optimizing it for both CCD imaging and visual use. Click here to see a plot of the reflectivity of XLT's Mirror Coatings.

2. Multi-layer anti-reflective coatings
Made from precise layers of MgF2 (Magnesium Fluoride), and HfO2 (Hafnium Dioxide) A rare element costing nearly $2000 per kilogram, Hafnium gives us a wider band pass than Titanium, used in competing coatings. Click here to see a plot of XLT's corrector transmission.

3. High Transmission Water White glass
Celestron Schmidt-Cassegrain optical systems with optional StarBright XLT coatings use Water White glass instead of Soda Lime glass for the corrector lens. Water White glass transmits about 90.5% without anti-reflective coatings. That is 3.5% better transmission than uncoated Soda Lime glass. When Water White glass is used in conjunction with StarBright XLT 's anti-reflective coatings, the average transmission reaches 97.4% - an 8% improvement! Click here to see a plot of water white glass versus soda lime.*

These three components of our StarBright XLT coatings result in one of the finest coatings available. The peak transmission for the systems is 89% at 520 nm. The overall system transmission is 83.5% averaged over the spectrum from 400 to 750 nm. The plot below shows the entire system transmission over the spectrum.

This plot is obtained by measuring the reflectivity of the secondary mirror and the primary mirror and measuring the amount of light transmitted through the coated corrector lens. Each of those values are multiplied together calculate the system transmission. The overall system transmission peaks at 88.9% while the average transmission is 83.5% over the spectrum from 400 to 750nm.

*Percent differences are calculated by taking the comparison data percentage divided by the baseline data. Example: Measured average system transmission for current StarBright is 72%. XLT average system transmission is 83.5%. 83.5% divided by 72% = 1.16 or 16% improvement. Measurement results are rounded to the nearest whole percentage.

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StarBright XLT vs. Current StarBright Coatings   TOP

StarBright XLT system transmission gives a 16% improvement compared to the current StarBright coatings. The average system transmission for the current StarBright coatings is 72% where the average system transmission StarBright XLT is 83.5%. Current StarBright uses soda lime glass correctors where StarBright XLT uses water white glass, which improves the corrector throughput dramatically.

The average system transmission of StarBright XLT 83.5% compared to current StarBright at 72%. StarBright XLT is a 16% improvement over current StarBright .* The peak transmissions of each being 89% and 80% respectively.

StarBright Mirror Reflectivity Comparison
StarBright XLT mirror reflectivity peaks at 95% and has an average reflectance across the spectrum of 93%. Click on the link above to show how XLT compares to current StarBright and UHTC.

Mirror Reflectivity for StarBright XLT and current StarBright. StarBright XLT reflectivity peaks at 95% and has an average reflectance across the spectrum of 93%. Current StarBright peaks at 94% with an average reflection of 91% across the spectrum.

StarBright Corrector Transmission Comparison
XLT's corrector transmission is 97.4% versus current StarBright with 87% and UHTC with 91% across the spectrum from 450 to 750 nm.

Average XLT transmission of 97.4% versus current StarBright with 87% across the spectrum from 450 to 750 nm. StarBright XLT is a 12% improvement over current StarBright transmission.* StarBright XLT has peak transmission at 99%, while current StarBright peaks at 91%.

*Percent differences are calculated by taking the comparison data percentage divided by the baseline data. Example: Measured average system transmission for current StarBright is 72%. XLT average system transmission is 83.5%. 83.5% divided by 72% = 1.16 or 16% improvement. Measurement results are rounded to the nearest whole percentage.

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Testing Methods:   TOP

Total telescope light throughput can be measured in two different ways; either by measurement of the assembled optical system, or by measurement of the reflectance of each mirror (or reflective element), and the transmission of each refractive element in the optical path. In the case of a Schmidt Cassegrain telescope, there are two reflective elements (the primary and secondary mirrors), and one refractive element (the corrector plate, or Schmidt Corrector). See diagram below:

Assembled Telescope vs. Individual Optical Element Analysis:

To measure the throughput of the assembled telescope, a beam of light is passed through the telescope and compared to a beam of equal intensity light passing through air only. Total telescope throughput is then the ratio of light intensity measured through the telescope divided by the light intensity measured through air. This is easily said, but very challenging to execute correctly. Great care must be taken to ensure that the reference beam is of constant intensity, and that its light is collected in a manner which does not bias the results. Errors introduced by beam geometry (f ratio) at the entrance to the detector, less than perfect alignment of the optical elements, including placement and dimensions of internal light baffles, will tend to reduce the intensity of light measured through the telescope.

The second method of measuring total telescope throughput, by spectrophotometric analysis of each element in the optical path, is not susceptible to these sources of error. Furthermore, individual element analysis provides specific information about each optical element, while measuring the throughput of the assembled optical tube does not. Results obtained in this manner represent an upper limit to the actual throughput of the assembled telescope. Total Telescope Throughput (%TT) is less than or equal to Corrector Plate Transmission (%TC) times Primary Mirror Reflectance (%RP) times Secondary Mirror Reflectance (%RS).

Corrector Plate Transmission (%TC):

We use a Shimadzu UV1601 spectrophotometer for analysis of corrector plate transmission. This is a double beam instrument with a spectral range of 190 to 1100nm. Transmission data is typically collected in the visible region from 400 to 750 nm. Small samples of corrector material called witness plates are included in each corrector coating run. In order to minimize handling and the possibility of scratching a full size corrector plate, we use these witness plates to represent the transmission characteristics of our correctors.

Our instrument is capable, however, of measuring the transmission of correctors up to 8" diameter. If this is necessary, the corrector plate is measured at 4 points roughly 90� apart, and the results are averaged. Before and after each measurement, baseline (100%) measurements are made to ensure light source and/or detector drift is negligible.

Primary and Secondary Reflectance (%RP, %RS):

The preferred method of measuring reflectance of primary and secondary mirrors involves the use of witness plates as well. These are small (1" to 2" diameter) flat polished glass substrates, which are coated along with the primary and secondary mirrors. Since the coating process is the same, and the surfaces are equally well polished, the reflectance of the witness plate is the same as that for the primary and secondary mirror. The reasons for using flat witness plates are 1) the primary and secondary mirrors are not themselves subjected to a measurement process which can potentially cause scratches, and 2) very simple test methods and readily available reference standards can be used to measure the reflectance of flat surfaces.

Typically, the reflectance of a surface is measured against a standard reference of known reflectance. Our standard reference is an enhanced aluminum coated quartz flat, calibrated against a NIST (National Institute of Standards and Technology) specular reflectance standard. To measure the reflectance of a flat sample, the baseline measurement is made using this standard, and the reflectance of the sample is compared to this baseline. The sample reflectance factor (%RS) is equal to its reflectance relative to the reference standard (%RSR) times the reference standard's known reflectance (%RR):

However, if the sample to be measured has a curved surface like a secondary or a primary mirror, and there is no witness plate available, then special care must be taken to ensure that the method used to measure reflectance is insensitive to this curvature. If we compared the reflectance of a curved surface directly to that of a flat reflectance standard, our results would not be accurate, since the converging or diverging beam generated by a curved surface would direct either less light (in the case of a secondary mirror), or more light (in the case of a primary mirror) onto the detector than was directed by the flat reference standard.

The most widely used tool for measuring the reflectance of curved surfaces is called an integrating sphere. This device collects and then measures the intensity of light in a manner which is insensitive to beam geometry, hence, insensitive to surface curvature of a reflective sample being measured. However, integrating spheres can be quite expensive, and they are time-consuming to set up and calibrate. We developed a method which is equally insensitive to surface curvature, but much less costly and time consuming to perform. We made our own reference standards from secondary and primary mirrors with the same surface curvature as those we wished to test.

We obtained samples of the secondary and primary mirrors which we wished to test, stripped the existing coating, and replaced it with one for which we also obtained flat witness plates. These flat witness plates were calibrated against a NIST specular reflectance standard. Since the flat witness plates were coated along with the curved samples, and since we have adequate data to show that our coatings are very uniform from part to part in any given coating run, we can apply this reflectance data to our curved samples. Using these curved surface reflectance standards we are able to measure other mirrors of the same curvature just as we use our flat reflectance standard to measure the reflectance of flat samples.

To perform these measurements, we use an Ocean Optics USB2000 Spectrometer with an LS-1 Tungsten Halogen Light Source. This is a single-beam instrument with a 0.3nm resolution, a scanning range from 340nm to 1024nm, and is equipped with a fiber optic curved-surface reflectance measuring probe.

Reporting the Data:

Collecting the data and reducing it to yield total telescope throughput (%TT) (system transmission) is simply a matter of multiplication. We find the average of each data set (%TC, %RP, and %RS) for each wavelength measured, and multiply them together.

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