
Mirror vs. Dielectric vs. Prism Diagonal Comparison
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Mirror vs. Dielectric vs. Prism Diagonal Comparison
By: William Paolini, 3/6/2014 v2
Back Row ( 2" )
Baader Zeiss Prism, Takahashi Aluminum Mirror, Astro-Tech Dielectric, Baader Dielectric, Astro-Physics MaxBright Dielectric
Center Row ( 1.25" )
Baader T2 Zeiss Prism, VERNONscope Enhanced Silver Mirror, Takahashi Prism, Tele Vue Everbrite Dielectric
Front Row ( 1.25" )
Vixen Prism, Astro-Tech Dielectric, Celestron Prism (vintage 1980s)
Figure 1
Diagonals used in this comparison
Image by the Author
Equipment courtesy of the Author, B. Brown, Daniel Mounsey, Roy McCoy, Steve Stapf (aka Starcam on CN), & Tamiji Homma
I. Introduction
The diagonal is often a little scrutinized component in the telescope's optical chain. Reflectivity efficiency and presumed wavefront accuracy are often the only attributes of attention. The purpose of this comparison was to instead determine if a diagonal's technology type (i.e., aluminum, silver, dielectric, or prism) plays any significant role in the telescope's performance, primarily for lunar/planetary observing and secondarily for deep sky object (DSO) observing. Note that only standard right-angle diagonals were evaluated. For lunar/planetary performance, the Moon and Jupiter were used (Jupiter was optimally positioned near the zenith). The primary criteria for lunar/planetary performance was: level and clarity of details observed, perceived contrast, presence of chromatic aberration, and scatter. On DSO, the primary criteria evaluated was threshold brightness of the furthest extents of nebula, ability to see stars at the threshold of vision, chromatic aberration on bright stars, and scatter around bright stars. The Takahashi TSA-102 f/8 Super APO was used for all tests. However, since it is popularly reported that a prism diagonal can induce chromatic aberration (CA) with faster focal ratios, for CA tests a Celestron f/6.25 80mm Onyx APO was also used. A TEC 140 APO was also used in a single-blind test with an experience observer to validate any significant outcomes seen in the TSA-102. All observations were conducted from a suburban location west of Washington, D.C., USA over a three month period from December 2013 to February 2014. The observing site had light-to-moderate light pollution with limiting magnitudes during observation evenings generally around magnitude 4.0 to 4.5. Finally, great appreciation goes out to the many generous members of the online astronomy community at www.cloudynights.com for their support and loan of valuable personal equipment as this comparison would not have been possible without them!
II. Diagonal Technologies
In today's market, diagonals are varied and come in four common technologies for amateur equipment: the aluminized (Al) mirror, the silvered (Ag) mirror, the dielectric mirror, and the glass prism. The aluminum and silver mirror technologies come in two main varieties: protected and enhanced. Generally, enhanced coated mirrors have greater reflectivity than protected ones, and silver mirrors have greater reflectivity than aluminum ones. In all cases, a protected aluminum or silver mirror is easy to scratch and needs to be cleaned carefully, whereas enhanced mirrors offer greater protection.
Mirrors advertised as being "protected" typically have a single coating applied to protect the mirror, which is usually silicon monoxide (SiO) or magnesium fluoride (MgF2). When the aluminum or silver mirror is advertised as being "enhanced", this typically means several dielectric coatings are added to both protect the surface, and also increase reflectance of the mirror by several percent over what can be achieved with only a protected SiO or MgF2 overcoat. The dielectric coatings used in enhanced mirrors are much harder than SiO or MgF2 and further increase the reflectance, typically allowing enhanced aluminum mirrors to have 96% or more reflectivity, and enhanced silver mirrors are typically 97%-98% reflective.
When dielectric materials are used exclusively to create a mirror, then 20 or more layers of dielectrics are usually needed to achieve a high reflectivity rate. The materials used for dielectrics are usually oxides of silicon, titanium, aluminum, and tantalum, or fluorides of magnesium, lanthanum and aluminum, and they are applied in alternating thin layers of high- and low-index dielectrics. While a fully dielectric mirror is very tough, making cleaning a care free chore, careful attention needs to be paid to the number of layers as more layers can compromise the overall wavefront accuracy of the substrate glass. As reported in the Photonics Handbook (www.photonics.com), "...a 100-layer coating with thickness variations of 2 percent across the surface (a typical coating uniformity tolerance) would distort the wavefront of the reflected beam by several wavelengths." Given this, it is important to know the final wavefront of the dielectric mirror *after* the coatings are applied as even the most precisely polished glass substrate can have greater than one wave of error if the many dielectric coatings are not applied with consistent high precision.
Finally, probably the most mature technology used for diagonals is the simple glass prism. BK7 is the typical material used for telescope diagonal prisms. Reflectivity internal to the prism is 100%, with no light loss. Light loss does however occur at the air-glass interfaces of the prism, like for all optics. With a prism's two air-glass interfaces, if a typical modern 99.5% efficient multicoating is used then the expected reflectivity should be 99% for the prism, not accounting for any loss to absorption by the glass, which is approximately 0.2% or less in the visible spectrum for each 10mm of glass path for BK7 glass (Ref: http://www.hoyaoptics.com/pdf/MasterOpticalGlass.xls).
III. Physical and Mechanical Examination
The diagonals used for this comparison ranged from inexpensive to premium, 2" sizes and 1.25" sizes, and represented all the major technologies (aluminum mirror, silver mirror, dielectric, and prism). The table in figure 2, summarizes the major marketing features advertised for these diagonals.
Diagonal |
Size |
Type |
Reflectivity Claim |
Wave Front Claim |
Body Build |
Astro-Physics MaxBright |
2" |
Dielectric |
>99% |
Inconsistent* |
CNC Machined |
Astro-Tech |
1.25" |
Dielectric |
99% |
1/10th wave before coating |
Machined Aluminum |
Astro-Tech |
2" |
Dielectric |
99% |
1/10th wave before coating |
Machined Aluminum |
Baader |
2" |
Zeiss Prism |
Not specified, Zeiss T-coatings |
Not specified |
CNC Machined |
Baader Clicklock |
2" |
Dielectric |
99% |
1/10 wave after coating |
Metal |
Baader T2 |
2" Capable** |
Zeiss Prism |
Not specified, Baader multicoatings |
Not specified |
Machined Aluminum |
Celestron |
1.25 |
Prism |
Not specified |
Not specified |
Plastic & metal |
Takahashi |
2" |
Enhanced Aluminum on Pyrex |
Not specified |
1/10th wave |
Metal |
Takahashi |
1.25" |
Prism |
100% internal reflectivity, multicoated |
Not specified |
Plastic & Metal |
Tele Vue Everbrite |
1.25" |
Dielectric |
99% |
1/10th wave before coating |
Solid Aluminum |
VERNONscope |
1.25" |
Enhanced Silver (Ag) |
99% |
1/20th wave or better after coating with Zygo report |
CNC Machined |
Vixen |
1.25" |
Prism |
Not specified |
Not specified |
Plastic & Metal |