The best of the best star diagonals

The best of the best star diagonals

As many observers, you probably note the poor quality of star diagonals supplied from factory with low-ends and mid-range scopes. The magic box hides a aluminium coated prism or a flat mirror at a few cents well unable to transmit all the incoming light falling from stars.

If that can be sufficient to show celestial objects to a casual amateur, when high-resolution is wanted, a low optical quality is no more acceptable.

Practically all optical systems through which the light has to pass "keep" some dozen percents of light. But whatever can say some experts, even a 10% light loss is invisible, like a surface polished at 1/4th wave (P-V) vs. the same at 1/10l.

Tele Vue 2" diagonal

However, the best solution to avoid asking the question is to be sure that 100% of the incident light is reflected by a high reflective surface offering, moreover, a high abrasive resistance to mechanical and chemical alterations (sand, moisture, etc). These mirrors are not more coated with aluminium but rather with dielectric coatings.

Noticeable difference between diagonals

Most mirrors star diagonals sold by major dealers between $50-100 provide good results and slim the one who can perceive a difference between brands. However, compared to a dielectric coatings star diagonal, a standard aluminium coated cannot compete on planetary contrast. The sky for example is more black through a dielectric coatings diagonal than in any prism model, including the Meade 2". The image is also noticeable crisper than from the standard Celestron diagonal and sharper at high magnification. Still better the collimation appears the same with a dielectric diagonal as it does when looking straight through the OTA of a SCT. The same could not be said using the standard prism diagonal supplied with Meade or Celestron's SCTs.

A difference can also be perceived using a long or a short focus scope. At f/12 for example most diagonals perform well but horribly at f/6, even in high-end refractors.

Intes

2" Deluxe

Astro-Physics

2" Maxbright

Tele Vue

2" Everbright

Celestron

2" diagonal

At last, in some occasions, using a Celestron C8 or a NexStar5 with a 0.63x reducer/corrector, observers related the collimation was thrown way off with the Tele Vue 2" star diagonal, slightly with the Meade but not with the one of Astro-Physics. In optical terms, nobody can really explain why some 2" diagonals are affected when reaching the end of back focus. We can blame the positioning of the mirror diagonal on its thin strips of foam - and Tele Vue accepts to replace the faulty mirror - but this means the performance of a reducer sometimes depends of a compromise with the 2" diagonal.

Performances

Several models of star diagonals share high appreciated characteristics like Astro-Physics, Intes, Lichtenknecker Optics or Tele Vue unit. All are mirror star diagonal protected with dielectric coatings and present a high reflectivity. Their flatness (wave surface) is ordinary higher than 1/10 wave. The material is Sital and Zerodur with a thickness of 10 mm by 2" size.

The Tele Vue model called "Everbright" shows an average reflectivity of 99% in white light from 400 to 700 nm.

The Astro-Physics "Maxbright" is in all points similar to the Tele-Vue with 99% of reflectivity while the Lichtenknecker Optics star diagonal works only on their System 64 but presents the same performances too.

The Intes Deluxe is also using coatings reflecting 96% of the light on the complete surface from one end to the other end and offers around 1/20 wave surface. Models reflecting 99% offers 1/10 wave in the central area of the mirror only, which is the used area by high power.

Some other models present a 96% of reflectivity, like the Intes standard with 1/15 wave accuracy or the old Brandon star diagonal.

Diagonal from Bradon/Vernonscope.

Last but not least, about their price you have to know that such star diagonals showing a 99% of reflectivity are about six times more expensive than a 96% overcoated enhanced... But you don't really need a reflectivity better than 96% as at the edge of many 2" diagonals the field quality drop drastically and is much worser than the value listed in advertisings.

Here also, excepting by the price difference, a 96 or 99% of reflectivity can not be perceived due to the eye's logarithmic response. You cannot see brightness difference. But knowing all this, if I have the opportunity of buying a 99% vs a 96% or a mirror at 1/4 wave vs the same at 1/10, why select the perfectible model, isn't it ? This debate is valid for everything so the question is relative and can depend of your budget too.

Dielectric coatings

First thing to know, dielectric coatings are resistant and they don't degrade like aluminium. Then they must not be serviced as the coatings cannot be removed. To remove this coating requires repolishing of the surface, and refiguring the surface if it is not exactly flat or spherical.

Roland Christen from Astro-Physics insists on the main advantage of these star diagonals : the mirror will have the same high 99% reflectivity 10 years from now when most aluminium coated will have dropped well below 90% and should be realumined. Markus Ludes from APM Telescopes confirms that testing reflection of good quartz protected aluminium coating showed after 10 years a reflection of about 83-85%. You can even get a worse reflection value when using poor coatings after many years of use.

Using excellent diagonals like the Maxbright for example, you can cleaned it repeatedly without developing a multiple of pinholes and sleeks. The same cannot be said of any aluminium coated diagonal, enhanced, protected or whatever.

Enhanced or not, aluminium coatings are very soft and do not stand up to repeated cleaning. As an added bonus, the dielectric coatings have much less scatter than any deposited aluminium coating.

According Astro-Physics, examination with a laser shows approximately a 5 fold improvement in surface scatter. This is because the aluminium layer consists of a series of droplets on the order of a wavelength of light that scatter it. On the contrary, dielectric coatings are multiple layers of thin film oxides made of very small particles deposited by an electron beam evaporator at a high temperature on the surface.

A microscopic image of a dielectric film surface showing the effect of film thickness on conglomerate growth and size. A left a 37 nm film thickness, at right a 380 nm film thickness. The image is the same 500 nm across the frame for both. Document OSA/Protostar.

The multilayer dielectric concept was developed by the US Army for mirrors optics used in desert warfare where blowing sand, grit or optics emersed in muddy water and then cleaning with a dirty rag would damage any other reflective coating. In fact these coatings are hard, even harder than the underlying glass (we can sometimes scratched). That technology has been refined by several commercial companies and is available to the amateur.

Better, Thomas Baader from the famous geman accessories showed to Roland Christen a dielectric diagonal coated at Leiboldt, Germany, and challenge him to scratch it by vigorous cleaning. But he couldn't. So after have worked with these diagonals, Roland Christen can say they are as close as anything to having nothing between the objective and the eyepiece. Like many advanced users I confirm his opinion.

Gold coating

Just a word about star diagonals reflecting a goldish color. Such a gold coated diagonal is mainly used by the Astropix Vernonscope 2" (Brandon). It is best suited for achromatic refractors to eliminate the focus shift between the blue/violet light and the center part of the visible spectrum. This diagonal is also an excellent filter when observing Mars due to its higher transmission than colored filters. Its price is $385.

I thank Roland Christen from Astro-Physics and Markus Ludes from APM Telescopes for their comments and corrections.

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