Frequently Asked Questions
What does it mean that the mirror surface is made with precision λ/8?
λ stands for the wave length of visible light, generally considered to be 550nm, where human eyes are most sensitive. No optical surface is perfect; you can always find tiny depressions and elevations on it. If it is stated that a mirror surface is polished with the precision of λ/8, it means that its deviation from the ideal shape doesn't exceed 1/8 λ. This criterion of surface evaluation is presented as a PV (after “Peak to Valley”). Geometric optics says that distortion of a reflected wavefront is the double of reflecting surface deformation. Wavefront distortion of 1/4 λ (which corresponds to 1/8 λ at the mirror surface) is known as the Rayleigh criterion. Lord Rayleigh was first who showed that wavefront distortion, not exceeding 1/4 wavelength of light, does not cause noticeable deterioration of the image. So it is some kind of a standard under which the quality of a mirror surface should not fall (often referred to as a so-called diffraction limit). Today, however, all optics manufacturers are trying to achieve better quality.
It should be added that the PV criterion can be quite misleading and not accurate. Without more detailed knowledge of the actual shape of an optical surface it is almost worthless parameter. Imagine that on a mirror surface would be for example a miniature bubble with the diameter of 0.1 mm and the depth of 0.1 mm. If we counted this bubble into the total value of PV, we would get an extremely poor result and the mirror would thus apparently look totally useless. But it is not true - the bubble is negligibly small compared to the size of the entire mirror and it has absolutely no effect on the image quality. Even the Rayleigh criterion was derived for one specific type of deviation of a mirror surface (spherical aberration) and so it should be thus understood, although it does not often happen in practice.
For this reason there are other, more precise, criteria used for quality evaluation of optical surfaces, such as the standard deviation (RMS) or the Strehl ratio (SR).
If we use the PV criterion, it is always necessary to recognize, whether we talk about the accuracy of a mirror surface itself, or about a mirror reflected wavefront distortion and also what wavelength L it is related to (it is not always the 550nm). Unfortunately, there is no agreement on how to use particular indicators and various producers and retailers can use different numbers of various meanings.
What is the Strehl ratio?
The Strehl coefficient (also Strehl Ratio or SR), is a frequently used measure of the quality of optical surfaces and it is probably the best parameter if we want to describe the quality of optics by a single number. Let us take a closer look at what it describes. At first glance it might seem that telescope objective display a star as a point. This, unfortunately, is not the case. The wave-like nature of light causes that an objective focused on the focal plane displays a point source (star) as a disk (so called diffraction or Airy disc). Diffraction rings appear around the diffraction disk then. Only 83.7% of light passing through the optical system will get into Airy disc in the ideal case. The remaining 16.7% of light is spread into the diffraction rings. This is, by the way, the reason why the image in the telescope can not be magnified too much - once you encounter an Airy disc then further enlarging does not show any further details. Every imperfection or defect then causes that even less light will get into the Airy disc and the more light will flow into diffraction rings. This further aggravates the situation.
An Airy disk and diffraction rings – on the left a perfect reflector with 30% central obstruction, on the right a half diameter refractor.
The Strehl coefficient is a dimensionless number of values from 0 to 1 (or from 0 to 100%), reflecting the ratio of intensities in the center of a diffraction disc of an optical system burden with defects and a perfect optical system. In other words, if the SR is 100% then the optical system is perfect. However, this value can not be reached in reality. Many producers are currently trying to produce mirrors of the highest quality, with the SR 95% or better. This is certainly very creditable and customers profit from thas. But the fact is that in practice it is not of so great importance. The difference between telescopes equipped with a mirror of the SR for example 0.85 and 0.95 is hardly observable. Poor atmospheric conditions in our countryside, bad seeing and light pollution are mostly the causes of this. This, therefore, substantially degrades such a high-quality mirror. That is why it is possible to recommend even a mirror with the SR around 0.9 in our observation conditions. Such mirror is certainly of sufficient quality and brings satisfaction to observers and it will save a considerable amount of the cost of a telescope. These funds can be invested then into a highly-reflecting coating, or to better equipment - eyepieces, filters, etc. Yet I offer such perfectly cut mirrors to those customers who believe that they can really utilize them.
What are the differences among glass types used for mirror manufacturing?
Many kinds of glass can be used for manufacturing astronomical mirrors. From plain and cheap "flat" glass (float), through standard borosilicate glass (sold under the names such as Pyrex, SIMAX, Borofloat etc.), various "optical" glass such as BK 7 to the most expensive and best quality materials : fused silica ("Fused silica" or "Quartz"), glass-ceramic (BVC - black vitreous ceramic) and ceramic (Zerodur used on the 8.2m VLT telescope) or ULE (Hubble Space Telescope). These excellent materials are extremely expensive and they are rarely used in amateur telescopes.
The number of differences among the glasses is of course high. In addition to a different chemical composition (for users of absolutely minor importance), they vary mainly in physical and mechanical properties. The most important of these is the thermal expansion of the material and the internal stress in the glass disc. A mirror with too high internal stress has a risk that the optical surface will collapse and the mirror will degrade over time. It could theoretically result in the rupture of the disc in extreme cases. The internal stress coefficient should not exceed 20nm/cm in high-quality mirrors.
The thermal expansion shows how a mirror behaves during temperature changes. In amateur conditions a telescope is moved from a house out to an observing site; in the professional conditions, where a telescope is located in a dome, it is exposed to temperature changes during a day. These temperature changes cause deformation of the glass disc, thus the deformation of the optical surface, until the temperature of the glass is (almost) the same as the themperature of surrounding air. Sometimes the temperature fall during the night is so fast that the mirror does not reach thermal equilibrium at all. The lower the value of thermal expansion of glass is, the smaller is the surface deformation during tempering and the better view the mirror gives us.
Examples of linear thermal expansion coefficients for different glasses:
Float, BK7: 7,5x10-6 K-1
Pyrex, Simax, Borofloat: 3.3x10-6 K-1
BVC: 2.4-2.8x10-6 K-1
Zerodur has virtually zero expansion
What is the difference between reflective coatings offered by you?
Reflecting surfaces of astronomical mirrors are formed by a thin layer of metal, fused to the polished side of the glass disc. The most commonly used material for this is aluminum, but in certain cases also silver, gold and other materials. Aluminum has very good characteristics, high reflectivity and it can be easily applied to the glass.
Unfortunately, it degrades relatively quickly in the air – creating a film of AlO2 on the surface, which progressively worsens reflective properties till the mirror is "blind". Therefore the aluminum layer is sometimes protected against the surrounding environment by application of one or more layers of transparent but chemically and mechanically resistant material, such as SiO, SiO2 or TiO2. The standard mirrors offered by me are aluminum coated with one protective layer of SiO2 overcoat. This combination is commonly used and it provides a good ratio of reflection (about 91%), service life (approximately 10 years) and price. Its application is not difficult, and it can be easily restored if necessary. However, it is possible to apply the basic aluminum coat in combination with 2 or 4 protective layers of SiO2 and TiO2. By good choice of thickness of individual layers you can achieve not only very high reflectivity (up to 98%), but also increased resistance of the mirror surface and its longer service life. The production technology of such reflective layers is quite demanding from the production process quality point of view. Application of these layers is provided by excellent optical workplaces. The disadvantages are a higher price and a difficult removal of such fused layers in case there is a need to recoat the mirror.
An often used combination with good cost-to-performance ratio, is a standard coating on the primary mirror and an enhanced coating on the secondary mirror.
Chart of the reflectivity of different coatings:
: Al+SiO2, Al+lamella, Al+double lamella
It is possible to equip a telescope with a navigation (DSC) system?
I have not equipped a telescope with a navigation system yet. I, myself, am the type of observer who likes looking in the map and finder and searches deep-sky objects on his own. Finding a galaxy of the 13th magnitude has always been a huge satisfaction for me. However, everything can be arranged by agreement. If you want to use the telescope with a navigation then I am able to make such adjustments to its structure that would allow easy installation of such system.
I use heavy equipment, is there any problem with weight balance of the telescope?
This is not a fundamental problem. I always try to outweigh a telescope with a medium-heavy eyepiece you will use. The telescope will withstand minor variations on both sides. In a worse case, when you want to use a dobson with now quite regularly used giant eyepieces weighing 1+ kg, the telescope can be fitted with a small counterweight which shall be attached to the bottom dish on the primary mirror side. The mount can be eventually equipped with small pressure breaks. However, I do not recommend such extremely heavy eyepieces to use with a lightweight dobson.
I would like to test one of your telescopes personally before ordering it. Is it possible?
Usually yes, but there is a small problem currently, as I do not have any finished scope at the moment. However, I have a design of a 18" telescope for my personal use and it is a matter of relatively short period of time (2-3 months), when it should be finished. Then it will be possible to watch the stars with my telescope before ordering one (you can visit me at home, I also attend various star-parties). In addition, I plan to make a highly compact 14" UL Dobson of for myself, so hopefully there will be some stuff to choose from :-)
