20"f/3 and 25"f/3 Dobsonian Telescopes: Optics





With its huge aperture of 635mm the 25“ f/3.2 dobsonian telescope of the Spacewalk Telescope Horizon series collects more than 11,000 times the light than the naked eye is capable of doing. This is impressive but not the only important aspect, the optical quality of the mirror is a crucial criterium. Only with top optical quality the maximum performance of such a large telescope can actually be tapped into, making it possible to use all of its potential. This means reaching cuiper belt objects, unvealing remote galaxy clusters and singular stars within the Triangulum galaxy as well as finest details within gaseous nebulae and galaxies, experiences that may have seemed impossible to you before.

This is why the core of all „Horizon“ telescopes is an optical system of very high quality which not only refers to the primary but also the secondary mirror. This combination will fascinate you with sharp high contrast images and show you countless wonderful details that will surprise you time and again.


1) The Primary Mirror


Every primary mirror within an „Horizon“ telescope is made in Germany within the facilities of Spacewalk Telescopes. The high quality blank disk is carefully ground, polished and given its parabolic form there, creating a precise and unique mirror for your telescope.

During the whole manufacturing process the optical quality of the mirror is constantly monitored. This includes inspections of the surface quality during grinding and polishing processes as well as several interfeometrical analyses of the parabola when giving the mirror the desired parabolic shape. Especially the final measurement is of great importance since this is where the information about the quality of the finished product is determined. This is why a lot of effort is put into the measurement, averaging many interferograms to define the Strehl-ratio and therefore the performance capability of the mirror as precisely as possible.

You will receive an intererometric test protocol including all information an a specific description of your unique mirror in print.

The primary mirror is declared finished when the Strehl-ratio measured by Spacewalk Telescopes is at least S=0.90 for the 20“ model and S=0.85 for the 25“ model .

Another important and often neglected aspect of a primary mirror is its edge thickness. The Spacewalk Telescope primaries have an edge thickness of 31mm or 35mm depending on the aperture and therefore qualify as very thin. As a result they cool down incredibly fast, giving you the opportunity to use even high magnifications already after a very short time.


2) The Secondary Mirror


Concerning the optical performance of a telescope not only the primary but of course also the secondary has to be considered. This fact is often being neglected but nevertheless critical.

The secondary mirrors integrated into Spacewalk Telescopes' instruments are sourced from a renowned manufacturer in the USA and have a surface accuracy of better than lambda/10. Optionally an even better quality is available at an extra charge.

Every secondary mirror also comes with a test protocol informing you exactly about its optical performance.