Ritchey Chrétien or Dall Kirkham ? 
 

Statements that can be read about Dall Kirkham telescopes aren't always applicable and correct. If we believed it, the Dall Kirkham telescope would be the ultimate telescope, the “photo machine” for   astrophotography featuring best image quality, biggest available usable field and best stellar images. However, reality sometimes looks different ..... 
Since not everybody is familiar with the mode of operation of different telescope systems we would like to discuss some differences between Dall Kirkham systems and Alluna-Ritchey-Chrétien systems in detail.. 

The Dall Kirkham system is a new system
It is argued, that a Dall Kirkham system with built-in corrector is a “new” and “revolutionary” telescope system. In fact, the DK system was developed in the 1940s independent of Horace Dall and Alan Kirkham. Back then already, experiments with lenses for correcting existing image errors and various conic constants were performed.
Although the optical mirrors are easier and cheaper to produce, the Dall Kirkham design never could establish in the up-scale astrophotography and particularly not in the research area. The reasons are the compulsory existence of optical lenses in the optical path and the poor usable overall balance of the Dall Kirkham design. It's not for nothing that all major observatories and research institutions more or less solely use telescopes based on the Ritchey Chretien design. 

The Field of Image
Often, Dall Kirkham systems are presented with a “huge” field of image. According to such statements a 20 inch Dall Kirkham telescope would have a field of image of 42 mm. Some other Dall Kirkhams are presented with a field of image of 50 mm in adverts. However, nowadays these are no really big fields of image any more! 
In our opinion a 20 inch Cassegrain telescope with a field of image of only 42 mm isn't acceptable any more and far from to be called future-proof. The diagonal of a current 40x40 mm chip is already 56 mm. CCD chips of the newest generation come up with an edge length of 45x45 mm. It requires a field of image of at least 64 mm for completely illuminating such a surface! 
Alluna Ritchey Chretien telescopes from 16 to 24 inches all feature a planar 65 mm field of image free of coma and astigmatism. Based on the field of image that's more than twice the size than that of a Dall Kirkham ! 

Vignetting 
The considerable vignetting of today's Dall Kirkham systems that is needed in order to maintain an f 6,8 aperture often remains unspoken. It further reduces the effective field of image for well illuminated astronomical exposures. As a matter of fact, most Dall Kirkham telescopes on the market show a vignetting of more than 15 % in case of a 42 mm field of image! 
Alluna Ritchey Chretien Teleskope show a vignetting of 4 % in case of 42 mm and only 10% in case of a field of image with 60 mm diameter! 

Spot Size 
In advertising Dall Kirkham telescopes with corrector lenses are often compared to Ritchey Chrétien without corrector. A “dramatic difference” is claimed in this connection ! Based on this montage the spots of a Dall Kirkham would be ten times better than those of a Ritchey Chretien telescope. 
However, comparing a DK with corrector to an RC without a corrector isn't a proper comparison and misleading. 
It is fact that real RC telescopes with corrector feature a smaller spot diameter throughout a larger field of image than most Dall Kirkham telescopes. The RC's RMS diameter is less than 5 µm polychromatic while the Dall Kirkham features more at the same distance. 
It is a further advantage of an RC telescope that it can be used on a large field of image without corrector  and that it reflects light from UV to long-wave infrared.  No colour changes are effected by a lens system. A Dall Kirkham doesn't allow this. The Dall Kirkham is a “cold”, modified system, in other words, not a reproducing system and without corrector lenses in the optical path it is blind and therefore unusable. 
The RC system, in contrast, can be used in almost all applications within the area of amateur astronomy. For example, it can be used with a spectrograph or in astrometry for receiving symmetrical stellar images. It's not for nothing, that the best astro-photos were shot with an RC telescope. 

Collimation 
It is often claimed that adjusting a Dall Kirkham telescope shall be much easier than adjusting a Ritchey Chretien telescope. 
It is a fact, however, that a DK has to be adjusted in exactly the same way as an RC and that the collimation mainly depends on quality and stability of the optical tube assembly. In addition, position and alignment  of the required corrector are quite critical in case of a DK and must match exactly. That's another reason why the modern and much more precise secondary mirror focusing isn't used in connection with a Dall Kirkham telescope. 
Alluna Ritchey Chretien telescopes can be adjusted perfectly within three minutes (here  you can find a manual, how easy it is.) Thanks to the high-quality optical tube assembly the collimation remains durably stable. The secondary mirror focussing together with a fixed and large-sized M100 connection at the backside of our RC telescope systems allow to use heavy weights as for example a CCD camera can be used without any influence on the collimation. Any equipment, be it a heavy CCD camera or a spectrograph, everything is fixed and connected to the telescope without any deflection. 

The Price
It is said that RC telescopes shall be quite expensive due to two hyperbolic mirrors. 
It's an assertion that every customer can easily figure out himself..... 
Every customer who is ready to invest into a telescope of this size category with quite considerable expenses should inform himself precisely in advance. Check the system's optical performance ! Will you be able to use current and future CCD cameras, too ? Compare the mechanical quality of the optical tube assembly, the functionality of the software and the technical data. 
Even if you don't like to think about it when you buy – which telescope features the better maintenance of value ? We are sure: a Ritchey Chretien telescope "Made in Germany" will be good value for many years. 

Why does Alluna not produce Dall Kirkham telescopes? 
It wouldn't be a big deal for us to build telescopes based on the simple and cheap Dall Kirkham design. 
However, the worse optical performance, the small usable field, the limitation with regard to the usage, and the construction that is not future-prof will always make a Dall Kirkham system a second-choice telescope. At Alluna Optics we set priority on all-purpose and future-proof telescope systems with unlimited usability and best optical and mechanical performance. 

Synergy at work 
By the way, Alluna-Optics is one of the few manufacturers who develop and produce their telescopes as overall systems from concept to realisation. Starting with the optics through mechanics up to electronic and software. Everything from one manufacturer. 
Each production process starts with the optics requested by the customer already manufactured and tested. The optical tube assembly will then be “tailor-made” on the basis of this optics. Thus the final product is perfectly adjusted with each of its components. 
Other telescope manufacturers mainly concentrate on the production of the optical tube assembly. Optical components such as the mirror set are then bought from component suppliers and simply mounted into the optical tube assembly. This so-called “assembling“ often results in too big production tolerances and too wide diffusion in the overall performance. That's the reason why one telescope works better and another worse. 
With an Alluna-Optics telescope you will receive a high-quality product without doubt.
 

Alluna Ritchey Chrétien Teleskope