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Ritchey-Chrétien or Dall Kirkham? 

Often you read about Dall Kirkham telescopes not always accurate and correct statements. Accordingly, the Dall Kirkham telescope would be the ultimate telescope, the "photo machine" for astrophotography with the best picture quality, the highest available field and the best star images. Unfortunately, the reality is different .....

Since not everyone is familiar with the effects of various telescope systems, we want to look in the following at some differences between the so-called optimized Dall Kirkham systems and Ritchey Chrétien systems.

Although the optical mirrors are easier and cheaper to produce, the Dall Kirkham design could not prevail in upscale astrophotography and especially in the field of research. This is due to the forced presence of optical lenses in the optical path and the associated chromatic aberration. The lenses also restrict usable bandwidth. The Dall Kirkham is a "cold" system without the built-in lenses is completely blind.
The Ritchey-Chrétien design with two hyperbolic mirrors offer the great advantage since no lens system is necessary and therefore no chromatic aberration (aberration by different wavelength or colour which is refracted differently) occurs. In addition, the coma aberration is corrected completely.
The shape of the off-axis star is much more rounded and punctual as for example in the classic Cassegrain, Dall Kirkham, or even more so in Newton.
All major observatories and research institutions around the world work, and. not without reason, almost exclusively, with telescopes of the Ritchey-Chrétien design.
 

The image field 
Optimised Dall Kirkham systems often have only one frame of 42 mm sometimes 50 mm, but with significant vignetting. As a minimum a field of 52 mm can be regarded as standard in this day and age. 52 mm corresponds to the diagonal of a full-frame sensor which is used in many DSLR cameras and astrocameras. CCD chip of the new generations that will probably be available at affordable prices on the market in a few years, have an edge length of 45x45 mm. This corresponds to a diagonal of 64 mm. Whoever wants to use a telescope with such cameras should already make sure that an image circle of at least 64 mm is approached vignette-free.
All Alluna RC telescopes from 16 to 24 inches can illuminate a plane, coma- and astigmatism-free field of view of 65 mm. For special applications even a field of 90 mm diameter is possible with a modified baffle and a 5 inch corrector, but at the cost of higher obstruction. This is calculated by the surface area more than three times as large as that of a Dall Kirkham.
 

Vignetting 
What is often concealed is the significant vignetting of current Dall Kirkham the expense of a f 6,8 aperture. It restricts the actual image field for well-lit astro images or photometric applications still further. The fact is that most Dall Kirkham telescopes on the market with an image field of 42 mm have vignetting of over 15%! Alluna Ritchey-Chrétien telescopes vignette 4% at 42 mm and only 10% with a field diameter of 60 mm! 
By the way, you can use each Alluna RC telescope at f 8,0 and with a reducer and fast f 6,0 aperture. There are no restrictions at f 6.0, the high image quality without astigmatism and field curvature are retained. 
 

The spot size 
In advertising Dall Kirkham are often compared with corrective lenses with the Ritchey-Chrétien without corrector. In this context one speaks of a of a "dramatic difference"! According to this montage the spots of a Dall Kirkham would be about 10X better than that of a Ritchey-Chrétien telescope. 
Comparing a DK with corrector (without it is blind) with an RC without corrector is not a correct comparison and is misleading.
The fact is, the real RC telescopes with corrector have a smaller spot size over a larger field of view than most Dall Kirkham telescopes. The RMS diameter of the RC is below 5 µ polychromatic defective while the Dall Kirkham at the same distance has more.
An advantage of an RC telescope is that it can be used without corrector on a large field of view and thus reflects light the UV up to the far infra-red. There are no colour changes on account of a built-in lens system as is essential in the Dall Kirkham. 
The RC system is universal, it can be used in almost all cases of amateur astronomy and astronomical research. It can be used, for example, with spectrographs or in astrometry to achieve symmetrical star images. And it is not without reason that the best astrophotographs are taken with Ritchey-Chrétien telescopes. 
 

Collimation 
It is often claimed that the adjustment of a Dall Kirkham telescope is much simpler than the Ritchey-Chrétien telescope. 
But the fact is that is that a CD needs to be adjusted just like an RC and the collimation is mainly dependent on the quality and stability of the OTA. Also, the location and the centring of the inbuilt corrective lenses in the DK is very critical and must be pretty exact. This is also a reason why no modern and more accurate secondary mirror focussing on the optimised Dall Kirkham is used.
The Alluna Ritchey-Chrétien high-quality telescopes can be adjusted perfectly within three minutes. The high-quality OTA collimation remains permanently stable. By the secondary mirror focusing and a fixed and large M100 connector on the back plate of our Alluna RCs heavy weights such as CCD cameras can be used without affecting the collimation. Each piece of equipment, whether heavy CCD camera or spectrograph, is connected firmly and without deflection with the telescope. 
 

Why does Alluna Optics not manufacture Dall Kirkham telescopes? It would be easy for us to build according to the simple and significantly cheaper Dall Kirkham design. The poorer optical performance, the mandatory presence of lenses in the optical path, the limitations of use, and not future-proof design make a Dall Kirkham system always makes for a telescope of second choice. Alluna Optics builds on quality and a universal and future-proof telescope system with unlimited use and superb optical and mechanical performance. The construction of Dall Kirkham telescopes would preclude this.

 

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