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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