After having good results with the design and construction of a 8" astrograph, I decided to start a project for a larger telescope that will allow to image smaller objects, like galaxies and planetary nebulas. My first approach was to replicate what I did previously with a corrected newtonian, but after talking deeply with many well know astrofotographers none of them recommended a newtonian configuration for small objects. There could be many reasons, from the optical design to mechanical complexity, that will drive you to different telescope designs, but fundamentally small objects require long focal length. Long focal distances are very demanding with tracking performance and always fighting against atmospheric seeing. It is highly recommended the use of adaptive optics, if available, which are not compatible with any newtonian configuration with wynne correctors due to the short back distances they have.
A large focal distance newtonian will have a very large tube. An apochromatic refractor will be prohibitive, if not possible, for 16 inch diameter. For large focal distances you should drop into one of the cassegrain categories. You can choose from many variants:
- Schmith cassegrain telescopes are very popular. They have very short tubes with moderate optical performance. The optical design consists of a schmith front corrector plate, a spherical concave primary and spherical convex secondary. The result is a large focal ratio in a compact design with moderate optical and chromatic correction. Recently, due to large area sensors, some of these popular telescopes are offering field lens correctors that are located close to the focal region, improving the correction of optical aberrations and offering better quality. This converts this telescope to subaperture catadioptic telescopes.
- Classical cassegrain consist of a parabolic concave primary and hyperbolic convex secondary. Cassegrain telescopes correct optical aberrations down to diffraction limit for small fields, making them ideal for planetary images at very long focal ratios. For fast configurations, the convex hyperbolic secondary turns to be very difficult to manufacture, and coma and astigmatism grow very fast. The addition of subaperture correctors converts a classical cassegrain telescope into a nearly perfect optical design for very large fields, at the cost of manufacturing complexity.
- Ritchey Chrétien telescope are the most complex to manufacture. They have a concave hyperbolic primary and convex hyperbolic secondary. The benefit of this cassegrain configuration is that coma is nearly nulled, only remaining astigmatism with moderate dimensions for medium fields. A RC scope has good optical performance without any subaperture corrector, and no chromatic effects, and thus a very wide useful spectrum. This is the reason why most of the professional telescopes are based on a RC configuration. The use of a subaperture correctors make them even better.
- Dall Kirkham are being more and more popular. They are the most easy to manufacture and the result is vey good. DK have a prolate ellipsoid concave primary, a shape between a spheroid and a parabola and thus very easy to manufacture. The secondary is spherical and thus very tolerant to misscollimation. In contrast to classical cassegrain or RC's, Dall Kirkham needs always to have a subaperture corrector to perform good optical quality. The use of two meniscus lenses result in a nearly diffraction limited telescope for large fields, at the cost of strict centering and tilting of them. A good mechanical design makes a DK one of the best choices.
Despite I was searching for a long focal length, and what I could afford from the manufacturing point of view, I liked the idea to design an hybrid telescope that could be turned onto a fast newtonian or a cassegrain. So I decided to go for the classical cassegrain design with a fast parabolid primary that can be used as an astrograph with a wynne corrector, and converted to a corrected cassegrain with a subaperture corrector. Exchanging the secondary mirror in a very fast and easy way will flip from one design to the other.