FOR PLANETARY IMAGING AND VIEWING
some thoughts by P. Clay Sherrod
In the modern age of computerized GO TO telescopes, the thrust for manufacturers and even Amateur Telescope Makers has shifted dramatically to smaller, more compact tube assemblies that can be easily fastened to a mounting that is essentially computer operated. With GO TO mountings requiring the skills and technology to produce them, there has been a regrettable shift in many optical production houses to shift emphasis on mechanics rather than optics in telescope design.
This is not to say that the optics are of poor quality, but the telescopes of manufacturer choice for such GO TO applications are typically Catadioptic design: either the Schmidt-Cassegrain or Maksutov or some radical off-spin of those two basic concepts.
Overlooked for some time is the outstanding performance and overall wide range of applications of a high quality Newtonian-design reflector. These work-horse telescopes are still on the top of the charts regarding overall optical performance to all designs except the expensive and necessarily-smaller-aperture high quality apochromatic refractors.
The Newtonian is good because it is basically simple: a parabolic primary mirror of exceptionally high quality and an equally high quality optical flat that is precisely matched to the user's intended area of astronomical interest.
The keys here are:
1) high quality is easy to build into a primary Newtonian parabola....and easy to verify and test;The overall design of a good Newtonian is paramount to achieving its near-perfect potential. First, buying a Newtonian off-the-shelf today is very difficult....no one makes them in sizes and selections that appeal to everyone. They are made in "one size fits all" categories for the modern "best bang for the buck" of the 21st century consumer. This is NOT the proper philosophy of a discriminating observer...whether it be for imaging, astrometry or for incredible high resolution views and images of the planets and lunar surface.
2) optical flats are available in a wide range of sizes and it is ultimately important, if not essential, to select exactly the right diagonal for the user's intended area of pursuit.
For the latter, there is not a better telescope design possible (other than an equal-aperture refractor of highest quality) than the basic Newtonian....provided it is put together properly for that purpose.
For example, one would not buy a Newtonian fitted with a secondary mirror that is necessarily large to permit wide field prime focus photography....the secondary alone would prohibit the highest resolution
possible from the quality primary mirror. Instead, this user must select the precisely smallest possible secondary mirror to intercept the light cone and determine where that mirror should be mounted relative to the primary for optimum imaging.
At that point the Newtonian becomes the leader in high resolution imaging.
My most outstanding views ever of Saturn might serve as an example here; after years of viewing in telescopes ranging from a 20" Clarke refractor, a 40" Boller and Chivens RC, and virtually every other type of scope on the planet, I happened to luck upon a night in a small private observatory which was equipped with a monstrously long Cave Astrola 10" f/7 Newtonian with an incredibly small secondary mirror....the system had been selected and assembled by none other that famed optician Gene Fair who fought for recognition of large and small Newtonians for quality planetary applications.
We were viewing the ringed planet at 400x magnification through a simple eyepiece...absolutely incredible views of many ring divisions. An incredibly steady night....so how much more magnification could this mirror take? We doubled the magnification with a cheap 2x Barlow lens....800x and then Encke's division was distinct enough to drive a spaceship through and yet another ring division was blooming just outside of that. Could it take more? Incredibly this gentleman just grinned and stuck yet ANOTHER 2x Barlow inside of the first, providing 4x amplification of the original magnification: 1,600x.
And I will never forget that view......nor the fact that this type of telescope is not available any longer to modern astronomers.