APO REFRACTORS, COMPOUND SYSTEMS and the SIMPLE NEWTONIAN
Are those little apochromatic refractors really better than reflectors? They certainly have been advertised as such. In fact, refractor manufacturers have always alleged that reflectors are, well... just a little less than the ultimate - workable, useable, but really not first rate - images just a little sour. And in fact, many a view through a reflector confirms the sour image reputation. Views through refractors are invariably sharp and crisp, neat and gratifying to the eye. But are reflectors really a poor man's telescope, a less than optimum instrument? As you might imagine, I don't think so. And here is why I think not and why the "super little refractor" thing is just another load of advertising hype.
First of all, refractors are generally "little" scopes; 3" to 4" is common. Even the much touted Questar is a little scope at 3.5". Okay, I'm obviously picking on little scopes. Yes, because it's really relatively easy to production manufacture high quality, little scopes that will yield diffraction disks and first order rings. Little scopes can use refracting objectives, which are easier to mass manufacture (no high accuracy aspheric surfaces to diddle with). Small refractor objectives come to thermal stability quickly, and the small atmospheric sections little scopes look through are stable and frequently result in essentially "perfect" seeing. Manufacturers can inexpensively make an attractive tube assembly out of nicely machined parts, and the whole thing put into an attractive, foam lined travelling box which can be thrown into the trunk with the rest of the suitcases; just another piece of luggage - very appealing all the way around.
For their size, little refractors perform excellently; really, they do. And the Questar catadioptric does remarkably well. But all these instruments are limited by their aperture. 3.5" has a resolving power, as defined by Dawes Limit, of 1.5 arc seconds - that's it. Not to mention the limited light grasp. By contrast, a 6" Newtonian has a resolving power of 0.75 arc second, twice as good.
If this is the case, why are so many people buying little "super scopes" and not 6" reflectors? Simple. Reflectors are almost always poorly made. Reflectors have always had the reputation of being an inexpensive scope and most were, and are, cheaply made. They have never captured the market for high quality telescopes. Perhaps some of the best reflectors ever made were the Cave 6" and 8" scopes in the 1950s and 60s, but even these were a price sensitive instrument. Only Questar emerged with a truly first-class instrument in the 1960s, and it wasn't a Newtonian, it was a Maksutov; small, sealed optics, clever mount, neat appearance - and it popped into that cute carrying case and disappeared to the closet. The amateurs dream; and many dreamed about owning one. But the price!. Wow! $1,500 and then $2,000. But... well, it's really a great scope. And in the far recesses of your mind you thought, "It's not a cheap, duddy Newtonian, it must be better, it must be something special, ... it must be worth it."
People still make comments about the Questar that suggest they feel there's some special magic about the instrument. Of course, there is none. Optics is not about magic, it's about design and manufacture. But many amateurs do not view optics this way. Many seem to think there's a certain magic to certain types of systems, something special about a Maksutov, or a Cassegrain, or a refractor, or some other exotic system. Something other than a Newtonian. Believe it or not, I can tell you that the most intrinsically perfect system for astronomical work is a Newtonian reflector. There is simply nothing better. The problem is that over the years people have come to see the Newtonian as an inexpensive instrument, and along with that comes the tail chasing logic confirming the truth of the assertion - Newtonians generally do not perform as well as other instruments. But why?
Let's look at some of the advantages connected with Newtonian reflectors that make them superior to virtually all other optical systems as a serious lunar/planetary telescope.
Aperture Size: The Newtonian reflector can be made in large enough sizes so as to satisfy the fundamental requirements for serious lunar, planetary and binary star observing without experiencing extremely high costs. Refractors become insanely expensive in sizes over 6". The same can be said for Maksutov's. High-quality Schmidt Cassegrain telescopes can be purchased in large sizes and at reasonable costs, but the optical quality is frequently low.
Quality of System: The Newtonian reflector, if the optics are fabricated correctly and the tube correctly designed, can produce a total system wave-front at the eyepiece of 1/8 wave, PV (peak to valley) or better. The reason this frequently fails to be the case is the fact that the primary mirror, the heart of the Newtonian system, is often not made up to proper standard, and the reason for that is the vast majority of purchasers will simply not pay enough money to ensure a properly made mirror. For example, I charge $795 for a 10" f/6 or f/8 mirror. A Maksutov system of the same aperture would certainly cost considerably more, and in the end not likely produce an equivalent wave-front. Not only that, but the secondary obstruction would probably be significantly larger, degrading the wave-front as far as resolving planetary detail is concerned. Another advantage of the Maksutov or Schmidt Cassegrain is it is a closed system; the tube is sealed at both ends from dust and dirt. But if the user of the Newtonian telescope understands his instrument, this is not a problem. It is only a problem if one wants a telescope you can put a cap on and stuff into a suitcase. But a 10" instrument is not something you stuff into a suitcase.
Secondary Obstruction: The Newtonian reflector requires a smaller secondary obstruction than any other reflecting telescope design. Compound reflectors of the Cassegrain and Gregorian types, either in classical or Schmidt Cassegrain or Maksutov configurations, require secondary obstructions having a physical size of at least 25% to 35% of the diameter of the primary mirror. The best one can reasonably hope to attain in a classical Cassegrain system is 25%. Anything smaller than that results in a huge effective focal ratio in excess of what is considered suitable for all-around use. If one wishes to have a system of f/25 or higher, then secondary obstructions of less than 25% can be obtained (but curvature of field increases far beyond the relatively flat field of a Newtonian). The same goes for any other compound system, Schmidt Cassegrain or Maksutov. Newtonian reflectors, by contrast, even of f/6 design, can easily accommodate secondary mirrors less than 20% of the diameter of the primary mirror and still give full illumination to a reasonable field of view. For focal ratios of f/8 and higher, secondary obstructions of 15% and smaller can be attained. No other reflecting optical system can do this without resulting in extreme proportions. The impact of the secondary obstruction on observing is most readily noticed in attempting to resolve fine, low contrast planetary detail. For binary star work some have claimed that a large secondary obstruction actually enhances performance since it slightly shrinks the apparent size of the Airy disk, throwing the light into the first order diffraction ring, and thereby somewhat increasing the apparent resolving power of the instrument. That one possible case inside, telescopes having central obstructions in excess of 25% begin to degrade the image noticeably in the arena of low contrast resolution. When the size of the secondary reaches 30% to 35%, as it does in many commercial Schmidt Cassegrain telescopes, the drop in the ability to resolve fine detail becomes extremely noticeable. No matter how good your optical system may be, the mere presence of such a large secondary mirror may degraded the final wave-front to an effective quarter wave, PV, or less.
Tube Length: the length of a Newtonian reflector is basically governed by the focal length of the primary mirror. The instrument is not a compound design, so the effective focal length is actually the focal length of the primary mirror. In order for the instrument to perform well for lunar and planetary observing the focal length must have a ratio of at least six and preferably eight times the diameter of the primary mirror. This simple equation demonstrates that an 8" instrument will have a physical length of approximately five feet to six feet and than a 10" instrument will have a length of approximately 5.5 to seven feet. By comparison, a Schmidt Cassegrain or Maksutov of equal aperture will have a length of from three to four feet. This is a deficit that manifests itself in difficulty of transportation and mounting. For transportation the problem can only be cured by making the tube so that it comes apart in two or more sections.. I have done this with my own 10" f/8 reflector and it works very well. The two halves of the tube are held together by four screws with wing nuts and disassembles and assembles very quickly.
Coping with the Open Ended Tube: The Newtonian reflector suffers from the fact that whether one uses a solid tube or a truss tube, the optics are open to the effects of dew, air currents, dust, dirt, and whatever comes along. This is considered to be a defect when compared to the closed system of the refractor, Schmidt Cassegrain or Maksutov design. The advantage of the closed system is that the primary mirror and secondary mirror are kept sealed and remain clean. Added to this is the idea that tube currents cannot form and degrade the final wave-front. Years of use and experimentation have shown that these problems can be dealt with effectively by designing a telescope tube so that the primary and secondary mirrors can be covered with a removable cap of some kind. A neat trick for the secondary mirror is to find a metal can or plastic, straight-sided bottle having an inside diameter slightly larger than the circular diagonal holder. If you are using a metal can it should be painted inside and out so is not to rust. If you are using a plastic bottle, cut off the top with a saw. The inside of the container is then lined with felt held to the sides of the container with double sided tape. The thickness of the felt is built up until the container fits snuggly over the diagonal holder. You now have a cover which can be quickly slid on and off the diagonal. Once you have made a cover for the primary mirror your telescope can be kept essentially sealed when not in use. I have designed these covers for an open truss tube telescope, but they can be made to work for closed tube telescopes as well.
Air currents are best dealt with by the use of a fan located at the bottom end of the tube blowing outward. Details regarding the construction and operation of such a fan are described in the Optimum Scope section. I have found the use of an exhaust fan to be the final step in producing a reflector which will give absolutely refractor-like performance. In the past, many have suggested the use of a plane parallel optical window to seal off the tube. I have studied this option and have concluded that while the use of a window will allow the secondary to be mounted in such a way so was to not produce spider vane diffraction, the window introduces its own optical imperfections not to mention the fact that closing up the tube does not solve the problem of thermal stabilization of the primary mirror. The primary mirror of a 10" instrument contains a considerable mass of material which will continually give off warm air for a sustained period of time. It has been found that the entire telescope must come to thermal equilibrium before the best results are obtained. Even my 6" refractor has openings at the top and bottom of the tube so as to allow the interior of the system to come to thermal equilibrium. This allows the objective lens combination to stabilize very quickly. Once the telescope has reached thermal equilibrium the openings are closed. Such a ventilation system would have to be installed in a reflector using a plane parallel optical window. But, I must emphasize that the problems would be greatly exaggerated in a reflector inasmuch as in a refractor the only optical element of consequence is located at the very top end of the tube, not buried at the bottom, and gives its heat off comparatively quickly.
Adjusting the Optics: The adjustment of the optics for a Newtonian reflector is extremely simple. The only optical element that requires regular attention is the primary mirror, and adjustment here is easily accomplished by twisting the three wing nuts or screws at the bottom of the cell. When using an f/6 instrument and above, collimation is a relatively painless affair. It is really not necessary to become deeply involved with ultra precise alignment techniques. Using an old film can with a 1/16" hole drilled through the cap and the bottom of the can cut out provides an adequate method of placing the eye at the optical axis. The only thing that needs to be done is to adjust the primary mirror until the diagonal appears to be centered within the mirror. The diagonal is adjusted so that the eyepiece holder tube appears to be centered within the diagonal (which will appear circular when viewed through the hole in the center of the film can cap). This process is most easily accomplished during broad daylight, but can also be done at night as well. How well Newtonian optics remain in alignment is strictly the result of how well the mirror cell is designed. Many mirrors cells are poorly designed and manufactured, are difficult to adjust, rapidly fall out of adjustment and lead to frustration in use. Once again, the telescope develops a bad reputation because it is frequently poorly constructed. For tips and suggestions on cell design the reader is referred to the section on A Simple Mirror Cell. As far as those reflectors that use globs of caulking material to hold a mirror to a particle board disk, I really don't know what to say here that would be fit to print - don't do it, please.
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While a Newtonian reflector of aperture and design proportions sufficient to function as a serious instrument for lunar and planetary observing is not going to be as readily portable as a small refractor or Schmidt Cassegrain or Maksutov instrument, such an instrument will optically match or out-perform all other forms of astronomical telescopes inch for inch of aperture in larger sizes. The problem is that such a Newtonian reflector requires slightly more care and consideration in use, but will be considerably less expensive to construct than any of the other telescope types. The point to emphasize here is that the Newtonian reflector is in no way a substandard instrument when compared to other compound reflecting optical systems or refractors. It is every inch the equal of these instruments, and, I believe, in many ways superior. Design the instrument well, construct it out of quality materials and with care, and fit it with quality optics. Give the instrument chance and it will absolutely amaze you.