Seeking an "Ideal" Telescope
Many of us have in mind what we regard as our "ultimate" or "ideal" telescope. For now it may be just an object of lust or perhaps you're lucky enough to already own it. In any event, it's interesting to consider the rationale for that decision, since each of us has our own reasons for wanting a particular type of instrument. The following article reflects my thoughts about telescope designs and the ideal telescope for me. It would be interesting to have more articles on this - to find out about your ideal telescope.
I like the sharpness of images in a refractor, but wanted more light grasp and resolution than in my trusty 4-inch achromat. And while it would be great to have the big light grasp afforded by a really large Newtonian, I get tired of the awkwardness of even my current 10-inch scope. I've also discovered that I have a problem maintaining my balance if I have to stand on a ladder in the dark to peer through the eyepiece of a large scope aimed near the zenith. Therefore, my ideal telescope would offer the best possible image quality in a fairly compact package. So I started looking at different telescopes and corresponding with people who have first-hand experience with the various products.
One design that I did not consider at all is the Schmidt-Cassegrain. They are compact and extremely versatile telescopes; some people have done excellent observing and imaging with them. But I have never seen an SCT that can compete with any other design in terms of image sharpness (assuming all the candidates are of good quality and properly collimated). This is mostly due to the large central obstruction. In the first edition of Dickinson and Dyer's Backyard Astronomer's Guide, they describe an experiment conducted using various sizes of central obstruction in front of a 7-inch Astro-Physics APO. A 20% linear obstruction emerged as the critical point where the image begins to degrade. Most SCTs have a 33% obstruction.
During my research, I often came across rave reviews of the Maksutov-Newtonian design. They said the image quality was the equivalent of comparably sized apochromatic refractors, with less weight and cost. Like refractor lenses, the Mak-Newt system uses spherical optics, which are easier to make to exacting standards than a Newtonian's parabolic mirrors. The central obstruction is also relatively small (~20%).
The drawback is that because of the thickness of the mirror and corrector plate, and length of the closed tube, the time required for these scopes to acclimate to the outside temperature is especially long. If there's a significant difference between indoor and outdoor temperatures, these scopes have to be left outside for an hour or longer before they give acceptable images. Some have also noted that the images deteriorate for awhile during the course of a night's observing if there is an appreciable change in temperature. This also applies to the Maksutov-Cassegrain design. A partial solution is to include a fan to induce greater airflow within the tube.
Matt Tarlach, formerly with Orion Telescopes in California, put it this way: "There do seem to be good reasons why the Mak-newts are inherently more prone to degrading tube currents. A refractor's lens is exposed up at the top of the tube, and can radiate directly to the sky. It also tends to have more glass-to-metal contact around the periphery of the objective, which helps conduct heat away. The mirror in a Mak-Newt is down in the bottom of the tube where it cannot radiate, even through the corrector, which is not very transparent in the IR. Also the mirror tends to have less contact with the tube so less opportunity to conduct the heat out. The Mak-newt also has the diagonal and support, of notable thermal mass, confined inside the tube. Finally, in the refractors the light path only travels the length of the tube once, and as it converges it quickly moves away from the tube wall. In the Mak-Newt the light path travels the length of the tube twice, with one trip being right along the side of the tube, so the effects of any tube currents are more than doubled compared to the refractor."Intes 7-inch Maksutov-Newtonian
There are several suppliers of Maksutov-Newtonians, and in most cases, the optics are made in Russia by either Intes or Intes-Micro (two different firms). Orion Telescopes formerly sold a very nice 6-inch Mak-Newt that was attractively priced. Another American supplier is ITE (Internet Telescope Exchange), who will customize the telescope with their own carbon fiber tube, which weighs less than half of the normal aluminum tube. Carbon fiber (graphite) is more dimensionally stable and allows the scope to be used on a lighter mounting. Moreover, they will assemble the telescope in any way you wish, optimizing it for either photographic or visual use, and ensuring that all your eyepieces will come to focus. Best of all, this scope in the 7-inch size is almost half the cost and half the weight of a 6-inch apochromatic refractor.
However, users report that since carbon fiber is a better insulator than aluminum, it actually adds to the cool down time of the optics. When I spoke to the owner of ITE, he said that the 7-inch version would be about as large as I could go, given the temperature differentials in the Midwest. An added consideration is that published analyses indicate that a typical compound telescope such as this will lose about 15% of the available light due to absorption and scattering.
Another telescope that ITE sells is the off-axis parabolic reflector. Here there is no central obstruction because the secondary mirror is mounted outside the light path.Diagram of off-axis parabolic reflector
The cost of the 7-inch off-axis scope is about $3500 just for the tube assembly, but that's still less expensive than a comparably sized APO refractor. Reports indicate that the images are outstanding. But this comes with a couple of drawbacks. Collimation is more sensitive and trickier than with the simple Newtonian, although the secondary mirror never has to be adjusted. Also, the off-axis configuration requires that the tube be larger than a normal reflector; thus even the 5-inch version of this scope has a tube eight inches in diameter. Finally, it will be affected by the same thermal issues as a conventional Newtonian.
There are some persuasive reasons why a really good refractor would be ideal. The Chinese-made 6-inch achromatic refractors sold primarily by Celestron are certainly a bargain. The image quality at low power is pretty good, but if seeing conditions allow high magnification, the image sharpness breaks down. This somewhat defeats the very reason for owning a refractor. Plus there is the false color typical of all medium focal length achromats. To get something relatively compact means it'll have to be apochromatic (APO), which also has the advantage of concentrating the most light possible into the Airy disk. But a 6-inch apochromat can be pretty heavy. Then there's the matter of its considerable cost. An alternative is the Meade ED refractors, but while they're advertised as being "apochromatic", they are not true apochromats. In addition to inducing more false color than a true APO, Meade refractors have suffered from poor quality.
As I considered the different telescopes, I realized that for every one of them, there was always a tradeoff. None were perfect. To arrive at an ideal scope, it's necessary to decide upon what attributes are most important to you and weigh them accordingly. One of the things that I most like is the ability to remain seated while observing in virtually any area of the sky. This is relaxing and helps your concentration. Another consideration is that I don't want a particularly large or bulky telescope. And image quality is more important to me than light gathering. Finally, my primary interest is visual observing, but if I choose to do astrophotography, refractors are easy to optimize simply with the positioning of extensions on the tailpiece. As a result of these preferences, I kept coming back to a refractor as my telescope of choice.
Another issue is light grasp, which is the overwhelming factor in deep sky observing. The various reviews have many comments about a 6-inch refractor being the equivalent of at least an 8-inch Newtonian. The following is the rationale:
- If a 6-inch objective lens has the normal 95-96% transmission and an 8-inch Newtonian has 85-88% combined reflection on both mirrors, then taking into account an average 22% secondary obstruction, both scopes have pretty much the same light gathering ability.
- A typical reflector has more light scattering.
- A refractor usually has a smoother wavefront than an aspheric reflector.
- A 6-inch refractor is less sensitive to atmospheric turbulence than a larger reflector.
- Without a central obstruction, more light is concentrated into the Airy Disk, thus providing superior contrast.
If you've managed to plod through this article so far, you've noted my bias toward a 6-inch apochromatic refractor. That decision was made a few years ago, and in September 1999, I placed my name on the waiting list to order an Astro-Physics refractor. A-P's waiting lists are legendary, and I have yet to hear from them. Bear in mind that the list is for the privilege of ordering an A-P scope; once ordered, you have to wait for another year (but probably longer) to actually receive it.
In August of 2000, I gave up on A-P and ordered a T.M.Back 6-inch APO. While Thomas Back designs the optics, they are manufactured in Russia by LZOS. The tubes are machined in Germany, and Thomas Back assembles and individually tests each scope at his location near Cleveland. Reports indicate that the TMB scopes are optically the equivalent of A-P and Takahashi. I received the TMB in March 2002.
For some of us, it also boils down to a certain aesthetic (or perhaps "emotional") sense with a refractor. One amateur astronomer, P. J. Anway, explains it this way: "Owning an 11" SCT for years and a 10" newt before that, I agree aperture has it's advantage; but there is a magical 'something' about looking through a good quality refractor. The visual 'wow' is definitely part of it. But for me, there is also a historical link that moves the soul. Galileo first discovering lunar craters with his 1.75"; Struve logging his doubles with the Fraunhoefer; Lowell checking on the Martian canals; Galle finding Le Verrier's planet; or even Peltier, carving another comet into his 4". All these enter the subconscious and sometimes the consciousness as one settles in behind the eyepiece. It's no wonder that it was about the refractor that Oliver Wendell Holmes penned the words: 'But most I love the tube that spies.....'."
Is it the perfect telescope then? Well, no. There is no such thing. A large Newtonian will blow the 6-inch APO out of the water when it comes to deep sky objects. A larger Newtonian with very good optics will also best a 6-inch APO on the planets because of its greater resolution. I sort of cheat when observing deep sky objects by using a Collins I3 Image Intensifier eyepiece that allows me to go much deeper to catch elusive faint fuzzies. The tube of the TMB 152 weighs as much as my 10-inch Newtonian and is heavier than comparable Takahashi and A-P refractors. The tailpiece with 4-inch focuser is designed to be absolutely rigid so as not to sag under the weight of a camera or binocular viewer. But the sheer weight of this tailpiece causes the focuser to move of its own accord when viewing objects overhead, so the focus lock has to be adjusted somewhat tightly. (I've now added an electric focuser.) Despite weight issues, the smaller size of the OTA does make it a bit more manageable than the 10-inch, however.
For me, it is a comfortable scope to use and gives great views, especially of the planets. It's not perfect, but it's probably as close as I'll get to my ideal telescope.Published in the July 2003 issue of the NightTimes