Copyright 2000 By
Mark Christensen, Ph.D.
Over the last few months we discussed the history of Bernhard Schmidt, the astrophotographic camera he invented, and the design revolution in astronomical optics that his creation sparked. From the standpoint of the amateur astronomer there are, however, several drawbacks with many of the designs that resulted. Thus in the case of the original Schmidt camera the focal plan is curved and, for any desirable focal ratio, all but inaccessible, making it essentially impossible to use a standard camera body to hold the film. This problem is especially severe for focal ratios (that is, the focal length of the camera divided by the diameter of the corrector plate) less than about f/3. Also, the tube must be at least twice as long as the focal length. As a result, for focal ratios greater than f/3, the tube becomes unwieldy. Finally, the mirror, while spherical and easy to make, must be something like 30%-50% larger than the corrector, adding extra weight to the camera and demanding a better mounting. If you add to these problems the fact that the aspheric corrector is difficult to make it is not hard to see why so few classical Schmidts cameras are in use by amateurs, as well as why so few new professional instruments are built. The other cameras based on the Schmidt principle have the same problems, with the sole exception of the Wright Schmidt.
While Celestron did briefly make a range of classical Schmidt cameras (if my memory serves they had clear apertures of 5"-8" and focal ratios of about f/1.5) and were sold with special curved film holders. Celestron ceased production of these cameras in the 1980's. To my knowledge, such cameras are not being currently manufactured for retail sale by anyone. The same company made a 5.5" f/3.3 Wright Schmidt camera at the same time that can be used with a standard 35mm camera.
It is likewise out of production. With CCDs currently being all the rage (despite their miserably small format and resulting small field) it is difficult to imagine that this will change.
Since these items are no longer made and nothing has replaced them the lens manufactures have moved into the market, selling very well correct 200 and 300 mm f/2.8 to f/3 lenses made of exotic glasses and priced to match, beginning in the realm of $1,500. The clear aperture of these lenses is in the range of 100mm, or 4 inches.
However, there is another way to achieve high speed (low f/-number) and wide fields. In the 1930's, while the Mt. Wilson observatory was under construction Frank E. Ross was asked by the director of the observatory to find a way to expand the photographically useful field of the large and fast (about f/3) reflectors in use at that observatory and planned for Mt. Palomar. As we discussed in the first article in this series, coma is the primary factor limiting the field within which the image quality of a parabolic mirror is acceptable. The problem is, the size of the useful field depends only on the focal ratio of the telescope, not the size. Hence the photographically useful field of a 100" aperture f/4 telescope is the same as that of a 4" f/4, namely, a little less than one inch in diameter. This is not very cost effective. When the large Mt. Wilson and Palomar instruments were first commissioned this was recognized. However, for the two other primary uses of such large instruments, photometry (measuring brightness) and spectral measurements coma does not matter because these measurements are performed on a single star. The star being measured can brought to the center of the field of view where the image produced by a parabola is absolutely perfect.
But wouldn't it be nice if such a large telescope could be used for photography? Hence the request to Mr. Ross. It is not known if the request was motivated by Schmidt's demonstration that aberrations could be reduced or eliminated by additional optics or not. The idea of canceling the aberrations introduced by one element in a design with another element was certainly well known to designers of photographic and microscope objectives at the time. It even has a name: "Aberration Balancing". In any case, the timing certainly suggests that Schmidts' discovery stimulated the astronomers to look at the issue afresh. Of course, eliminating or reducing the aberrations of a large parabola by placing a full size (or larger) corrector in front of the mirror was and is out of the question. The glass to make such a device does not exist.
This drove Frank Ross to consider placing some auxiliary optical elements near the focal point, the final result being the Ross Corrector, a group of two or three small (compared to the mirror) lenses placed a short distance in front of the film.
The coma correctors Ross developed were designed to be used with specific telescopes so he was able to optimize the design to work with those instruments. He discovered that, by correctly choosing the radii of the lenses, he could almost completely eliminate coma over a very wide field. For example, he expanded the useful field of the Mt. Wilson telescope from a fraction of an inch to about five inches. While designing the corrector he learned that if the coma was to corrected he had to introduce a small amount of spherical aberration. Spherical aberration shows up as a symmetrical spreading of the image. However, he was able to achieve a reasonable balance between the remaining (or residual) spherical and chromatic aberrations. By making the focal length of the corrector infinite (also known as a zero power lens assembly) he was able to use a single type of glass in the design. If the design had a non-zero power it would introduce false color, the reduction of which to manageable proportions would require other elements or different types of glass.
Today there are several Ross-type correctors available to the amateur astronomer. Lumicon and Tele-Vue both produce correctors. These have additional elements to allow them to adapt to a variety of telescopes. In their most recent catalog Orion introduced a corrector matched specifically to an 8" f/4 parabola. Likewise, some of the focal reducers sold for use with Schmidt Cassegrain telescopes at least partly correct the coma of those systems. The price of these units ranges from $99 to $300 dollars. They all seem to be optimized for use with 35-mm cameras. Since they are based on the Ross corrector they will all have a small amount of residual aberration. Since they are commercial items it is difficult to obtain detailed specifications.
After Ross introduced his (coma) corrector several other ideas appeared. Maksutov, in the same article in which he described his full aperture corrector, also described a corrector composed of a single small thick meniscus corrector which, when used with a parabolic mirror in the same general arrangement of the Ross corrector, also reduces the coma.
Unfortunately, the Maksutov design, while easier to make than that of Ross is not as well optimized, producing markedly inferior images. As a result few if any have been built.
Of course, since Ross's approach of introducing a few lenses near the focus worked so well in reducing the coma the natural question is can the parabola be eliminated, replacing the primary mirror with a sphere and 'tweaking' the corrector to match? After all, making lenses, with spherical curves, is very repeatable and cheap on a production basis, while aspherizing a large mirror remains a bit of an art. Thus in 1957 Jones designed a two element corrector which was matched to a spherical mirror. Unfortunately, the corrector has negative power similar to a Barlow lens, although a Barlow lens is designed to work best with a parabolic mirror. As a result a f/4 mirror and the Jones corrector results in a f/10 system, which is not the direction we want to move in when doing astrophotography. Plus the images suffer severely from chromatic aberration (that is, color). Brixner refined the design somewhat in a Sky and Telescope article in 1966 but the performance remained mediocre at best.
Jones described the final variation to this approach in 1970 in the same magazine. In this version (known as the Jones-Bird design) the final focal ratio is f/6. However, the images are again far from perfect. Thus it seems that the approach of correcting a spherical mirror solely with simple a field corrector is a dead end.
Returning the Ross's approach, the final twist in this story is, of course, how to get rid of the spherical aberration introduced by the Ross corrector. In the May 1985 issue of Sky and Telescope magazine John Richter described a classical Ross corrector applied to a f/4.5 parabola. He mentioned that the spherical aberration re-introduced by the corrector could be eliminated if the parabola were slightly overcorrected. That is, if the mirror was a weak hyperbola instead of a parabola (the parabola is in fact the dividing line between an ellipse and a hyperbola) the net effect would be zero spherical aberration. Of course, the coma of a hyperbola is not exactly the same as that of a parabola but it turns out to be a very minor difference.
Mr. Richter's suggestion seems to have laid fallow for a number of years but in the December 1996 issue of Sky and Telescope Paul Lind described a mirror-corrector system he had designed and built using this concept. He used an 8" f/4.5 mirror that he had built some number of years ago. He wanted a f/3.6 system and so, to avoid re-grinding his mirror, he modified the Ross design slightly to use glasses of two different types. This allowed him to design a corrector of positive power, thereby reducing the focal ratio to what he wanted, while still correcting the system for color. Once he designed the system he had to make the two lenses and re-figure his existing mirror. The net effect was near perfect images over a 120 film format, which works out to a 2 ¼ inch square field.
Alejandro Di Baja described a simpler design in the May, 1999 issue of the same magazine. He built a 9 inch f/3.9 astrograph using the classical Ross corrector formula (that is, zero power using the same glass for both lenses) and a hyperbolic mirror. He is also using the 120 film format. Again, the image quality is excellent over the entire field of view. The limiting factor in these hyperbolic camera designs is likely to be the size of the diagonal or film holder. In addition, some form of vacuum back is needed to hold that large span of film flat. This is not necessary with the 35 mm file format except on the most humid of nights, when the film can actually bulge away from the pressure plate in the back of the camera.
If you are interested in an 8" f/4 system giving the Orion corrector a whirl seems reasonable. This would produce a 750-mm focal length f/4 system, which is very attractive. Correcting the small amount of spherical aberration introduced by the corrector would require overcorrecting the primary mirror slightly (that is, turning it into a hyperbola) but it is likely that it is not necessary, since the Orion system is designed for a 35 mm frame and hence can under correct the coma slightly, trading that off for better spherical aberration performance – it's called aberration-balancing, remember? A few test shots would answer the question. If I had such a mirror in hand I would certainly try it.
Finally, the Takahashi company of Japan makes a hyperbolic camera with a lot of zeros in the price tag. Other firms offer variations of the Wright Schmidt or the Maksutov Newtonian. Again, the price tags are as impressive as the performance.
In conclusion, I would like to suggest the following guidelines, all tailored to the 35 mm film format and finite financial resources. First, if the focal ratio is f/4 or above do some test shots with your parabola. You may be completely satisfied with the results, especially if the focal ratio is 5 or greater. Second, if you want to do work faster than f/5 you should consider the Orion corrector that is designed to work with an 8" f/4. If that focal length, focal ratio, or size telescope is unappealing, then you might want to consider one of the variable coma correctors. However, before doing so you should talk to the technical support of your supplier to make sure you can insert the corrector into the optical train of your telescope without major mechanical surgery. Finally, if you have the available funds, you might consider one of the high speed lenses or commercial astrographs described above.