Classic & APO Refractors
By George Kohl
I was a young amateur astronomer in the 1950s and 1960s that enjoyed reading the Unitron refractor ads in the pages of Sky & Telescope magazine. The Unitron refractor was a dream telescope for me, but was too expensive to buy and like many other young amateurs I used a Newtonian reflector.
At the time terms like chromatic aberration didnít mean very much to me. I just knew that the pictures of the Unitron refractors and all their extra equipment looked like the ultimate telescope. As time went by, I learned more about telescopes by going to star parties, looking through a lot of high quality telescopes, and attending the Riverside Telescope Makers Conference. I began to appreciate how a good Newtonian could perform compared to other more expensive designs. This was at the time more compact catadioptric systems like the Celestron Schmidt Cassegrain were becoming popular and Newtonians on big heavy German equatorial mounts were going out of style.
At about this same time a genius of simplicity named John Dobson came up with a very simple and inexpensive way to mount a large aperture portable telescope. This design and the Newtonian were a marriage made in heaven and we all know how the story turned out. The Newtonian reflector was once again in style for portable scopes. A third revolution got started in the 1970s with new refractor designs. Thanks to fluorite and new types of glass it became possible to design short focus refractors with little or no visible chromatic aberration. The refractor design was back in style and became very popular. The only problem with the new APO refractors was their high cost. In the last few years thanks to companies like Orion good quality APOs are now available at a reasonable price. Please note that I did not use the word cheap. In the world of optics there are good reasonable prices and cheap optics. There are good deals in optics, but there is also a lot of cheap optics that are bad optics.
Because of the new APO refractors I finally decided to buy a refractor. I wanted a lightweight portable scope so I decided on a 80mm. The first 80mm APO I purchased had good optics, but I didnít like some of the features so I sold the scope. The second 80mm APO I purchased was a TMB 80mm SS f/6.3. I mounted the scope on a Meade LXD75 GoTo mounting and have been very happy with the combination. I use the scope for visual observing, ccd photography, and ccd photometry of variable stars using a V filter. One of the problems with ccd photometry is the small size field of view. I decided to use my APO because it gives me a large enough field of view to work with the variable, comparison, and check star.
I still remembered the Unitron ads in Sky and Telescope. I have had quick looks through long focus refractors but my only real observing experience has been with a friend who built a 6-inch f/15 folded refractor using a Jaegers objective. The Jaeger objectives were high quality achromatic doublets. We spent a lot of time observing Jupiter with the scope and it gave a beautiful detailed view of the planet. The first time I looked at Jupiter I immediately thought of the Jimi Hendrix song Purple Haze. A 6-inch classic refractor even at f/15 has chromatic aberration. The Sidgwick standard for classic refractors is to take the square of the aperture in cm and multiply it by 1.2 to get the minimum needed focal length in cm to have good control of chromatic aberration. This simple rule is practical up to about a 5-inch refractor. A 6-inch classic refractor would have to be f/18 and an 8-inch classic refractor would have to be f/24. Unless you fold the optics the tube is just too long to be practical. In the April 2004 issue of Sky and Telescope there was an article by Ernie Pfannenschmidt where he measured the amount of chromatic aberration in various telescopes. He tested a 5-inch f/15 refractor and it had good color correction. The same issue had a very interesting review of various filters used to correct chromatic aberration in short focus refractors and their relationship to true color. I decided to buy a good long focus refractor at a reasonable price. I saw an ad for a Sears refractor # 6344 that was made in the late 60s / early 70s. The scope appeared to be in very good condition and it was a 76mm f/15.8. I had already read two reviews of this type of scope and decided to buy it. The company in Japan that assembled / made the scope was called Yamamoto (SYW). The 76mm objective is a Fraunhofer design in an adjustable push-pull cell. The Fraunhofer design is one of the best for an achromatic doublet. It came in a nice wooden storage case with a German equatorial mount, 5 eyepieces, Barlow lens, star diagonal, image erector, Moon and Sun filter (Never use the Sun filter), solar projection screen, astronomy book, and instructions. The star diagonal, image erector, Barlow lens, and eyepieces that came with the 76mm scope are all standard Japanese size .965-inch. You can buy an adapter to use 1.25-inch eyepieces in a .965-inch diagonal / .965-inch focuser or another option is to use a hybrid diagonal. In both cases you have a reduction to .965-inch in your optical path. The focuser tube on the 76mm scope has an inside diameter of 1.39-inch and uses a screw on adapter that reduces the diameter to .965-inch. I was going to have a custom adapter made to use a 1.25-inch diagonal but I found out on one of the Cloudy Nights classic telescope forums that Vixen made an adapter I could use. Thanks forum members! I purchased the Vixen adapter # 3720 and it screwed into my focus tube and allows the use of any 1.25-inch diagonal and eyepiece with no vignetting.
One advantage of a long focus telescope is that eyepieces work better. An f/15.8 cone of light is easier for an eyepiece to handle than an f/6.3 cone of light. I compared the performance of my 24mm Panoptic and 20mm Meade Research Grade Erfle on both scopes. On the 80mm scope the Panoptic has much better star images near the edge of the field than the Erfle. On the 76mm scope both eyepieces have good star images close to the edge of the field. I can use Barlows with my APO and turn the light cone into an f/12.6 and f/18.9 and get the same edge of field performance as a long focus refractor. I use Antares 2x and 3x Barlows with University Optics Orthos for high power observing with my 80mm scope. This arrangement gives excellent image quality with no internal reflections or ghost images. A good Barlow does a lot more than just increase the magnification.
I use Astro-Tech dielectric star diagonals on both telescopes for maximum light transmission. I can use 2-inch eyepieces on my 80mm APO that give a maximum field of view of 5.5 degrees. I like using a Panoptic 24mm eyepiece that gives 21x and 3.2 degrees. On the 76mm refractor the Panoptic 24mm gives 50x and 1.4 degrees. I used various high quality eyepieces and Barlows when I compared the two scopes on different objects:
Star Image Quality: For me the real test of any telescope is how starlight comes to a sharp focus. On a good steady night a high quality telescope should produce pinpoint star images. Both refractors give pinpoint star images at focus and show good inside and outside focus ring patterns. Both refractors have good contrast. I checked both scopes for chromatic aberration by looking at Vega. Using various magnifications I could see no obvious chromatic aberration with either scope. By no obvious chromatic aberration I mean that I couldnít see any chromatic aberration for sure. The reason the 76mm refractor has such good color correction is because it meets the Conrady standard that is stricter than the Sidgwick standard. For example a 4-inch classic refractor can be f/12 to meet the Sidgwick standard, but to meet the Conrady standard it would have to be f/20. The easiest way to remember the two standards: (Sidgwick standard) Aperture in inches times 3 equals focal ratio. (Conrady standard) Aperture in inches times 5 equals focal ratio.
Lunar Observing: I never get tired of looking at the moon. At a public star party it is the one thing that impresses the public the most because of the great amount of detail visible. In both refractors the moon was very sharp with good contrast and no chromatic aberration on the limb or terminator when using high magnification.
Planetary Observing: I spent a lot of time looking at Jupiter with the two refractors. Jupiter because of its large size in arc seconds is a good test of telescope optics because you get a nice large image without using too much magnification. Both refractors showed good detail on the disk of the planet with no chromatic aberration. The image was brighter with the 80mm APO (multi-coated objective), but the image quality was a little sharper with the 76mm scope. The 76mm scope also had a darker background. Both scopes can handle 60x per inch of aperture with no problem. For planetary observing I prefer a magnification of 100x to 200x on any size telescope I use. The only exception is on rare nights of very steady seeing.
Double Star Observing: This is a traditional use for long focus refractors. I spent a few years measuring the separation and position angle of binary systems using a filar micrometer. I used an 8-inch f/7 Newtonian. It really made me appreciate the skill of great double star observers like S.W. Burnham. Burnham could detect a binary down to 0.2 arc / second separation by the elongation of the image using a 6-inch Alvan Clark refractor. You can use other types of telescopes besides a long focus refractor, but for this kind of work a good long focus refractor really does seem to have an edge. I enjoy looking at colored doubles. When I first got my 80mm APO I compared it to my 6-inch f/8 Newtonian on the same colored doubles to make sure I was getting true colors. The colors were the same. Of course with colored doubles observers donít always agree on the same colors, but at least I know the scope doesnít add its own tint. The 76mm scope shows the same true colors. One thing you notice when using a good refractor is how clean and well defined star images appear. When I looked at the famous double double in Lyra with high magnification the stars were airy disks with diffraction rings. Both scopes gave a very pleasing view.
Deep Sky Observing: This is one area where aperture is king. Several years ago I spent a night observing with a friend who had built a professional grade 24-inch Classical Cassegrain on an equatorial mount. We were observing under very dark skies and M51 looked like a picture. Observers that use big Dobs know what I mean. When I got my 80mm Apo I was surprised by how good it did on some deep sky objects. You are limited to brighter objects, but you still can get some beautiful views under dark sky conditions. I have to say that the 80mm APO wins on deep sky observing because of its large field of view and brighter image. The 76mm scope gives very good pinpoint star images and contrast but the large field of view of the 80mm APO works great on large open clusters like the Pleiades and other objects that are too large for bigger scopes.
Final Conclusions: The 76mm classic refractor and 80mm APO both have very good optics. It is easy to tell the difference between good and bad optics. But when comparing good optics the differences can be so small that it comes down to a matter of personal preference. If I had to pick a winner between the two scopes I would have to pick the 80mm APO just because of the convenience of the short tube and its versatility. I have fun using both of the scopes and that is what amateur astronomy is really about.
Clear Skies & Good Observing