Wild Card 005.1 “John Kraus Passes On” PLUS “More on Achromats vs. Apos”
In “Wild Card 002”, I wrote about the great early radio telescope built at the Ohio State Radio Observatory in the 1960s. It was known as “The Big Ear”. The man who built it, Dr. John D. Kraus, passed away on Sunday, July 18, 2004, at his home near Delaware, OH. It seems appropriate that we remember him in this column.
Alas, although I worked in Radio Astronomy for a time, I never had the honor to meet John Kraus. However, what I know about him suggest that he would have fit in with amateur astronomers and telescope makers. The reason is that he refused to worry about whether or not funds were available to do the research he wanted to do. Instead, he rolled up his sleeves and figured out ways to get things done. Here are a few examples:
Dr. Kraus came to Ohio State in 1946. He wanted to do radio astronomy research, but knew that OSU would be far down on the list of schools that were competing for funding for instrumentation. Besides, few sources of funds even knew what radio astronomy was, much less were willing to fund that kind of research. Kraus would have to build what he needed, using any resources he could find. Over his long career, he built plenty.
The “Big Ear” is the equivalent of a 175-foot diameter radio telescope. It was built in a corn field in Delaware County, OH, and pointed due South parallel to the ground. An enormous flat reflector was jacked up on hinges to reflect the radio radiation from the sky into the fixed radio telescope. In this way, the declination in the sky could be selected. The telescope continually surveyed a strip of the sky at that declination as the Earth turned on its axis.
I’ve not found any figures on how much the thing cost, but I’ve read in various places that it cost only a penny or two on the dollar when compared with what a fully-steerable instrument of the same size would. It was built with a combination of student, volunteer, and (very little!) paid labor, and was in service for more than 30-years. Over its operational life, “The Big Ear” completed a survey of most of the entire sky visible from Ohio. The “OH” catalog of radio sources is 20,000 objects long! [Only an ill-considered action by the president of Ohio Weslayan University, which owned the land on which it sat, resulted in “The Big Ear” being dismantled. The president of OWU didn’t even have the courtesy to tell Kraus what he was doing. (That president soon became an ex-president….)]
By the time “The Big Ear” had completed the various surveys of the radio sky and the other studies for which it had been designed, it was really obsolete. But John Kraus and his associates simply shifted gears and modified the telescope to continue surveying the radio sky, but this time with a receiver designed to search for signals broadcast by extraterrestrial civilizations. The original receiver only allowed 20 channels to be checked at any one time and didn’t have the sensitivity needed. So Kraus encouraged two undergraduate students in one of his classes to undertake the task of designing, building, testing, and installing a 50-channel receiver, and Kraus served as their undergraduate faculty advisor. That receiver was used for the remainder of the SETI program at the OSU Radio Observatory. It is the one that deteceted the famous “WOW!-source”, the only known possible evidence of an extraterrestrial signal.
In the 1970s, how many university EE departments were teaching budding engineers how to do things by allowing them to build research equipment? I don’t know the precise figures, but I suspect that the answer is “damn few”. Since then, I hope that more departments have learned from Kraus and his cohorts the value of this kind of student work.
Over his career, John Kraus invented four significant antennas. “The Big Ear” is the best known, and this type is now known as a “Kraus Antenna”. However, radio amateurs also know that Kraus invented the “W8JK Flat-Top Beam” antenna before WWII. Variations of this antenna are in use by hams today. These two antennas would be enough for some folks, but John Kraus also invented the corner reflector antenna, which is in use in many of the UHF-TV antennas available today, and the helix antenna, which is in wide use in the spacecraft communications industry.
I’ve just given you a brief sketch of John Kraus’s accomplishments. You can read a long list of books written, students encouraged, awards received, etc., at the website “www.bigear.org”. But the true measure of the man is the generous gifts John Kraus and his late wife, Alice, gave to both Ohio State and Ohio Wesleyan University. OSU is where he taught the last 58-years of his life, so the gifts to that school are easy to understand. He and his wife donated an 80-acre tract of land to OWU for the Kraus Wilderness Preserve in 1976. In light of the unpleasantness associated with the destruction of his beloved “Big Ear”, it would have been understandable had John Kraus been less than enthusiastic about continued giving to OWU. But John and Alice Kraus continued their support of Ohio Wesleyan University for the rest of their lives.
It’s easy to focus on the long lists of accomplishments of John Kraus. I prefer to just say that his was a life well-led….
“More on Achromats vs. Apochromats”
Last week’s column engendered several forum posts, several from someplace in California called “Yuba, City”. I also had a long conversation with an amateur on the East Coast who has a 4” apo, but is considering a 5” f/12 achromatic refractor for more serious planetary observing. He had read last week’s column and asked what I would suggest.
I replied that he would likely be satisfied with the image quality of a 5” f/12, but that, if he was used to the color performance of an apo, it might take a while to get used to the “blue haze” that is found with an achromat of that focal ratio. I then pointed out that a 5” f/24 refractor could be folded to achieve the same tube length as an f/12. And, 120” of focal length is easy on eyepieces, and makes it easy to obtain powers high enough to get a good view of the Moon, planets and double stars.
But would the color be acceptable? I can’t tell you how much color is acceptable. I’d likely find the 5” f/12 achromat my friend mentioned reasonably acceptable. But I’ve never used an apo for a long period of time. So my friend might not. Would he find the 5” f24 acceptable? To answer that, I decided to look at the several “rules of thumb” that have been around for a while:
One “rule of thumb” that I’ve heard for years, but can’t track down just now is:
MinFratio = 5*D,
where D is the diameter of the objective in inches. (I doubt if this rule of thumb has been in use outside of the US for a good while!) This says that a 3” would need to be f/15, with which many folks would agree. We see almost no color in a good 3” achromatic refractor. (The apos of that aperture are of much shorter focal length.) A 4” would need to be f/20, and a 5” would need to be f/25. So my suggestion of f/24 for a 5” isn’t far off. But this formula leads to very long focal lengths for refractors above 5” in aperture.
Sidgewick (“Amateur Astronomer’s Handbook”), and Rutten & van Venrooij (“Telescope Optics”) make the same assumption in coming up with the equations they present. They say that, if we focus an achromat on a star for the green, and if the blur of the red and green images of the star is no more than 3-times the diameter of the Airy Disk, the objective is reasonably achromatic. Based on this standard, the following would be the equation:
MinFratio = 3*D,
where D is again the diameter of the objective in inches. (Note that Sidgewick comes up with 2.88, and R & vV comes up with 3.09. I’ll just say 3.) This equation would suggest that a 5” f/15 would be reasonable, and that the 6” f/18 I used as my example would also be a reasonable achromat. I’ve looked through achromats of those focal ratios, and they still have more color than I suspect that “ApoFolks” would tolerate.
The strict definition of an achromatic lens is that it bring two wavelengths to the same focus. If it does that, a lens is “achromatic”. So, an f/1 lens can be achromatic, at least according to this definition, but no one would want to look through one. What the above equations are trying to do is establish the minimum F-ratio at which an objective is “sensibly achromatic”.
I think what many folks seek is the minimum F-ratio for an achromatic doublet at which its performance is “sensibly apochromatic”, an F-ratio at which secondary color isn’t noticeable in all but the most critical applications, such as viewing the star Sirius or looking at the limb of Venus.
I’d suggest that the standard for an objective to be “sensibly apochromatic” would be if, when it’s focused for the green, the size of the blur of the red and blue images is no more than the diameter of the Airy Disk.
I could have just done calculations to derive an equation for this, but I wanted to see for myself what the progression of minimum F-ratios I would get that would just meet the above criterion. So I used my trusty optical design program, SODA, to design a 5” f/24 achromatic objective. (I chose those numbers because I had already mentioned that as a good size for a folded refractor.) This objective has no coma, spherical aberration or astigmatism over a modest field. Purely by accident, this objective just meets the above criterion for being “sensibly apochromatic”. I then scaled the objective down to be a 2” f/12, and found that it just barely met the criterion. Scaling up to an 8” diameter, I found that it needed to be f/36 to just meet this criterion. This led me to notice that the following equation will satisfy the “blur the size of the Airy Disk”-criterion:
MinFratio = 12 + 4*(D-2),
where D is still the diameter in inches. Here’s a non-table of the results:
A refractor of 2” diameter would need to be f/12, and
A refractor of 3” diameter would need to be f/16, and
A refractor of 4” diameter would need to be f/20, and
A refractor of 5” diameter would need to be f/24, and
A refractor of 6” diameter would need to be f/28, and
A refractor of 7” diameter would need to be f/32, and
A refractor of 8” diameter would need to be f/36!
I’ll tell you three times: These are long refractors! These are long refractors! These are long refractors!
But, if we fold them in half, they’re not nearly so long. I wouldn’t recommend that anyone run right out and obtain an 8” f/36 achromatic objective, but, if one fell in my lap, I’d fold it either once or twice to get the tube down to a manageable size and to get the eyepiece in a reasonable position. To do this, I’d need one or more excellent flats, because the Strehl Ratio of the objective would be multiplied by the SR of each successive flat to get the System Strehl Ratio.
I think my friend who was looking to build a 5” f/12 refractor would do well to consider having a 5” f/24 objective made. He could then fold it in half with a high quality flat, and use a small diagonal to being the light out the side of the tube, just as if he were making a Newtonian. Such a telescope would leave little to be desired for Lunar, planetary, or double-star observation. And, it would be no longer than the original 5” f/12. A 6” f/28 objective could be folded so that it’s tube would only be about 7-feet long, which is still a manageable size.
Of course, I didn’t invent the folded refractor. But I made one a lotta years ago, and I liked its performance. If you don’t need the small size and wide-field performance of a true apochromatic refractor, you might find a very long focus achromat to be just what you need.
NEXT WEEK: A survey of unobstructed reflector telescopes, plus the first “Wild Card Almost Free (The Catch Is You Gotta Pay Shipping) Optics Contest”.
RICK SHAFFER is an astronomer, teacher, writer, and designer/builder of telescopes and museum exhibits. He lives and works in Sedona, AZ, where he folds light paths in different ways. He claims to have invented the term “OptiGami” to describe these techniques, but no one pays him any attention….
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