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- THE BURGESS 24MM MODIFIED ERFLE & 10MM ULTRAMONO
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- Comparison of the Boltwood II and Sky Alert Cloud Sensors
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The Definitive Newtonian Reflector
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The Definitive Newtonian Reflector
Amateur astronomers and telescope makers have debated from time immemorial the advantages and disadvantages of different telescope designs. In particular, mountains of hard copy and electronic articles are available on the merits of refracting and reflecting telescopes, more recently, apochromatic refractors vs. Newtonian reflectors. This debate has become rather rancorous (Newtonian telescopes as APO “killers” comes to mind.) and unscientific, to say the least. And when all is said and done, in a discourse without loaded words and acrimony, the discussion actually devolves to one concerning perfect optics. And isn’t this what we all want or wish we had?
In this discussion, I intend to stay away from belief or opinion and stick to Science.
I need to set up some ground rules so I can present some interpretations and my conclusions in the clearest light. I will be using endnotes so that you, the reader, can check the validity of any conclusions made. As an aside, I have to be extra careful in using online sources; they tend to be ephemeral, hard to track and often downright inaccurate.(1) Experts seem to come and go. In addition, I wish to set up some restrictions before stepping into the discussion of Newtonian reflectors, refractors, and my 6” Definitive Newtonian Reflector, to be described and discussed in this article.
Telescope performance should not be judged according to the cost of the instrument, only its aperture. In addition the amount of accessories available to the amateur does nothing to enhance the underlying optical performance. As far as eyepieces go, a Plossl is the best thing going at so low a price, and provides the ideal view in evaluating the performance of any telescope.
In my discussion, I argue that only one additional element, related to aperture determines optical performance – the quality of a stellar diffraction image.
All else is derived from this. A careful reader here might ask, “What about the planets and the moon?” Any extended object, such as the moon, is composed of myriad diffraction images, whole bushel baskets of them! And, the finer each image of the many “points” in that image, the clearer that overall picture is going to be. Any degradation of that basic point source, (a star), is going to do bad things to an extended image, thousands of times. As a part of the process, I wish to present to the amateur telescope making community my 6” f/8 Newtonian used at f/16 that will seek to address every one of the Newtonian’s faults—a telescope that I call the Definitive Newtonian Reflector, or DNR. This “image of a star” is actually a small dot and surrounding rings of light; the Airy Disk. Without getting into the mathematics of this image, it is the result of the wave nature of light; the diameter of a star is solely due to the aperture of any given telescope. Even when a star is a nearby red-giant, it isn’t resolvable by any telescope in the amateur range, and most if not all professional instruments. This will change with the huge aperture telescopes envisioned by their planners. A most interesting rule states that the diameter of the diffraction image and its associated rings gets smaller as aperture increases. As noted before, my tongue-in-cheek remark about bushel baskets of diffraction rings shows that there are even more in those baskets when these diffraction images get smaller. And with these images, there are more of them to make up an image of the moon or planets. Think of it this way: these minute points represent the grain in the image of Jupiter, analogous to the grain of photographic film, as old fogies from the last century fondly remember. In this modern era of digital photography, one can see a similar analogy with the number of pixels in a digicam. The more pixels the better!
As I begin to describe my telescope below, I will discuss the diffraction image of a star not only in terms of its diameter but also the rings surrounding the central image. This central image contains 84% of the light, while the other 16% goes into rings of decreasing intensity. It is worth noting that this first ring contains by far, most of that 16%. This will matter later in my discussion. And here’s my telescope with its answers to the design of the Newtonian reflector.
It is the job of an amateur telescope maker to make a telescope that produces the finest images for its size. This is actually easier if the amateur makes his own mirror; commercial optics are too uneven in quality; the custom-made optics, while mostly excellent, carry a price tag that is too large for the average amateur’s wallet. For my telescope, I chose to make a 6” f/8; this f/ratio is the best one for ease of construction. Figuring such a mirror is relatively easy and it is possible for the amateur on a second or third try to do much better than making a merely adequate mirror. With some care, and not cutting corners, an ATM can make a mirror that can exceed 1/8 wave or better, the wavelength being of green light. I think that I have done better than this, having fifty-six years experience pushing glass. I use the Foucault test and the Everest pinstick method.(2) I have little confidence in the Ronchi test for producing the finest work as it is a qualitative rather than a quantitative test. It is better to take the time to learn the proper methods; then make fine optics. And from the discussion above, in order to get the best imagery, one needs all the skills one can get. The glass used here is Pyrex, known for its ability keep its figure in widely changing temperatures. Despite the fact that Pyrex blanks are no longer made in the U.S., there are large numbers of blanks on the secondary market. As a concession to those who choose to make ultra-thin mirrors, a process for which I have little confidence, I chose a blank of ¾” thickness that could adapt to ambient temperatures more rapidly than a thicker Pyrex blank or plate glass. Coincidentally, a ratio of blank thickness of 1 to 8 is still within the range of acceptable, according to Porter and Texereau.(3)
For further evaluation, I rely upon the works of H. R. Suiter who gives all the information an amateur could ever need to evaluate his telescope’s performance to the nth degree.(4) In two of these works, Suiter gives the criteria for judging one’s optics; and using these criteria, I have conflated the two sources to avoid repetition. I find in my conflation that there are five subtle issues that beg discussion and action to eliminate them; other amateur telescope makers have dealt with the major issues to an admirable degree.
The first issue is scattered and stray light -- a problem in all Newtonian reflectors, the sources are many; one being caused by having a tube which is too short, where the eyepiece holder is almost on the edge of the tube. Light gets in very easily as the eyepiece holder is not shaded. This problem is worsened when a Dobsonian Newtonian lacks a tube due to weight considerations. In such a case, light can reach the eyepiece in just about every way, in particular the area behind the diagonal as viewed from the eyepiece. Some ATMs put a tube assembly of an eyepiece holder, spider, diagonal and finder on a tube near the front of the telescope which does much to alleviate the problem. But some light still gets through – from the back and sides. In the case of reduced diagonal size, this problem is at its worst. For this and many other reasons, I chose to use a solid tube and avoid most of the problems. In addition, I utilized a 14” long “dewcap” that put the eyepiece holder quite a ways back on the tube. On the other end, I made a back plate for the mirror cell that, while having a hole for air circulation from a high speed fan, was constructed to allow almost no light to shine through. Finally, I resorted to a trick I first used in 1987 for a 6” f/10 reflector. I like to call this device my “Dark Bucket.” Any light that bounces around in this telescope is not going to do anything at my eyepiece holder; it simply cannot! When all else fails, the light gets lost here. This device consists of a 4” diameter can which is epoxied to the main tube. It provides a very dark, shaded recessed area directly behind the diagonal as viewed from the tube of the eyepiece holder. Of course, the whole interior of the telescope tube has been coated with a thick layer of black flocking material; any remaining shiny spots were given small application of Krylon #1602, Ultra Flat Black. Scattered light doesn’t have a chance. I had one person, in broad daylight, when the mirror wasn’t intalled, stare down my dewcap and exclaim, “I can’t even see the bottom of the tube!” Hardly certain, but it was nice to hear. Of course, I blackened the edges of all lenses (and the window) in my system.
I can relate a cogent example about stray light. Having spent many years of observing time looking at the moon, I’ve almost never had to use my finder because I could generally aim the telescope and find the moon by seeing increasing amounts of scattered light as I closed in. I can’t do that any more!
Second, some comments about turned edges -- such an error on an amateur or professional mirror is a death knell to good definition, no matter how well the overall figuring is done for the rest of the surface. This “hook” at the mirror’s edge seems far too small to cause so much trouble, and sadly there seems to be an attitude in recent literature that turned down edges are incurable; the only easy “cure” is to stop down the mirror to cover this defect. As an old-timer, I wonder how much corner cutting, or how much time it took to make an edge turn that much. Mirrors coming out of my workshop have stopped having TDEs about forty-five years ago. I admit that there have been a few that needed to have their edges ground away, but never more than 1/50th of an inch. I’ve been told that such a TDE wouldn’t really count for much, but I like to make things the best I can. And in this case, I wanted my mirror to be as close to perfect as possible for this discussion of the diffraction image in my DNR.
Third, I’d like to talk about primary and secondary ripple -- Texereau shows the dramatic differences in surface smoothness using different lap materials and different polishers.(5) While it is true that the results may seem trivial to most amateurs (pros too), I find that if one wants the very best definition, one must go for materials that produce surfaces that approach perfection. For my mirror, I used cerium oxide and tempered pitch until a black” polish was obtained; then I polished for another two hours, just to be sure.(6) Then, I made a new lap and polished for two hours more, using rouge, to get the very best surface as illustrated by Texereau. Only then did I figure the mirror as accurately as I could to 1/16th wave. Scratches and sleeks had to be minimized as they scatter light.
The fourth issue is seeing related problems—tube currents and some solutions: All telescopes are afflicted by seeing, that pesky wavering of our atmosphere. Here is a variable that can afflict a telescope in grotesque ways to soften and blur the clean diffraction disk we’d like to to see in our telescope. Some of this bad seeing takes place in front of our telescope; that we can do nothing about. But what occurs at that first optical surface and inside it is certainly our business.I’ve used several approaches to deal with these effects. My large “dewcap” not only helps with scattered light, it also provides a shield that prevents warm eddies of air from the observer wafting across the entrance of the telescope. Since most air currents within the tube tend to cling to its inner diameter, I have allowed for .75” clearance between the tube and the mirror’s edge. While others favor a 1” clearance, I was forced to use the clearance that was available with the tube I had.
The fourth issue is seeing related problems -- tube currents and some solutions:
All telescopes are afflicted by seeing, that pesky wavering of our atmosphere. Here is a variable that can afflict a telescope in grotesque ways to soften and blur the clean diffraction disk we’d like to to see in our telescope. Some of this bad seeing takes place in front of our telescope; that we can do nothing about. But what occurs at that first optical surface and inside it is certainly our business.
I’ve used several approaches to deal with these effects. My large “dewcap” not only helps with scattered light, it also provides a shield that prevents warm eddies of air from the observer wafting across the entrance of the telescope. Since most air currents within the tube tend to cling to its inner diameter, I have allowed for .75” clearance between the tube and the mirror’s edge. While others favor a 1” clearance, I was forced to use the clearance that was available with the tube I had. In this case, my experience has been that even a .5” space has worked well. While my tube is metal, the thick flocking inside serves as a form of insulation. As a habit, I never put my hands on any telescope tube while observing for the same reason.
The next attempt to alleviate this problem has been to install a fan on the outside of the mirror cell to force air into the tube. For those who have taken a close look at my “dark bucket,” they may have noticed that the rear of this device has a door that is removable to allow air to circulate out of
this enclosed optical system. I suppose that the removal of the eyepiece cover would allow air out, but why take chances? I chose a computer fan from Surplus Shed that runs off a 12 volt battery.
Fifth on the list of problems is spider diffraction. I dealt with this problem by eliminating the spider entirely. To do this, I made an optical window with a hole ground through the glass to hold a threaded bolt and its diagonal holder. No struts are ever needed to hold the diagonal in place and it is these struts radially extended outward from the diagonal holder that cause spider diffraction. The window was given a single layer antireflection coating on both sides to eliminate stray reflections and allow the window to pass more light to the mirror. As an aside, several ATMs referred me to the optical scientist William Zmek, whose studies are said to reveal that an optical window degrades the final image in an optical system due to surface scratches, haze and polishing errors. My many web searches came up with this prolific researcher, but trying to go through his many papers failed to find a clear connection to what I was quoted by others. While I was looking, it seemed to me that while a window has two offending surfaces, a refractor lens has two, four or perhaps six air/glass interfaces. I believe that this fact alone renders this question about image degradation moot.
And so, my window gets rid of the problem of spider diffraction, especially detrimental to imagery. For those who doubt, try looking at Jupiter with a Newtonian with a 4-vaned spider. While some observers actually like the spikes emanating from stars, this is one of the reasons why an ordinary Newtonian cannot perform up to refractor standards. As the picture below shows, how could it? Behold the image of a star:
Some may rightly point out that my window seals off the tube and totally interferes with the flow of air though the tube. But please remember that my dark bucket has a removable back to let more than enough air out its side to cool the setup down and eliminate those tube currents. An hour of using the fan to drive air through the mirror cell, past the mirror, and out the side cools things down within. And with that, I can close the door to the dark bucket and turn off the fan. Note that the door to the dark bucket rests against an internal ring that prevents light leakage from this quarter, just as I used as a “stop” for my mirror cell to get the same effect.
You may have noticed that I have been mentioning some of the less obvious defects of an ordinary Newtonian reflector. The reason for this is my wish to get all of these cleaned up to show a clear diffraction image of a star unencumbered by these errors. We now have the best diffraction image we can get at the eyepiece. Think of it this way; if the large numbers of diffraction patterns that define a planet’s image all have spikes; the nearest analogy is looking at a soft half-tone image from a newspaper. Curved spiders spread these spikes’ light through the entire field with a faint haze. I think of the above problems as a subtle “mud” that degrades contrast. Interestingly, these effects can not be readily subjected to mathematical analysis. One might not directly sense this haze, but it is there and surely lowers contrast. It is dramatically shown below:
Omega Centauri Hubble Space Telescope
Reproduced by permission—Space Telescope Science Institute
This type of problem can only be entirely eliminated with an optical window. It is assumed, even among experienced ATMs that making a window is a very hard chore, but nothing could be farther from the truth. Actually, neither of the window’s faces need to be flat to 1/20 wave as many have often stated. I have seen optical windows that were out of flatness by 5 waves on each surface with absolutely no effect on diffraction patterns. It is true that optical glass is needed, and one must learn the techniques that are readily available. But the main importance in the production of an optical window is that it needs to be of uniform thickness, and that its surfaces must be regular with no zones. Such a window can be made by an experienced ATM if he would only try.
This discussion continues with the issue of obstruction in this telescope. Many Newtonians of the short focus variety often have obstructions of 30% or more. Longer focus telescopes often get by quite well with a 25% obstruction, yielding images that are quite passable. Suiter in Star Testing Astronomical Telescopes states that an 18% or lower obstruction, yields diffraction images that are imperceptibly different than those in an unobstructed refractor. ATMs have struggled to attain this number in their own instruments, by using low profile focusers and smaller tube diameters to lessen the distance between diagonal and prime focus. This latter strategy conflicts with the problem of air currents and narrow tube clearances. But with the effort, the 18% ratio can be achieved.
For my telescope, the number is 13.7%. How is such a thing possible? The answer to this question lies in the use of a Barlow lens; a transfer lens would serve just as well—either one dramatically changes just how images are formed in a Newtonian.(7) Look at it this way: Say that I wish to have a fully illuminated field in my telescope of ¼ degree. Clearly, an amateur working with my numbers would come up with a necessary diagonal minor axis of 1” or a little more and certainly not what I claim here. Now for a shift in perspective. The fully illuminated field I want is .25” before the Barlow lens. Let’s place this lens such that it has a magnification of two times, meaning that it intercepts the light at the appropriate place so the .25” field is magnified twice to a diameter of a .5” at the eyepiece. In any case, my configuration yields a half inch fully illuminated field for a 1.25” diameter eyepiece. Not bad.
By the way, my Barlow lens is apochromatic, an easier feat at so small an aperture—one inch. As a final bit of figuring, any residual spherical aberration caused by this lens has been dealt with by some judicious refiguring of the main mirror when the whole optical system is inspected under a dual-pass autocollimation Foucault test. And the beauty of all this is that such a test is a null test, ensuring the highest accuracy. My experience has been that such slight “touching up” of the mirror’s figure still keeps the mirror well within superb optical tolerances. There is another advantage to this setup. Diagonals can be made to remarkably high standards, but a lesser known fact is that diagonals, by virtue of how they are made, often have turned down edges that, as mentioned before, can ruin imagery in any Newtonian. It is often advised that an ATM buy a diagonal that is slightly larger than need be, so that its edges do not contribute to the final image at the eyepiece. It happens that, coincidentally in my telescope, this outer annulus of the diagonal is unused in helping to produce the fully illuminated .5” field.
What does all this mean to that diffraction image of a star so talked about early on in this article? As Suiter reasons, 18% obstruction ought to be enough and here I am with 13.7%. For those who doubt the utility of a Meade apochromatic Barlow lens and potential color errors, a switch to a 1” minor axis diagonal without the Barlow would yield an obstruction of only 16.7%.
Theorists in the hobby raise the issue of the degradation of imagery caused by a relatively small 18% obstruction in a Newtonian Reflector. Supposedly, images are trashed as some light in that diffraction image of a star migrates from its center to intensify the thickness of the first diffraction ring discussed earlier. Not so surprisingly, it is these same people who need to take a look through a properly designed and accurately made Newtonian.
And now for a story. I had great interest in making variable star magnitude estimates in the 1960s. An article by Leif Robinson in Sky and Telescope about Orion nebular variables inspired me to try to make estimates along the same lines as his. The charts were more or less centered on the Orion Nebula and the area was simply loaded with stars of all brightness ranges and periods. It was a great area of sky to be in, and one could comfortably work with a 6” telescope in skies that today’s observers dream of. There were stars that were suspected of making such rapid magnitude changes that an observer could repeat observations every quarter hour or so. The list of stars for this area was long enough that an observer could run all the way down the list just in time to start over. A number of estimates done over a month’s time showed all sorts of minor variations. I began to suspect that some of those variations could be due to my own inaccuracy in estimating magnitudes. It even seemed probable that my estimates were not accurate to .1 magnitude at all. It was even possible that a lot of the variations I recorded were due to my personal equation, a certain measure of inaccuracy of my own unconscious making. It was a given that the stars on that chart did vary, but my results precluded any accuracy being better than .2 magnitude. At the same time, I became aware that there were stars of magnitudes (marked on the chart as “A”, “B”, and “C”) that on some nights did not reflect the sequence of brightness that they should have displayed. So over the months I did intensive work and recorded those sequences. Surely something was going on!
NU_ORI.jpg AAVSO, used by permission
A few years later, the AAVSO came out with a new map, and I saw my three stars marked as magnitudes A(9.3), B(9.2) and C(9.0) and none of them variable at all.
Years passed and in 1996, David Lane of Halifax, Nova Scotia created software called ECU (Earth Centered Universe) that allowed an observer to create maps of one’s own choosing—finder charts for M-Objects or anywhere else one observer might desire, including a chart of the Orion Nebular Variables. The beauty of this software was that it incorporated data that included guide stars for the
Hubble Telescope down to 16th magnitude. And, by merely placing a cursor over a star, one obtained its magnitude. Naturally, I moused my way to those three stars and discovered that NONE of them were variable at all. In fact, none were equal in brightness, the stars having magnitudes of A(9.2), B(9.0), and C(9.0). Surely this was proof in my case that I wasn’t as good as I thought. The AAVSO database puts one of these stars as non-varying C(9.4), and I’ll leave it at that. At least I could be consoled that I had serious proof that that my estimates were only accurate to .2 magnitude.
And I was a conscientious variable star observer. Studying light curves in the AAVSO web site database, I realized that the AAVSO was living with the same kind of visual accuracy with numerous observers, and achieving a better result by averaging. A study of the AAVSO database and light curves reveals considerable spread in visual estimates; before the averaging takes place. It is clear that at least one conscientious variable star observer would find it impossible to judge subtle differences in diffraction pattern intensities.(8) And here’s the stake to put in the heart of this previously unscientific debate. I have seen references on Cloudy Nights that 30% vignetting at the edge of an eyepiece is recommended to keep diagonal size down in a Newtonian since an observer won’t notice it. Other recommendations run as high as 50%. If our eyes can see and measure levels of light so well, why isn’t this vignetting noticeable?
I’ve heard reports of observers who can see the contrast differences between a 5% and no obstruction at all, although such claims are subjective, and in my opinion, unlikely. It’s very hard to imagine how an observer could accurately hold the memory of that first image in his mind as he walks over to a second telescope to make the comparison. For all the forceful argumentation about telescopic contrast that occurs between reflector and refractor people, this argument is based upon what?
Conclusions and Some Questions
A Definitive Newtonian Reflector reveals diffraction images of stars and all else, indistinguishable from those of an equal aperture apochromatic refractor. Side by side, there is perhaps one in a thousand observers who might see a slight difference. And guess what? Manufacturers cannot stay in business making optical devices for the one person who has the sensory equipment to tell the slight difference between the Definitive Newtonian Reflector and an Apochromatic Refractor of the same aperture, but they can profit with a far larger group of people who can be made to feel certain that they can tell.
Many ATMs make a fetish of accuracy in making their mirrors; witness the discussions on the Cloudy Nights’ ATM forum. In the meantime on the Refractor forum, it is rarely discussed at all. Are APO owners making assumptions about accuracy of their optics? What do the APO makers say?
Has anyone found in the literature that there can be color errors in an apochromat (9), that spherochromaticism can be a serious problem in their design where spherical aberration varies according to wavelength (10) and finally, due to the exotic glasses used, there can be errors induced by changed temperature (11)?
Telescope owners must realize that any 1/8 wave total wavefront error for any reason, in any telescope, alters the diffraction image of a star, more than my Definitive Newtonian Reflector does with its insignificant obstruction. And remarkably, this DNR scales up with even better obstruction ratios as its aperture is increased! It is clear that an Amateur Telescope Maker, with experience, can make optics that approach perfection better than this 1/8th wavefront criterion.
There is no reason why anyone needs “an inch or two of extra aperture” to get the performance of a Newtonian up to the standard of a smaller APO. One can no longer assume that either type is better than the other in equal apertures, in the formation of stellar diffraction images, the fundamental basis on which all other images are built. And the Definitive Newtonian Reflector is great news for telescope makers. With more effort and more experience, any ATM can duplicate my work. It is almost, but not quite, easy.
The powers of human perception to estimate small differences in brightness in 1) the diffraction image of a star and its rings, 2) the differences in the shadows found in Foucault testing and 3) small differences in brightness as required for variable star observing are a lot less accurate than most workers in these areas of optics assume. This makes the Definitive Newtonian Reflector the equal of any other telescope, imagined or executed, of equal aperture.
I repeat here: Of equal aperture.
As Robert Browning once penned:
199 River Rd
Lincoln, RI 02865
|1)||"Never believe everything you've read on the Internet."|
|-- Abraham Lincoln (Original source unknown.)|
|2)||Amateur Telescope Making, Book 2. Scientific American, New York 1959: 3-48.|
|Hereafter designated ATM-2. My use of the Scientific American editions, rather than the newly edited|
Willmann-Bell printings, is a matter of my personal habit of many years. One other in this series will be
cited as ATM-3. Incidentally, the ATM-1 I use, but will not cite, is dated 1959. Earlier editions reveal
interesting concepts deleted in later texts. These are quite worthy of study!
|See also: Texereau, Jean. How to Make a Telescope. Interscience Publishers, New York 1957: 27.|
Hereafter cited as Texereau.
|4)||Suiter, H. R. (Click to access the "Top Ten" aspects of improving a Newtonian.)|
|Star Testing Astronomical Telescopes. Willmann-Bell Richmond, VA 2003.|
Seronick, Gary. "The Big Red One: My Optimized 6" f/9 Reflector." http://www.garyseronick.com
|5)||Texereau: 83-5. Note illustrations: 85.|
|See also: Suiter, H. R. "Modifications to the Diffraction Image Caused by Surface Roughness in Mirrors."|
Telescope Making 28: 24-31.
|6)||Texereau: 55. figure 27.|
|7)||ATM-3. 1961: 278-9.|
Dall, Horace. Personal Conversations. October 7, 1967, June/July 1973, July, 1991.
|8)||AAVSO Web site: http://www.aavso.org/vsp|
|9)||Cox, Robert, compiler. "Optical Experts Comment on the Texas Star Party," Telescope Making 31: 50-5.|
|10)||Kingslake, Rudolf. Lens Design Fundamentals. Academic Press New York 1978: 136.|
Baker, James G. "Planetary Telescopes," Applied Optics Volume 2, Number 2: 128.
|11)||Jones, Mike. Thermal Modeling and Athermalization in Telescope Optical Design|
When I began thinking about this project in 2008, I had just been diagnosed with Rheumatoid Arthritis. By the time I started construction in 2011, the course of this affliction had already taken its toll, and I was no longer able to walk and the construction of a telescope began to take large amounts of time with my increasingly incapable hands. Finally, when 2014 arrived, I was no longer able to make telescopes in the manner I had cherished for over fifty years. Because of this, I wish to gratefully acknowledge and commend the help I received as this project approached, for me, its bittersweet end.
To Al Hall, for machining a hardwood cell for the optical window.
To Dick Parker, for coring out the hole in that window.
To Robert Horton, who took over the polishing of the mirror and window when I no longer could, graciously leaving the figuring to me while I could still do this most important part. Also, for gathering supplies when I couldn’t leave the house or drive due to wintry conditions.
Friends like these are entitled to my gratitude and undying joy for having such people in my life.
A few words on construction
I don’t know how many potential ATMs are put off by not only the grinding and polishing, but also the gathering and constructing parts for his telescope. For my own, I kept an eye open for complete telescope assemblies that needed only a new mirror to finish the job. As luck would have it, I ran across parts from a company that had failed and were selling either complete tube assemblies and/or the parts I needed. And all these parts fit. Very nice.
At other times I have seen complete telescopes that needed a bit of mirror work and some touching up to make an impressive instrument. A lot of these are foreign made, and quantities of them pass through EBay daily. I like to see these telescopes as “kits” for the aspiring telescope maker who wants to take a shot at optical work by grinding a mirror for one of these. My only advice on doing this is to avoid cutting corners on mirror production such as polishing pads, machine polishing and figuring, using excessive manual speeds to rush the work, and using ridiculously thin blanks. Buy a telescope making book and stick with that book because asking for help on the internet is only going to confuse rather than help you. The problem? Too many experts!
I have to admit one difficulty in replacing the mirror in your telescope “kit.” It can be difficult to replace the primary mirror with one of your own with the same focal length. In one case, it is easy to shorten your tube, but if your mirror’s focal length is too long for the tube, your only alternative is to regrind your mirror to a shorter focal length. So be careful. I have to admit that after all these years, I fell into this trap!
And by the way, please remember that, if you are so inclined, you are entering this hobby not to save money, but to learn the skills that will give you not only a good telescope but with practice, a superb one. Admittedly, some of this is helped with the choice of a 6” mirror with an f/8 focal ratio.
Note that the rack and pinion on this telescope is only a one speed manual device. No bells no whistles! The secret is that you make things a lot easier for yourself when you choose f/8 as the f/ratio for your mirror. To focus an f/8 mirror is not the delicate chore you have to endure when your mirror is an f/4, f/2.5 or some such number. At f/8, or even f/6, an observer can do very well with a manual rack and pinion with no motors, and no astronomical price.
My dark bucket presents a challenge in two related ways. First, one has to purchase a hole saw to cut into the side of his main tube opposite the eyepiece holder. After googling for such a device, I found a hole saw that matched the diameter of a 100mm can I selected. The second challenge is that one must insert the can into the tube and trace out the parts of the can which project into light’s way. This excess must be cut from the can before it is welded or epoxied to the main tube. All of this requires some ingenuity. Of course, one doesn’t need to have a dark bucket if it really isn’t wanted.
My mounting is an equatorial costing me a total of $80. The crutch tripod comes from my idea first mentioned in a January, 1997 Sky and Telescope article, page 31. The crutches were cheap and easily found at local yard sales.
Concerning the 12 volt battery, I have read on the Cloudy Nights website many accounts of battery packs of different configurations involving D cells. Instead, I chose to go with a rechargeable 12 volt cell that ran fifty-seven hours continuously until I, and not the battery gave up! Please note its placement as part of my mount’s counterweight system.
This mounting can easily hold 60 kg. The 30 mm pipe threads were lapped together with a mix of cutting oil and 500 carborundum, then cleaned and oiled to produce very smooth bearings on both the Right Ascension and Declination axes. While this mount was actually for another telescope, I adapted it to serve admirably for this one.
Re: Testing the window. When the mirror was fully polished, it was figured to an accurate sphere and set aside. The window was then fully polished and placed in contact over the mirror and the two elements subjected to the Foucault test. Two aspects were noted: First, I measured the change in focus due to some power in the window as it was not scrupulously flat on either surface -- a few waves error in flatness is perfectly acceptable. In my case the change in focus at the knife edge was less than 12 mm, even as light passed through the window twice, doubling this aberration. The change at the prime focus was negligible due to the large airspace between mirror and window in actual use. Second I checked this combination for any deviations from the mirror’s spherical figure caused by zones in the window’s surfaces. There were none requiring more than minor touching up. Finally, I parabolized the spherical mirror in the usual fashion. Then I figured the entire system to a null via autocollimation with a few finishing touches. Job done!
- Jeff Morgan, Jon Isaacs, NGC007 and 45 others like this