Some very nice folks from the Australian Broadcasting Corporation came and interviewed me on film for a bit on folks who make their own telescopes to see the great August 2017 eclipse. Here is the link:
artificial star, celestron, classical cassegrain, couder, double pass autocollimation test, ealing, FigureXP, focus, foucault, hyperbolic, optical tube assembly, parabolic, primary, refurbishing, ritchey-chretien, Ronchi, schmidt-cassegrain, secondary, spherical, Telescope
We at the Hopewell Observatory have had a classical 12″ Cassegrain optical tube and optics that were manufactured about 50 years ago.; They were originally mounted on an Ealing mount for the University of Maryland, but UMd at some point discarded it, and the whole setup eventually made its way to us (long before my time with the observatory).
The optics were seen by my predecessors as being very disappointing. At one point, a cardboard mask was made to reduce the optics to about a 10″ diameter, but that apparently didn’t help much. The OTA was replaced with an orange-tube Celestron 14″ Schmidt-Cassegrain telescope on the same extremely-beefy Ealing mount, and it all works reasonably well.
Recently, I was asked to check out the optics on this original classical Cassegrain telescope, which is supposed to have a parabolic primary and a hyperbolic secondary. I did Ronchi testing, Couder-Foucault zonal testing, and double-pass autocollimation testing, and I found that the primary is way over-corrected, veering into hyperbolic territory. In fact, Figure XP claims that the conic section of best fit has a Schwartzschild constant of about -1.1, but if it is supposed to be parabolic, then it has a wavefront error of about 5/9, which is not good at all.
Here are the results of the testing, if you care to look. The first graph was produced by a program called FigureXP from my six sets of readings:
I have not yet tested the secondary or been successful at running a test of the whole telescope with an artificial star. For the indoor star test, it appears that it only comes to a focus maybe a meter or two behind the primary! Unfortunately, the Chevy Chase Community Center where we have our workshop closes up tight by 10 pm on weekdays and the staff starts reminding us of that at about 9:15 pm. Setting up the entire indoor star-testing rig, which involves both red and green lasers bouncing off known optical flat mirrors seven times across a 60-foot-long room in order to get sufficient separation for a valid star test, and moving two very heavy tables into said room, and then putting it all away when we are done, because all sorts of other activities take place in that room. So we ran out of time on Tuesday the 5th.
A couple of people (including Michael Chesnes and Dave Groski) have suggested that this might not be a ‘classical Cassegrain’ – which is a telescope that has a concave, parabolic primary mirror and a convex, hyperbolic secondary. Instead, it might be intended to be a Ritchey-Chretien, which has both mirrors hyperbolic. We have not tried removing the secondary yet, and testing it involves finding a known spherical mirror and cutting a hole in its center, and aligning both mirrors so that the hyperboloid and the sphere have the exact same center. (You may recall that hyperboloids have two focal points, much like ellipses do.)
Here is a diagram and explanation of that test, by Vladimir Sacek at http://www.telescope-optics.net/hindle_sphere_test.htm
FIGURE 56: The Hindle sphere test setup: light source is at the far focus (FF) of the convex surface of the radius of curvature RC and eccentricity ε, and Hindle sphere center of curvature coincides with its near focus (NF). Far focus is at a distance A=RC/(1-ε) from convex surface, and the radius of curvature (RS) of the Hindle sphere is a sum of the mirror separation and near focus (NF) distance (absolute values), with the latter given by B=RC/(1+ε). Thus, the mirrorseparation equals RS-B. The only requirement for the sphere radius of curvature RS is to be sufficiently smaller than the sum of near and far focus distance to make the final focus accessible. Needed minimum sphere diameter is larger than the effective test surface diameter by a factor of RS/B. Clearly, Hindle test is limited to surfaces with usable far focus, which eliminates sphere (ε=0, near and far focus coinciding), prolate ellipsoids (1>ε>0, near and far foci on the same, concave side of the surface), paraboloid (ε=1, far focus at infinity) and hyperboloids close enough to a paraboloid to result in an impractically distant far focus.
We discovered that the telescope had a very interesting DC motor – cum – potentiometer assembly to help in moving the secondary mirror in and out, for focusing and such. We know that it’s a 12-volt DC motor, but have not yet had luck tracking down any specifications on that motor from the company that is the legatee of the original manufacturer.
Here are some images of that part:
If it’s been a while since you spent time looking up at the heavens with your naked eyes, binoculars and telescopes, looking at planets, stars and galaxies, then this Saturday might be your night.
The Hopewell Observatory is having an open house on Saturday, July 2, 2016, and we have a variety of scopes to look through. Some of the scopes will be under our roll-off roof and some will be rolled out onto the small lawn outside the observatory itself.
Mars, Jupiter and Saturn will be very conveniently placed for viewing right at sundown, and if it’s dry and clear enough, we should be able to see the Milky Way. Many nebulae, open and globular clusters, galaxies, and double or triple stars will be visible as well.
You are invited! And it’s free!
The location is about an hour due west of Washington DC by way of I-66, near the town of Haymarket, VA. For detailed directions, follow this link, which I posted for one of the dates which got canceled because of bad weather. Ignore the date, but do pay attention to the fact that we have no running water! We have bottled water and a composting toilet and hand sanitizer. Plus makings for coffee, tea, and hot chocolate – all gratis.
The picture above is of one of our telescope mounts, which carries several telescopes and was set up to take astrophotographs at the time. Below is a picture of the outside of the observatory shortly after a snowstorm.. Notice that there is no dome – instead, the galvanized steel roof rolls back on the rails and columns to the right of the picture when the scopes are in use.
If you have your own telescope, feel free to bring it. If it needs electricity, we have an outdoor 120VAC outlet, but you should bring your own extension cord and plug strip. If you want to stay all night, that will be fine, too! If you feel like bringing a cot or a tarpaulin and a sleeping bag, that’s equally OK by us! Show up at or near sunset, and stay until the sun comes up, if you like!
Warning: the area definitely has insects, such as ticks and chiggers, which appear to avoid everybody else and to do their best to attack me. I strongly recommend long pants, shoes/boots, and socks that you can tuck the pants into. Tuck your shirt into your pants as well, and use bug spray, too. I have personally seen plenty of deer, cicadas, moths, wild turkeys, squirrels, and birds, and I have heard from a neighbor that a bear tried to eat his chickens, but other than the insect pests, the wildlife stays out of your way.
Again – for detailed directions, look at this link.
We found these two beautiful moths that flew into the operations cabin at the Hopewell Observatory a couple of nights ago, and we have no idea what type they are. Never seen them before and can’t find any images identical to them. (One species is similar, though.)
Any suggestions will be welcome.
Ain’t they purty li’l things?
And when they opened their wings they were even more spectacular, but I didn’t get a good shot.BTW the yellow-and=red moth is sitting on the struts of a telescope made by Alan Bromborski.
All Newtonian telescopes require a secondary mirror — a flat mirror held at roughly a 45-degree angle to reflect the light from the primary out to the side. Generally this secondary mirror is an ellipsoid, in order to waste as little light as possible.
One major problem is figuring out how to hold this secondary mirror in place securely without interfering with the passage of light from your distant target. The secondary mirror can be held on a stalk, or on crossed arms like a spider’s web.
The images below show how Ramona D made a spider using a piece of extruded aluminum tube with a square cross section, several bolts, a spring, a piece of plastic dowel, some pieces of steel strapping tape, a few thumbscrews, and various small nuts and bolts. She did a very neat job, including threading and tapping several small holes in the aluminum tube.
The idea is not original to me: I got the idea from somebody else on line, but unfortunately, I don’t recall the name of the person to whom I should give credit.
Here are some photos that probably do a better job of explaining how to make it than I could explain in many, many paragraphs.
Steve S recently finished a telescope with help from the DC-area amateur telescope making (ATM) workshop that I’ve been running at the Chevy Chase Community Center (CCCC) for several years (I took over from the late Jerry Schnall around the turn of the century) with help from several local ATMers and under the auspices of the National Capital Astronomers (NCA).
Steve had made the mirror quite a long time ago (not here in DC). The optics are quite good according to my tests, and if you look at the photos, I think you will agree that the body of the telescope looks excellent as well.
As you can see, he used more-or-less dimensional wood rather than the more conventional plywood. Or should I say, clear pine that had been glued into boards at the lumber factory. He made the cradle with a bolt that allows one to loosen or tighten the grip on the tube so that one can rotate it or shift it forward or back to take care of any changes in balance.
It may not be obvious, but the wood is in fact coated with varnish.
The rocker box is held onto the azimuth bearing with sturdy wingnuts so that it can be more easily transported. The two circular sections of the azimuth bearing were table tops purchased at Lowe’s (IIRC).
I help run the amateur telescope-making workshop at the Chevy Chase Community Center in Washington, DC, sponsored and under the auspices of the National Capital Astronomers. Both the NCA and its ATM group have been on-going since the 1930’s, well before I was born. In our ATM group, have the somewhat esoteric thrill of manufacturing incredibly accurate scientific devices (telescopes), from scratch, with not much more than our bare hands and a few tools. And then we go and use them to observe the incredible universe we come from.
Since these telescope mirrors are required to be insanely accurate, we need extremely high-precision ways of testing them. However, we don’t have the tens or hundreds of thousands of dollars needed to purchase something like a professional Zygo Interferometer, so we use much cheaper ways of testing our mirror surfaces.
Some of those methods are associated with the names Foucault, Couder, Bath, Ronchi, Ross, Everest, and Mobsby, or are described with words like “knife-edge”, “double-pass” and “wire”. They all require some relatively simple apparatus and skill and practice in measurement and observation.
We are of the opinion that no one single test should be trusted: it’s easy to make some sort of error. (I’ve made plenty.) You may perhaps recall the disaster that happened when the Hubble Space Telescope mirror passed one test with flying colors, and other tests that weren’t so good were ignored. When the HST finally flew in orbit, it was discovered that the mirror was seriously messed up: the test that was trusted was flawed, so the mirror was also flawed.
We don’t want to do that. So, at a minimum, we do the Ronchi and Foucault/Couder knife-edge tests before we say that a mirror is ready to coat.
But the ultimate test of an entire telescope is the star test.
In principle, all you need for that is a steady star, your telescope, a short-focal-length eyepiece, and a copy of Richard Suiter’s book on star-testing optical telescopes.
Unfortunately, around here, it’s often cloudy at night, and if it’s clear, it might be windy, and around the CCCC building there are lots of lights — all of which make star-testing a scope on the two evenings a week that we are open, virtually impossible. We aren’t open in the daytime, and even if we were, I don’t see any ceramic insulators on any telephone poles that are both small enough and far enough away to use as artificial stars in the manner that Suiter describes. (There are a few radio towers visible, but I doubt that their owners would let us climb up one of them and hang up a Christmas tree ornament near the top!)
So, that means we need to make an artificial star.
I’ve been reading a few websites written by folks who have done just that, and it seems to be a bit easier than I thought. The key is to get a source of light that acts like a star at astronomical distances — but close enough that we can fit it inside the basement of the CCCC, probably not in the woodshop where we make the scopes, but more likely out in the hallway or in the large activity room next door, both of which are about 40 or 50 feet long.
So here are my preliminary calculations.
First off, it appears that the resolving power of a telescope equals the wavelength being used, divided by the diameter of the objective lens or mirror, both expressed in the same units. The result is in radians, which you can then turn into degrees, arc-minutes, arc-seconds, or whatever you like, but it’s perhaps easier to leave in radians. In any case, the larger the diameter, the tinier the angle that your telescope can resolve if it’s working properly.
I am going to use a 16-inch mirror diameter, or about 0.4 meters, as an example, and I will use green light at about 560 nanometers (560 x 10^-9 m) because that’s pretty close to the green mercury line we have in our monochromatic light box. I then get that the resolution is 1.4×10^-6 radians.
(We can convert that into arc-seconds by multiply that by 180 degrees per PI radians and by 60 arc-minutes per degree and by 60 arc-seconds per arc-minute; we then get about 0.289 arc-seconds. If we were to use an 8-inch mirror, the resolution would be half as good, meaning the object would need to be twice as big to be resolved, or about 0.578 arc-seconds.)
I read that one can make an artificial star by using an ordinary eyepiece and a small illuminated hole that is put some distance away from the eyepiece. The entire setup is aimed at the telescope, and then you have an artificial star. Here is the general idea:
Supposedly, the equations go as follows, with all of the dimensions in the same units. I think I will use millimeters.
We want to make it so that the size of the artificial star will be small enough to be below the limit of resolution of any telescope we are making. I am pretty sure that we can set things up so that there is 40 feet (13 meters) between our telescope rig and the table or tripod on which we sill set up this artificial star.
I also know that I can find an eyepiece with a focal length of 12 mm that I’m willing to use for this purpose, and I also purchased some tiny little holes from “Hubble Optics” that are of the following sizes: 50, 100, 150, 200, and 250 microns, or millionths of a meter. Those holes are TINY!!! So that takes care of H and F. I still need to figure out what SS should be.
A few lines ago, I found that for a 16-inch telescope, I need a resolution of about 1.4×10^-6 radians. The nice thing about radians is that if you want to find the length of the arc at a certain radius, you don’t need to do any conversions at all: the length of the arc is simply the angle (expressed in radians) times the length of the radius, as shown here:
So if our artificial star is going to be 13 meters away, and we know that the largest angle allowed is roughly 1.4×10^-6 radians, I just multiply and I get 1.82×10^-5 meters, or 1.82 x 10^-2 millimeters, or 18.2 microns.
Which means that I already have holes that are NOT small enough: the 150-micron holes are about 10 times too big at a distance of 13 meters, so my premature rejoicing of a few minutes ago, was, in fact, wrong. So, when I make the artificial star gizmo, I’ll need to figure out how to make the ‘star size’ to be roughly one-tenth the size of the holes in the Hubble Optics micro-hole flashlight.
Or, if I rearrange the equation with the L, H, F and SS, I get that L = H * F / SS. The only unknown is L, the distance between the hole and the eyepiece/lens. For H, I have several choices (50, 100, 150, 200 and 250 microns), SS is now known to be 18 microns or so (36 if I want to test an 8-incher), and I plan on using a 12.5 mm eyepiece. If I plug in the 150 micron hole, then I get that L needs to be about 104 millimeters, or only about 4 inches. Note that the longer L is, the smaller the artificial star becomes. Also, if I replace the 12.5 mm eyepiece with a shorter one, then the artificial star will become smaller; similarly, the smaller the Hubble Optics hole, the smaller the artificial star. This all sounds quite doable indeed.
ESSENTIAL STEPS FOR MAKING YOUR OWN TELESCOPE
By Guy Brandenburg
[Guy is the main telescope-making instructor at the amateur telescope-making (ATM) workshop sponsored by the National Capital Astronomers and currently housed at the Chevy Chase Community Center in Northwest Washington, DC. This guide is based on what he has gleaned by reading a variety of books on telescope making, as well as instruction by Jerry Schnall (his predecessor in this workshop), watching demonstrations at Stellafane, his own participation in two Delmarva Mirror-Making Marathons, visits from other ATMers to our workshop (including John Dobson), a tour of a commercial optical factory near Bel-Air, MD, and a visit to the Chabot telescope-making workshop in Oakland, CA. He has tried to steal good ideas from all of them and not to insert too many bad ideas of his own.]
- Plan your project by looking at various home-made and commercial telescopes, either in person or in books or magazines or on-line. Star parties put on by local clubs often will have some home-made telescopes on the field, and their owners/makers will be delighted to tell you what worked well and what they would wish to modify one day. Decide what size and focal length you can afford to make and can actually manage to carry around in your car or your hands, or install somewhere permanently. A Newtonian reflector is by far the easiest type of telescope design to make yourself. If you keep at it and don’t drop the mirror on a hard floor, you are pretty much guaranteed success and a well-performing telescope. All of the other designs (refractors, cassegrains, catadioptrics, etc.) are much, much harder to make and can fail for reasons that are very hard to figure out. Larger-diameter scopes are more expensive, heavier, and take more time to make, but you can see more detail and dimmer objects, too.
- Costs: If you are thrifty and crafty, you can definitely make a telescope of a given size for less money than one you purchase, but if you insist on the finest and most expensive components (exotic wood, for example), you can end up spending much more. Fifty years ago, the only way that the average person could afford to own a telescope was if they made it themselves; commercial 6-inch scopes sold in the 1950s for prices that are equivalent to about $2,000 in today’s money. Today, a six-inch commercial telescope and mount costs much less than that, and the prices of Pyrex-equivalent mirror blanks have recently tripled. A recent study in Sky & Telescope found that the biggest potential savings are for large telescopes, but a lot depends on one’s ability to scrounge and find inexpensive, but good-quality, components.
B. Time: It’s not possible to say exactly how long any project will take. However, I always find that everything that I make, takes longer than I originally estimate, which is known as Hofstadter’s Law. When I made my first telescope, a 6” f/8, I kept notes on how much time I spent on grinding, polishing and figuring, and later added that all up: 30 hours. My second telescope, an 8” f/6, took me 40 hours. But keep in mind, those totals only count the time I spent actually pushing glass, not the time planning things, thinking about things, discussing the project, taking Ronchi and Foucault measurements, setting up the work, and cleaning up after a grit. They also did not include any of the time I spent on making the tubes and mounts or aluminizing the mirrors or learning how to use the telescopes in the first place. Obviously, you might take less time than me, or longer.
C. Longer focal length for a given diameter means sharper images of stars and planets, and usually an easier job of grinding and figuring, and they work well with less-expensive eyepieces, and the scope will work even if not perfectly collimated (aligned internally), and you won’t have problems with coma. However, it also means dimmer images with narrower fields of view, and objects will remain visible for shorter lengths of time, requiring constant adjustment; also the scope will be longer, heavier, and harder to maneuver. A focal ratio of 6 is considered by many ATMers to be an intermediate one.
D. A shorter focal length for a given diameter means brighter images, a wider field of view, and a shorter, lighter telescope that is easy to carry around. However, the figuring process will be more difficult, collimation will be more critical, and coma will be a problem at the edges of your field of view. You may need to use additional tests to verify that your mirror is in fact well-figured.
E. The difficulty, cost, weight, and amount of time needed to make a scope is roughly proportional to the cube of the diameter, which means that a scope with a 12-inch diameter mirror will be about 8 times harder, heavier, and more expensive than a 6 inch diameter telescope. On the other hand, its light-gathering power is proportional to the square of the diameter, so the 12-inch mirror will gather four times the amount of light than a 6-inch mirror, which in practice means that you will see stars and other objects a magnitude or two fainter with the larger scopes.
F. As you can see, in astronomy, there are trade-offs. There is no car, no boat, no garment, no athlete, and no telescope that is BEST for ALL purposes. For example, a scope that is really good at providing a wide-field image of the Pleiades won’t do well at imaging the bands and festoons on Jupiter. Just setting up a huge scope 20” or more in diameter is a major undertaking and probably requires a step-ladder, whereas a 6” Dob can be set up and ready to go in about a minute by a sixth-grader. A large Dob is going to do its best work at a dark-sky location like the Rockies or parts of West Virginia. A computerized go-to scope, which you are probably not going to be able to build yourself unless you have some amazing skills in machining, computer programming, and electrical engineering, can work anywhere. Your goal should be to build a telescope that will be used!
G. Rather important: a home-made Dobsonian-mounted Newtonian reflector is not really suitable for astrophotography. For that, you will need a very expensive equatorial mount as well as a digital camera of some sort (DSLR or CCD or dedicated web-cam) and, usually, a computer and a power supply. A “Dob” is GREAT for optical viewing with your eyes, and you don’t have to spend the entire night doing polar alignment, taking flats and darks and multiple guided exposures of the same object for hours on end with an expensive DSLR or CCD camera or dedicated web-cam; nor do you have to spend hours and hours learning how to digitally process all those digital images. Instead, you LOOK with your eyes, and you can look at dozens of different objects in a night, without fussing with electronic gadgets…
H. You will need to decide on the materials for your mirror: plate glass (relatively cheap but usually no more than ¾” thick), Pyrex or its generic equivalents, or a relatively exotic material such as ‘fused silica’ (i.e. quartz) or a really exotic material like Zerodur, Cer-Vit, or BVC. For several decades, ATMers used full-thickness Pyrex mirror blanks produced in massive amounts by Corning Glass. They were relatively inexpensive, durable, and had low thermal coefficients of expansion. Unfortunately, Corning has stopped making Pyrex, and the generic replacements known as Borofloat and other trade names are now made overseas in considerably smaller batches, so the price for a full-thickness borosilicate crown glass equivalent to Pyrex has tripled or quadrupled. It turns out that modern float glass (also called plate glass) works quite well for making telescopes, unlike the wavy window-pane glass of 80 years ago. The term “float” means that the glass is made in a continuous process and is annealed and manufactured to be extremely flat and homogeneous by, yes, floating it on a bed of pure molten tin. You do have to be more careful about preventing astigmatism, and must cool the glass down to ambient room temperature in a bath of cool water whose temperature you have measured, but many excellent mirrors have been made using ¾”-thick plate glass cut into disks by the water-jet method at a local glass fabricator.
I. Instead of making your own mirror, you could buy a used or new one that someone else made. There are generally several used primary mirrors for sale at any given time, of various sizes and prices, at websites like eBay, Cloudy Nights Classified, or Astromart. Astromart has the widest selection, but you have to pay a fee (about $10-$20 per year) to post or respond to ads. Cloudy Nights has a lot of forums where people express their opinions on subjects astronomical, and many of them are probably correct. (Definitely worth reading in any case!) Some of the scopes you can find are real bargains, but you don’t know what the quality is unless you or someone you trust tests it. Here at the NCA ATM workshop, we generally have some mirrors of various sizes that were either made by one of us, or which were donated to us, or which we bought through one of those online sources.
J. Record-keeping: you will be engaging in a scientific project, so you will need to do what scientists do: keep records of what you do. Write down what grit size you are on, what sort of stroke you are using, how long you spent on the various steps, and how you overcame the various problems that arose. If possible, take photographs of your work (especially of the ronchigrams during the figuring process). Also make plenty of diagrams and sketches and calculations, and label them, too! Your notes will let you know what works and what doesn’t, and will save you a tremendous amount of time. At the CCCC, we have a file cabinet and hanging file folders in which you can keep your notes.
K. If you have decided to make your own mirror for your own telescope, then there are four major steps: Rough grinding or “hogging out”; medium and fine grinding; polishing; and figuring. The hardest part is the figuring, but fortunately by the time you get there, you will have learned a lot about how to treat the glass. Let’s examine all four steps, in my next post. ==>