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:
This ultra-short scope, by Todd M, has a mirror of 4.25″ (108 mm) and a pretty short focal length – about 2 feet (60 cm). He made just about everything, right here in the NCA ATM workshop at the Chevy Chase Community Center. He ground, polished, figured, and even helped aluminize the primary mirror; made the primary cell AND the spider and secondary holder; made all of the rest of the mount that you see; and even made the focuser itself from some plumbing parts!
It’s a very nice job, meriting a lot of praise. In case you were wondering, the paint was a special, very-high quality and very expensive top-of-the-line alkyd enamel, costing about $200 per gallon – and we have two of them. Explanation: it was an ‘oops’ can that was specially ordered and mixed for someone who changed their mind and couldn’t return it. In exchange for a non-profit donation receipt in the name of NCA, Bill R was able to get the person to donate both gallons to us.
The spider and secondary holder are very similar to the one made by Ramona D that you can see here. The major differences are:
(1) Todd used busted bandsaw blades rather than steel strapping tape for the vanes. (Both were the same price: free.) After looking at both projects, which both turned out quite nicely, my conclusion is that if you want to use bandsaw blades, you have to heat-treat (anneal) them so they will have less of a tendency to break right at the location where you are trying to bend them by 45 degrees. (Heat it up to cherry red and then let it cool slowly in the air, making it softer and less brittle, I am told…)
(2) And of course, it certainly helps to grind down the teeth of the bandsaw blade both for safety and to reduce weird reflections. Strapping tape is about the same thickness as many band saw blades, but the tape is wider and hence more stable and less prone to turn crooked (I think).
(3) Todd used ordinary 1/4″-20 machine screws (aka bolts) to attach the vanes of the spider to and through the walls of the tube. He cut off the heads of the bolts and ground one side flat near the head, and then drilled a little hole in that flat part, tapped (threaded) that, and used a tiny little machine screw to attach the vane to the specially-prepared screw, in a process that I hope is clearly shown in these three drawings.
(4) Ramona, however, used thumbscrews instead of doing all that cutting, filing and tapping. Actually, our little tiny tapping drills didn’t play well with our bit holders – they kept slipping. So she just drilled holes in the center of each thumbscrew head, and bought three very small nuts and bolts and used them in the place of the little screw that Todd used.
(Thumbscrews like these:)
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.
Have you ever tried to make a convex optical surface?
If so, you know that it’s much more challenging than a concave one, since the rays of light do not come to a focus at all.
Some of us* at the Amateur Telescope Making workshop here in Washington DC have made several attempts at doing this, pretty much without success. I would like to show you some weird images that we got when we tried to ‘figure’ the convex surface by performing a Ronchi test from the back side, looking through what was supposed to be a flat.
What we find is that even though the glass itself is very clear and free of visible strain when seen by the naked eye or when using crossed polarized filters, it looks like we are looking through an extremely murky and totally un-annealed piece of ancient Venetian glass, causing all sorts of weird striations in what should otherwise be nice, smooth Ronchi lines.
These pictures go in order from outside the radius of curvature to inside the ROC.
You might well think that the glass itself has lots of strain left in it, causing the very weird patterns that you see here. I can prove that this is not the case by showing you a short video that we made with crossed polarizing filters of the 5-inch diameter blank itself and two pieces of plastic (the protective covers for one of the filters). Judge for yourself.
This is not the first time that this strange phenomenon has occurred.
Any suggestions from those with actual experience would be extremely welcome.
* Me, Nagesh K, and Oscar O.
I put all of my tickets (over 30!) into the raffle for a 100-degree-apparent-field-of-view eyepiece at AHSP but didn’t win it. I probably should have put a few tickets in some other raffles. They had a whole lot of different stuff being raffled off. The eyepiece I wanted was donated by Hands On Optics. At AHSP they give you ten tickets as part of your registration, and then you can buy more of them. Elizabeth Warner and her husband left the morning of the raffle (Sunday) and gave me theirs, which was very nice of them.
I’ve been trying to make an optical flat for some time now. It’s not easy, even if you are starting with a piece of ‘float’ glass – modern 3/4″ thick window sheet glass that is manufactured by floating a layer of molten glass on a bath of molten tin.
The test apparatus consists of a supposedly-flat 12-inch diameter and a monochromatic light box, and my own gradually-increasing understanding of what the interference lines actually mean. Essentially, they are like contour lines on a topographic map, but the trick is to figure out which sections represent valleys and which ones represent hills. It’s taken help from other amateur telescope makers (particularly Philip P) and sections of Malacara’s book on Optical Testing and http://www.lapping.com .
It’s pretty amazing how we can measure stuff that is soooooo small!
Here are some photos in chronological order of my working on them. I would paste some videos but WordPress won’t allow them and I don’t feel like uploading them to YouTube. BTW: I am not done!!!
Up until this point I was trying to make the flat more perfect by using a hard Gugolz lap of full size (6 inches in diameter), much as we do with parabolizing concave mirrors. I don’t think I made a whole lot of progress. Then I read some of the papers that Philip P sent me, and re-read the Malacara, and decided to think of the contour lines in terms of measures of height, and decided to use a two-inch-diameter lap only on the parts that appeared to be “high”. I marked the back of those regions with a Sharpie permanent marker (which comes off easily with isopropyl alcohol when needed) so I could see where to work and could see if what I did made any difference.The places that I marked with the letter H were High spots, kind of like you see on a weather map that is plotting isobars (lines connecting places with the same barometric pressure). The lower right-hand corner was one of those places, as was the smudged region at about 9 o’clock.
BTW I got the green color by using ordinary fluorescent lamps and two carefully-selected theatrical lighting gels to filter out all the light with wavelengths either longer than or shorter than the green Mercury vapor line of 5461 Angstroms.
By the way: I’ve discovered that the 12-inch-diameter optical flat that is underneath my 6 inch test flat isn’t as flat as I thought. Boo.
Will work on this some more this afternoon.
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.