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Monthly Archives: September 2018

How Thick Are the Coatings on the Mirrors We Aluminize?

09 Sunday Sep 2018

Posted by gfbrandenburg in Uncategorized

≈ Leave a comment

At the NCA ATM class at the CCCC, we are the fortunate inheritors of a 1960s-era vacuum chamber and aluminizer that was twice given away as surplus (first by the Federal Government or US military, and then later by American University), but which still works.

Much of the credit should be given to Dr. Bill Pala, who snagged it for AU from the US surplus system; the late Bob Bolster and Jerry Schnall, who together ran it for a long time; Dr. John Hryniewicz; Alan Tarica; and several others whose names unforotunately escape me at the moment but who have given excellent advice on repair and maintenance and even provided replacement parts.

Here are four photos of the rig, followed by two of our finished mirrors.

IMG_0069
IMG_0070
IMG_0071
IMG_0072

IMG_0064

032

The question came up: (1) how thick are the coatings we generally put on our mirrors, and (2) how efficient is it — that is, of the aluminum that we vaporize, how much of it actually lands on the mirror?

Thankfully, John H actually measured how thick the coatings are as we coated a mirror. He found that the average thickness is about 93.4 nanometers (billionths of a meter, or thousandths of a micrometer, or millionths of a millimeter), and that the coatings looked like this when blown up sufficiently: ”

 I have attached some scanning electron micrographs of the top of the film.  The grain structure is very fine, you can compare the size with the scale bar at the bottom that shows you the length of the total bar (10 ticks, between each pair of ticks is 1/10 of that number).  There are some particles or perhaps larger grains on top.  They are still very submicron, a couple of tenths at most.

170901_ext__0156_SE(U)
170901_ext__0157_SE(U)

The way the mirrors get coated is basically three steps:

(1) We get the mirror very, very clean, using both with a special detergent (Alconox) bbefore it goes into the vacuum chamber, and high-energy electron bombardment while the pumps are working;

(2) We get the pressure in the chamber very, very low, so that there are relatively few air molecules or atoms between a coiled tungsten filament and the mirror. (We get the pressure down to somewhere in the range of 7*10^-5 to 4*10^-4 Torr, depending on which gauge you believe. This is quite low indeed – roughly the air pressure at the altitude of the International Space Station; this is needed so that the aluminum atoms won’t tend to bounce of the molecules of nitrogen and oxygen and lose their energy.

(3) We melt, and then boil off, a small quantity of pure aluminum from the filament, which goes off in all directions, fairly evenly; the Al atoms that happen to be going in the right direction ashere to the mirror. There, they form a very even, reflective layer.

You may wonder, how do we prevent this layer of aluminum from oxidizing once it comes back into contact with the normal atmosphere? Answer: we don’t. Aluminum oxide is the main component of rubies, sapphires, and corundum, which are very hard. Since the stuff we deposit is relatively pure, it doesn’t have the red or blue color of those pretty and precious gems, and it is transparent, so it forms a hard, transparent, protective layer all by itself. If your coating tarnishes or gets extremely dirty, the aluminum-and-gunk layer is pretty easy to remove with a little bit of hydrochloric acid mixed with copper sulfate. Then you clean it off and re-aluminize.

(Yes, commercial labs do overcoat their mirrors with stuff like Silicon Monoxide and Silicon Dioxide (aka quartz), but we haven’t collectively figured out how to do that with our minimal budget.)

So, again: how efficient is it? What percentage of the atoms of aluminum headed to the mirror, actually adhere to the mirror?

To answer this, it helps to pretend that the filament is at the center of an imaginary sphere, shown below, and that the mirror (facing down, towards the mirror) happens to be at the top of this sphere. Recall that to a good approximation, the aluminum that evaporates off of the coil goes in all directions, i.e., it coats this entire imaginary sphere equally – or it would, if there wasn’t all sorts of pipes and wires and glass bell jars in the way.

The filament and aluminum is located at the center of this sphere.

I measured the distance from the filament to the mirror, and found that it’s just about 20 inches, or roughly 500 millimeters. Archimedes figured out long ago that the surface area of a sphere is equal to four times the area of any circle contained in the sphere, or 4*pi*r^2 in our current notation. So that imaginary sphere, on which the aluminum is deposited, has an area of about 3.1 or 3.2 million square millimeters.

imaginary sphere for aluminization

We currently use slugs of aluminum that are about 15 mm long (give or take a couple of mm) and cut (not at right angles, because the pliers won’t do that) from wire with a diameter of 5 mm (radius 2.5 mm). If we pretend the slugs are cylinders then the math is much easier: we can use the formula pi*r^2*h to get a volume of about 295 cubic millimeters, and we will pretend that all of the aluminum boils off (and none of it sticks like glue to the tungsten) and goes equally in all directions. (Probably not the case, but in practice it doesn’t seem to matter much.)

Now if we divide the 295 mm^3 of aluminum by the total surface area, 3.2 million mm^2, we get the average thickness. I get a result of about 9.1*10^-5 mm, which converts to 91 nanometers. Which is very close to the result that John H found.

On the other hand, most of that aluminum is wasted, because it’s NOT aimed at the mirror. If you have an 8-inch diameter mirror (about 20 cm diameter or 100 mm radius), its area is 10,000*pi square millimeters, or about 31,000 mm^2 – and that’s only one percent of the area of the entire imaginary sphere.

Oh, well, aluminum wire is quite cheap.

 

 

 

Difficulties in Using the Matching Ronchi Test on a 12″ Cassegrain Mirror

08 Saturday Sep 2018

Posted by gfbrandenburg in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making

≈ 2 Comments

Tags

Astro Bananas, cassegrain, couder, double pass auto collimation, ealing, foucault, Hopewell Observatory, matching Ronchi, Mel Bartels, Ronchi, ronchigram

The other regulars and I at the DC ATM group at the CCCC have been trying to test a 12 inch Cassegrain mirror and telescope manufactured nearly 50 years ago by a company called Ealing and currently owned by the Hopewell Observatory, of which I am a member. It hasn’t been easy. I discussed this earlier on Cloudy Nights.

Reports from several people, including Gary Hand and the late Bob Bolster, indicated that the optics on this mirror weren’t good at all. Apparently the folks at the University of Maryland’s observatory were sufficiently unhappy with it that they either sold it or gave it to National Capital Astronomers, a local astronomy club, who in turn gave it or sold it to Hopewell Observatory.

With a plain-vanilla Ronchi test, we could see that the mirror was very smooth and continuous, with no turned edge, astigmatism, or bad zones. With the Foucault/Couder zonal test (aka “Foucault” test) , I got results indicating that it was seriously overcorrected: some sort of hyperboloid, rather than the standard paraboloid characteristic of classical Cassegrain telescopes, which have a parabolic primary mirror and a hyperbolic secondary mirror.

However, I have begun losing my faith in my zonal readings, because they often seem to give results that are way out of whack compared to other testing methods.

So we decided to do some additional tests: the Double-Pass Auto-Collimation (DPACT) test used by Dick Parker, as well as the Matching Ronchi test (MRT).

The DPACT is very fiddly and exacting in its setup. We used (and modified) the setup lent to us by Jim Crowley and illustrated by him at his Astro Bananas website. Our results seem to show that the mirror is in fact NOT parabolic, rather, overcorrected, which confirms my Foucault measurements. If it were a perfect paraboloid, then the ronchi lines would be perfectly straight, but they definitely are NOT: they curve one way when inside the focal point, and curve the other when the tester is outside the focal point.

We also tested the entire setup on a radio tower that was about half a mile (~1km) distant. We found that the images were somewhat blurry no matter what we did.

We also attempted the MRT on the same mirror. However, requires very accurate photography and cutting-and-pasting skills in some sort of graphics programs. What you are inspecting is the curvature of the Ronchi lines. Here is the result that Alan T and I got last night:

matching ronchi for 12 inch cass

In black is the ideal ronchigram for this mirror, according to Mel Bartels’ website. The colored picture is the one we made with either my cell phone or the device I finished making earlier this week, shown in my previous post. Here are the two images, separated rather than superimposed:

IMG_1337

ideal ronchigram 12 inch cass ealing

The mirror’s focal length is 47.5″ and the grating has 100 lines per inch, shown somewhat outside of the radius of curvature. The little ‘eyelash’ on the lower left is simply a stray wire that was in the way, and doesn’t affect the image at all. The big hole in the middle is there because the mirror is a cassegrain.

I don’t know about you, but I don’t really see any differences between the real mirror and the theoretical mirror. Do you?

Conclusion

So, what does this all mean?

  • One possibility is that the mirror is in fact perfectly parabolic (as apparently shown by the MRT, but contrary to what I found with Foucault and DPACT) but there is something wrong with the convex, hyperbolic secondary.
  • Another possibility is that the mirror is in fact NOT parabolic, but hyperbolic, as shown by both my Foucault measurements and the DPACT (and contrary to the MRT), which would mean that this telescope was in fact closer to a Ritchey-Chretien; however, since it was marketed as a classical Cassegrain, then the (supposedly) hyperbolic secondary was in fact not tuned correctly to the primary.
  • The answer is left as an exercise for the reader.
  • A star test would be the best answer, but that would require being able to see a star. That hasn’t happened in these parts for quite some time. In addition, it would require an eyepiece holder and a mount of some sort. Or else setting up an indoor star…

Latest Ronchi or Knife-Edge Tester for Mirrors and Other Optics Using a WebCam

07 Friday Sep 2018

Posted by gfbrandenburg in astronomy, Optics, science, Telescope Making

≈ 1 Comment

Tags

brightness, color balance, exposure, focus, foucault, gain, knife edge, Ronchi, testing, webcam

Here is the latest incarnation of my webcam Ronchi and knife edge (or Foucault) tester. It’s taken quite a few iterations to get here, including removing all the unnecessary parts of the webcam. I attach a still photo and a short video. The setup does quite a nice job of allowing everybody to see what is happening. The only problem is setting the gain, focus, exposure, brightness, color balance, contrast, and so on in such a way that what you see on the screen resembles in any way what your eye can see quite easily.

IMG_1335

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