More about spray-coating astronomical mirrors with silver!

Here is a batch of articles and links concerning the spray-on process for making astronomical mirrors reflective using protected silver solutions.

Long ago, I translated Foucault’s monograph on making paraboloidal, silvered astronomical mirrors. Part of his article described the process that he and Steinheil developed for silvering, which involved using silver nitrate solutions and various other reagents. It looked quite tricky, and also required further polishing! Plus, our telescope making workshop here in Washington DC had a Navy surplus vacuum chamber that was (and still is) quite effective at putting on good-quality, inexpensive aluminum coatings for any mirror up to 12.5″ diameter.

However, I and a couple of other ATMers (Bill R and Oscar O) are working on mirrors in the 16 to 18 inch range, and they simply won’t fit. So I was quite intrigued to watch how Peter Pekurar and some other folks coated a couple of rather large mirrors right in front of a small crowd of onlookers in a tent at this summer’s Stellafane.

I have a few videos on my webpage (here).

There is also an article on the process in the January 2020 Sky and Telescope, and a webpage (here) on the topic run by Pekurar and Howard Banich and others.

Not to mention a bunch of posts on Cloudy Nights (here) and a nice PDF explaining it all, (here).

What is really, really amazing is that the webpage by Pekurar and Banich also has interferograms showing that the overcoating has absolutely no effect on the sub-microscopic, geometrical figure of the mirror! Unfortunately, it’s only effective against chemical attack, not against dirty fingers or scratches. They also did some careful experiments on reflectivity at various wavelengths with various treatments of the surface.

A couple of local ATMers and at least one professional at Goddard Space Flight Center have told me about their experiments with the process; they found that it is easy to mess up if you aren’t stringently clean and also easy to waste materials.

James Tanton : what is K-12 math?

Jim Tanton is a very deep thinker and communicator about many aspects of mathematics. He recently was in residence in the DC area for a few years and was a mentor at Math for America – DC (based at the Carnegie Institution for Science), where I attended several of his highly entertaining and inspiring talks for new and experienced DC secondary Math teachers.

This article by him goes into what mathematics is all about, and how we teach [a part] of that in school. Here is the link:

Problems Solved with the Old 6″ Refractor?


, , , , , , , ,

I found a few things that may have been causing problems:

(1) Whoever put the lens cell together last didn’t pay any attention at all to the little registration marks that the maker had carefully placed on the edges of the lenses, to show how they were supposed to be aligned with each other. I fixed that, as you see in the photo below. The reason this is probably important is that the lenses are probably not completely symmetrical around their central axes, and the maker ‘figured’ (polished away small amounts of glass) them so that if you lined them up the way he planned it, the images would be good; otherwise, they would probably not work well at all and could very well be causing the poor star test images we saw.


2. The previous assembler also put eleven little tape spacers around the edges, between the two pieces of glass. More is apparently not better; experts say you should have three spacers, each 120 degrees apart from the other two. Done.

3. The bottom (or ‘flint’) element is slightly smaller than the other one (the ‘crown’), so it probably shifted sideways. That alone would be enough to mess up the star tests in the way that we saw. So I wrapped two thicknesses of blue painter’s tape around the outside of the flint, and put some three cardboard shims between the edges of the ‘crown’ and the aluminum cell.

4. There were no shims at all between the flint and the aluminum ring that holds it in place underneath. This caused some small scratches on the glass, and might have been warping the glass. I put in three small shims of the same type of blue painter’s tape, lined up with the other spacers.

We will see if these improvements help. I really don’t want to haul this all the way out to Hopewell Observatory and struggle with putting it back on the mount for a star test. That was just way too much work, much more than I expected! The next test will be with an optical flat placed in front of the lenses, and a Ronchi grating.

I would like to thank Bart Fried, Dave Groski, and several other people on the Antique Telescope Society website for their advice.


By the way, these photos show how we held the refractor on the mounting plate for the Ealing mount at Hopewell Observatory.

Trying to Figure Out Problems With a Century-Old Refractor


, , , , , , ,

I am disassembling the lens cell of the >100 year old 6” f/14 Kiess refractor that produces horrible results on star tests.

There is absolutely no information inscribed anywhere inside the cell, inside the tube or outside it, nor on the edges of the lens elements. I can only guess as to what type of glass they used, and figuring it out won’t be easy. The least destructive method I can think of beginning to do this is by weighing them and calculating out their precise volumes, and from that calculating their densities. A graduate gemologist could probably calculate their indices of refraction, but not me.

Tomorrow I plan to measure the curvatures of the lens elements; perhaps someone familiar with old telescopes will then have clues as to who might have made this particular type of optical prescription.

The shims seem to me to be intact, so I think I can rule out astigmatism from lens elements put in crooked. [OTOH, someone on the Antique Telescopes Facebook group says that the large number of small black spacers in between the lenses may itself be causing the massive astigmatism problem that we found in the star test. I don’t have enough experience to be able to tell whether that’s correct or not.]

The small chips on the edge of the second (meniscus? Flint?) lens element were already there when I got it. I was also surprised to find that the first (biconvex, crown?) lens element has a small bubble very close to the center. It’s probably not significant, but I will check for strain as well.


12-inch Ealing-Made Ritchey-Chretien Telescope is Sold [EDIT]


, , , , , , ,

EDIT: It has now been sold to an ambitious telescope maker in Italy. 

We had a 12-inch Casssegrain optical telescope assembly for sale at an extremely attractive price: just two hundred dollars (or any reasonable offer). You pay for shipping.

The full-thickness primary mirror alone is worth much more than that as a raw piece of unfinished Pyrex! (United Lens charges $450 for an equivalent, 12.5″ diameter, roughly 2″ thick, raw, unfigured, disk of Borofloat!)

The telescope was part of a package (mount-cum-telescope) that was purchased from the Ealing company back in the 1960s by the University of Maryland. The scope itself never gave satisfactory images, so the UMd observatory sold it off in the early 1990s, and it ended up at the Hopewell Observatory about a decade before I became a member. Hopewell kept the mount, which still works quite well, but removed the telescope and replaced it with a 14-inch Celestron Schmidt-Cassegrain.

I recently examined the telescope itself (the one we are selling) and found that it indeed has a hyperbolic primary with a focal length of about 4 feet (so it’s f/4). Presumably, the convex secondary is also a matching hyperboloid, to create a Ritchey-Chretien design, but I don’t feel like perforating a large spherical mirror to create a Hindle sphere to test it properly. In any case, using a 12-inch flat, I was unable to produce decent Ronchi images.

As you may know, figuring and collimating a Richey-Chretien require a LOT of patience, more than I have. My suggestion would be to refigure the primary into a paraboloid, procure a standard flat, elliptical diagonal, and repurpose this as a Newtonian. Refiguring this mirror a task that I don’t feel like taking on, since our observatory already has a 14″ Newtonian, a 14″ SCT, and I already have built a 12.5″ Newtonian of my own. Plus, I am finding that figuring a 16.5″ thin mirror is plenty of work already.

So, our loss could be your gain! Make an offer!

I attach a bunch of photos of the OTA from several viewpoints, including a ronchigram. The mirror has been cleaned off since these picture were made; the little electronic motor was for remote focusing of the secondary.

A Weekend at Almost Heaven


, , , , ,

I spent Labor Day weekend at the Almost Heaven Star Party very close to Spruce Knob, the highest ridge in West Virginia. When the skies cleared at night, the stars and Milky Way were magnificent, but that only happened about 1 night out of three. My 12.5″ home-made Dobsonian telescope performed very well; in fact, because its primary and secondary mirror are almost fully enclosed by the light shrouds and upper cage, I was able to keep observing long after all the other refractors and Schmidt-Cassegrains were closed down by the heavy dew. (To keep the dew off of my finder scope and Telrad, I used large rubber bands to wrap chemical hand warmer packs around them, and that crude and cheap arrangement worked very well!)

Here are three photos taken by me:

All but the photo with the sextant were taken by Oscar.

Cleaning Up a Century-Old Refractor


, , , ,

Last week, I was helping staff and students at the University of Maryland’s Observatory to clean out a storage trailer.

We noticed a seven-foot-long, 6-inch diameter telescope that had been sitting in a corner there, unused, ever since it was donated to the National Capital Astronomers (NCA) club nearly ten years earlier by the son of the original owner, Carl Kiess,  who had worked at the Lick Observatory in California and the National Bureau of Standards in or near DC, but who had passed away nearly fifty years earlier. I figured I could put it on a motorized telescope mount at Hopewell Observatory and at a minimum test the optics to see if they were any good. The current officers and trustees of NCA all said they thought this was a good idea.

One thing that caught my eye was how filthy and flaky the coating was on the tube itself, although the lens appeared to be in good shape.


The drive, while impressive, does not have a motor, requires a pier, and is extremely heavy. I decided not to mess with the drive and to put it temporarily on our existing, venerable, sturdy, motorized, electronic drive we have at Hopewell Observatory.

So I experimented with various abrasives and solvents to clean off the nasty green coating; a fine wire wheel inserted in an electric drill did the best job. Here it is partly cleaned off:

I then used Brasso for a final polish, followed by a final cleaning with acetone, and then applied several coats of polyurethane to keep it looking shiny for a number of years. (The lenses stayed covered for all of this!) So this is how it looks now:

The next task is to make a temporary holder and then put it on the mount, and then test the optics.

Silvering Mirrors, and More, at Stellafane

For me, these were the two most significant demos at the 2019 Stellafane Convention in Springfield, Vermont:

(1) Silvering large mirrors, no vacuum needed

We had a demonstration by Peter Pekurar on how to apply a layer of Silver (metallic Ag, not aluminum) onto a telescope mirror, accurately, with a protective, non-tarnishing overcoat, that works well. I looked through such a scope; the view was quite good, and I was told that interferograms are great also.

What’s more, the process involves overcoating a mirror with spray bottles of the reagents, without any vacuum apparatus needed at all. Note: Silver coated, not aluminum coated. This is big for me because the upper limit at our club’s aluminizer is 12.5″, but some of us are working on larger mirrors than that; commercial coaters currently charge many hundreds of dollars to coat them.

You can find information on some of these materials at Angel Gilding. Peter P said he will have an article out in not too long. Here are a few photos and videos of the process:


Finished mirror; notice it’s a little blotchy




(2) Demo and links for Bath Interferometer (see )

How to set up and use a Bath interferometer to produce highly accurate interferograms of any mirror for many orders of magnitude less cash than a Zygo interferometer. As I wrote earlier, Alan Tarica had taken the lead on fabricating one at the CCCC – NCA ATM workshop, and we eventually got it to work, but found it rather frustrating and fiddly to use.

The presenter is a HS teacher, and it shows: he explains things very clearly! On his website ( ) you can get plans for 3-D printing the parts for the Bath device, if you have any access to a 3-D printer, so you can print the parts out for yourself. He also has links to vendors that are selling parts for it, such as certain small lenses, mirrors and beam splitters. He shows you where you can get them for very little money from Surplus Shed and such places. Or you can purchase his really inexpensive kits that he’s already 3-D printed for you. Plus parts for an XYZ stage, which you will need for fine focus. The whole setup (not counting mirror stand and two tripods, which he assumes you have access to already) is under $130.

I will need to look carefully at our setup as built almost completely by Alan, and see how it differs and what we would need to do to make it better. The problem is that there are lots of little, tiny parts, and many of them need to be adjustable. We saw him doing LOTS of little adjustments!

Before his talk, I had absolutely no idea how this (or similar interformeters) really worked. Now I understand: the interference fringes that we see are really contour lines – like we see on on a USGS topo map, only with the mirror tilted in one direction or the other. A big difference with the USGS topo map is that there, the contour lines (isohypses – a new word for me today) are often 10 feet to 100 meters apart. In interferometry, the contour intervals are either one or one-half lambda (wavelength of light) apart – a really tiny amount! We need that level of accuracy because the surface we are studying is sooooooo flat that no other measuring system can work. His explanation of this whole thing now makes perfect sense to me. And the purpose of the software (free!) is to un-slant the mirror and re-draw it using the countour-line information.

Beautifully clear explanation!

Caution: a friend who works professionally in optics told me his team had made three Bath interferometers, using cheap but good quality ebay xyz stages, and found that they were just too much trouble; so they borrowed a very expensive commercial interferometer (costing many tens of kilobucks) from another department and are using that instead. I’m not selling my house to get a Zygo interferometer!!! But I will try the Bath interferometer instead.