I am posting some photos and videos of the demonstration of a Bath interferometer on Saturday at the 2019 Stellafane convention.
I am posting some photos and videos of the demonstration of a Bath interferometer on Saturday at the 2019 Stellafane convention.
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:
(2) Demo and links for Bath Interferometer (see http://gr5.org/bath )
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 ( http://gr5.org/bath ) 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.
Gordon Waite is a commercial telescope maker who has made a number of very useful YouTube videos on his grinding, polishing, parabolizing, and testing procedures. I thought some of my readers might be interested in viewing them. The link is here, or else you can copy and paste this:
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:
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:
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?
So, what does this all mean?
I continue to try to determine the foci of the apparent hyperbolic primary on the Hopewell Ealing 12inch cassegrain, which has serious optical problems.
My two given pieces of information are that the mirror has a radius of curvature (R) of 95 inches by my direct measurement, and its Schwarzschild constant of best fit,(generally indicated by the letter K) according to FigureXP using my six sets of Couder-mask Foucault readings, is -1.112.
I prefer to use the letter p, which equals K + 1. Thus, p = -0.112. I decided R should be negative, that is, off to the left (I think), though I get the same results, essentially, if R is positive, just flipped left-and-right.
One can obtain the equation of any conic by using the formula
Y^2 – 2Rx + px^2 = 0.
When I plug in my values, I get
Y^2 + 190x -0.112x^2 = 0.
I then used ordinary completing-the-square techniques to find the values of a, b, and c when putting this equation in standard form, that is something like y^2/a^2 – x^2/b^2 = 1
Omitting some of the steps because they are a pain to type, and rounding large values on this paper to the nearest integer (but not in my calculator), I get
y^2 – 0.112(x – 848)^2 = – 80540
(x – 848)^2 / 848^2 – y^2 / 248^2 = 1
Which means that a is 848 inches, which is over 70 feet, and b is 284 inches, or almost 24 feet. Since a^2 + b^2 = c^2, then c is about 894. And the focal points are 894 inches from the center of the double-knapped hyperboloid, which is located at (848, 0), so it looks a lot like this:
Which of the two naps of this conic section is the location of the actual mirror? I suppose it doesn’t make a big difference.
Making that assumption that means that the foci of this hyperbolic mirror are about 894 – 848 = 46 inches from the center of the primary mirror. I don’t have the exact measurement from the center of the primary to the center of the secondary, but this at least gives me a start. That measurement will need to be made very, very carefully and the location of the secondary checked in three dimensions so that the ronchi lines are as straight as possible.
It certainly does not look like the common focal point for the primary and secondary will be very far behind the front of the secondary!
Bob Bolster gave me an EXTREMELY fast spherical mirror that is about f/0.9 and has diameter 6 inches. I didn’t think at first that would be useful for doing a Hindle sphere test, since I thought that the focal point in back of the secondary would be farther away. But now I think it will probably work after all. (Excellent job as usual, Bob!) (I think)
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:
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’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.
Will work on this some more this afternoon.