03 Thursday Jan 2019
Posted in astronomy, astrophysics, flat, optical flat, Optics, Telescope Making
Tags
figuring, flats, Gordon Waite, machine, optical, Optics, parabolizing, Polishing, Telescope Making, testing optics, Waite Research, youtube
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:
08 Saturday Sep 2018
Posted in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making
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:
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?
08 Sunday Oct 2017
Posted in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making
Tags
cassegrain, completing the square, ellipsoid, hyperbola, hyperboloids, Optics, parabola, sphere, testing
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
I got
y^2 – 0.112(x – 848)^2 = – 80540
and eventually
(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)
29 Saturday Apr 2017
Posted in astronomy, monochromatic, optical flat, Telescope Making
Tags
Oscar Olmedo and Jeff Dunn both took the opportunity of a clear night last night to achieve first light with the telescopes that they have been working so hard on. The target was Jupiter. The location is right outside the Chevy Chase Community Center, and time was just about 10:00 pm. The fact that you can see so much in this iphone image shows that light pollution is a real problem there.
We also managed to do a star test using an artificial star on Oscar’s 6″ f/3. I made the testing rig with considerable help from Alan Tarica and Bill Rohrer. We reflected the light off of a known optical flat so as to double the testing distance. We had everybody in attendance at the telescope=making workshop examine the inside- and outside-of-focus images, and we all agreed that using the images in Richard Suiter’s book, it’s a bit overcorrected, probably somewhere near 1/4 wave of green light, which was what we were using — a green laser pointer attenuated and stopped down to about 100 micron hole. But good enough.
Next step for Oscar is to aluminize his mirror in our vacuum chamber.
Congratulations to both gentlemen!
14 Saturday Nov 2015
Posted in astronomy, flat, optical flat, Telescope Making
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.
13 Friday Feb 2015
Posted in flat, monochromatic, optical flat, Telescope Making
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!!!
That’s me looking skeptically at my cell phone, pretending to look skeptically at the glass.
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.
