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2012-08-07, 01:56   #12
ewmayer
2ω=0

Sep 2002
República de California

7×23×71 Posts

Quote:
 Originally Posted by only_human They were looking for interference bands in laser light. There was splitting and recombining the laser light and observing things through an eyepiece that was part of the test rig. That is what my hazy and flawed memory thinks I was shown. I've never known what test was applied or term is appropriate for what I saw. I should have paid much closer attention. I would have liked to have seen some mirror grinding too but don't remember seeing any.
My senior engineering project in college was the design, building and collimation of a beam splitter for a laser Doppler anemometry system to be used in fluids-turbulence research, so I have a little experience with applied laser interferometry. In my case, HeNe beam passed through a half-silvered 1cm^3 mirror (the cube silvering plane intersecting 2 opposing faces of the cube, via one diagonal of the respective faces), then we used several secondary mirrors to cause the 2 resulting beams to exit 10 cm apart (taking same-length optical paths in doing so) and reconverge several meters outside the beam splitter, in the place where the nanoparticle-seeded fluid chamber would later be. Placing an eyepiece at the convergence "point" (which due to nonlinear Gaussian optics that happens at near-wavelength scales is actually roughly in the shape of a 3-D ellipsoid) allowed us to project an enlarged image of the cross-section of the focal volume onto a nearby wall. Before final collimation, one could see two ~10cm-diameter red light splotches (brightness distribution following a radial Gaussian normal distribution) with the "edges" separated by several cm, with just a faint hint of interference fringes in the intervening gap. Once we got the collimation right by getting the two light splotches to merge into one, we got a beautiful circular set of fringes, each fringe several mm wide in magnified wall-projected terms.

In a mirror-collimation setting, perhaps one could test symmetry of the mirror in similar fashion: bounce 2 collimated beams off radially opposed points of the mirror, place an objective or electronic coherence checker at the reconvergence point, if the mirror is off by more than a quarter-wave, no fringes. It seems inefficient, but just saying, such interferometric collimation is indeed possible.

2012-08-07, 02:08   #13
only_human

"Gang aft agley"
Sep 2002

EAA16 Posts

Quote:
 Originally Posted by ewmayer It seems inefficient, but just saying, such interferometric collimation is indeed possible.
I regret using the word interferometry. I don't know what they were doing with those lasers.

2012-08-07, 06:37   #14
xilman
Bamboozled!

May 2003
Down not across

2×5,101 Posts

Quote:
 Originally Posted by Uncwilly I have been inside the dome of a 4.1 m scope and been inside the largest solar scope. Also have been to a company that produced an ~1 m Dobsonian back in the day. Have looked through the telescope that holds the record as the most viewed through (more people have looked through it than any other).
In January 89 we spent two weeks on La Palma --- our first visit to that island. An old friend of mine was chief scientist at the William Herschel telescope and his job was to get the thing commissioned. Jean and I were give a tour of the various domes (incidentally, the 2nd time I've seen the INT, the first being when it was still at Herstmonceaux) and I got to climb on the square frame of the WHT's Serrurier truss, from where I could look straight down to the 4.2m mirror. Somewhere I still have the photos.

I've looked through a 17th century telescope. It's part of the collection of the Museum of the History of Science in Oxford. The scope isa small refractor, perhaps 50mm aperture but it still works fine. All we could see was part of the roof of Blackwell's bookshop opposite the museum. One of William Herschel's Newtonian reflectors is in the museum. The roughly 20cm primary speculum is still in good condition but, unfortunately, the diagonal has been lost.

My Dob was much smaller --- only 46cm --- and was sold off about 20 years ago to a couple of people from the Rutherford Appleton Lab who wanted a half-metre class mirror to build a lidar for investigation of atmospheric haze.

Paul

2012-08-08, 10:27   #15
only_human

"Gang aft agley"
Sep 2002

2·1,877 Posts

My earlier post said Questar in error. I toured Celestron.

The Celestron founder, Thomas J. Johnson, died in March: Celestron founder dies at age 89.

This must be the innovation they were talking about when I visited:
Quote:
 Johnson’s biggest challenge with Celestron telescopes was to find a way to efficiently produce the Schmidt corrector plate used in the top-of-the-line catadioptric telescopes (hybrid of a reflector and a refractor). With his designers in 1970, the founder began making a telescope that took amateur astronomy by storm: the Celestron 8. This instrument revolutionized the hobby by bringing compact, affordable telescopes to the marketplace and led to more adaptations, making Johnson’s company a leader in the industry.
As for what they were doing with the lasers during my tour, it's obvious that I don't have a clue. My guess is this: Classical Cassegrain Collimation
Quote:
 If you are using a single beam collimator with a Cassegrain that has a corrector plate, beam centering on the secondary must be judged by eye (viewed by reflection in the primary). This is not the best practice, but by moving your gaze in a circle around the secondary while checking centering, accuracy can be improved. With the holographic collimator you can see if the target pattern is centered on the secondary. The next step is angular adjustment of the secondary so that the beam reflected from the center of the secondary is centered on the laser aperture on the face of the collimator. This sets the secondary square with the optical axis. The holographic lens should be threaded into the collimator for the next step. The secondary will now reflect and project the holographic pattern upon the primary, and the primary should be centered within the pattern. The pattern is now reflected by the primary and projected in a collimated beam out the front of the telescope. The angular alignment of the primary is adjusted by projecting the pattern onto a surface or screen in front of the telescope and adjusting the primary to center the pattern on the shadow of the secondary. In the case of a corrector plate, if the pattern projected from the scope is undistorted, the corrector alignment is fine. If not, adjust the corrector to remove pattern distortion.
In 2010, Celestron put together a 17.6 minute video" "In Celebration of its 50th anniversary, Celestron presents":
The Path Of Light (Episode 1) by Celestron Telescopes
Quote:
 "The Path Of Light" is a documentary celebrating astronomy, telescope making and human ingenuity. Episode 1: “Generation of Dreamers” focuses on Celestron's founder, Tom Johnson, an electrical engineer, who developed the techniques for mass producing Schmidt-Cassegrain Telescopes, making these powerful astronomical instruments accessible to amateur astronomers.
It is a bit grandiose at the start but has a lot of background of Celestron and their early years and briefly mentions their telescopes use at El Camino College.

Last fiddled with by only_human on 2012-08-08 at 11:16

2012-08-11, 00:11   #16

"Richard B. Woods"
Aug 2002
Wisconsin USA

170148 Posts

Quote:
 Originally Posted by only_human My earlier post said Questar in error. I toured Celestron.
Ahh ... that explains a bit, when combined with:

Quote:
 Johnson’s biggest challenge with Celestron telescopes was to find a way to efficiently produce the Schmidt corrector plate used in the top-of-the-line catadioptric telescopes (hybrid of a reflector and a refractor).
Questars were also catadioptric, but never larger than 6-inch aperture in the 60s-70s IIRC. Once a Celestron 8 could compete on price with a Questar 4, there was a great boom in amateur telescopy.

 2012-08-13, 02:25 #17 ewmayer ∂2ω=0     Sep 2002 República de California 262478 Posts A seemingly unrelated topic led me back to the subject of hand-grinding mirrors this afternoon. I previously discussed this elsewhere on the forum, but too lazy to dig out the thread - anyway it's irrelevant. Briefly: I have around a hundred packaged chips (non-working, these were from a defective test batch) from a defunct startup I worked for a few years back. These are packaged much like these Intel CPUs but have the silicon chip proper in the middle coated in a very hard and chemically resistant (I've tried fuming ntiric, sulfuric & hydrochloric acids to no avail) layer of black epoxy resin potting compound. I also have one of the same batch which was sent out to a specialty firm which precision ground off the potting goop and exposed the underlying chip, with the attendant iridescent diffractive effects. I would like to similarly remove the potting epoxy from the batch of chips I have and turn them into a piece of artwork, but such microgrinding is very expensive, so I need an inexpensive way of DIY removal. Today it occurred to me that placing a bunch of the chips on a precision-flat metal surface and using a flat-surfaced mirror blank and some grinding compound might work - the multi-chip array would make it easy to grind the batch evenly, and the glass blank would allow me to see when I get down to the silicon, at which point I would switch to successively finer-grit abrasive. The only problem that occurs to me is that many (maybe most) blanks are pre-curved. Maybe just use a piece of plate glass? (It could even be rectangular here.) Thoughts? ----------------------- p.s.: Interesting page I found by a fellow who makes his own blanks out of scrap glass. Last fiddled with by ewmayer on 2012-08-13 at 02:37
 2012-08-26, 01:32 #18 ewmayer ∂2ω=0     Sep 2002 República de California 7·23·71 Posts Just took delivery of a pair of 12"-dia. (x 3/4" thick) mirror blanks I purchased from this fellow, for just $60 +$11 shipping. (They both fit neatly in the flat-format one of the two USPS medium flat-rate boxes). They both have some decent-size nicks around the edges, but each has at least one nice undamaged flat plate-glass-smooth surface. Also bought a selection of four 1lb bags of various-grade grinding grits on eBay, cost \$23. Spent a couple hours this afternoon cleaning the glass surfaces and the metal backs of the chips I intend to grind the potting compound off of, spread a very thin layer of binary epoxy on one of the glass surfaces, and glued 32 of the chips (each ~1.75" square) down, in the pattern of a 6x6-tiled square with the 4 corner tiles missing. Once the epoxy cures, I can start grinding. Hopefully the epoxy will adhere to the smooth glass the way I intend. The chips are very uniformly manufactured, so the grit will hopefully remove the tiny raised-solder-dot I/O pins and the raised surface of the potting epoxy uniformly. I can track the progress through the translucent back of the glass grinding tool. Wish me luck! When I'm done using them for the chip-related project, they will serve nicely as a blank/mirror pair for an eventual 12" Dobsonian, my "retirement scope".
 2012-08-27, 04:26 #19 cheesehead     "Richard B. Woods" Aug 2002 Wisconsin USA 170148 Posts How will you (or, will you) keep the grit from grinding a curve into the glass as well as grinding the epoxy?
2012-08-27, 18:38   #20
ewmayer
2ω=0

Sep 2002
República de California

7·23·71 Posts

Quote:
 Originally Posted by cheesehead How will you (or, will you) keep the grit from grinding a curve into the glass as well as grinding the epoxy?
a) The surface being ground is flat to begin with, except for the slightly raised epoxy rectangles and solder dots surrounding thrm, which are what will get ground off;

b) I will use a grinding stroke - perhaps a simple spinning will suffice - different that the telescope-mirror ones which are designed to impart curvature.

2012-10-10, 19:37   #21
xilman
Bamboozled!

May 2003
Down not across

2×5,101 Posts
Stars are variably starry

Quote:
 Originally Posted by ewmayer Draco is one of my favorite constellations.
[aol]Me too !!!?!!!???!!![/aol].

Back in the good old days I used to observe AB Draconis as often as I could. An excessively faint (often invisible even in my 46cm Newtonian) eruptive variable of the U Gem type which would flare up very rapidly every now and again.

As Draco is circumpolar in these parts it can be observed essentially all year round.

2012-10-10, 21:16   #22
ewmayer
2ω=0

Sep 2002
República de California

7×23×71 Posts

Quote:
 Originally Posted by xilman Back in the good old days I used to observe AB Draconis as often as I could. An excessively faint (often invisible even in my 46cm Newtonian) eruptive variable of the U Gem type which would flare up very rapidly every now and again. As Draco is circumpolar in these parts it can be observed essentially all year round.
My late father was a lifelong member of the AAVSO and huge fan of the eruptive variables, SS Cyg and U Gem types, mainly. He got me interested, as well, IIRC I had roughly ~2 years' of solid observing ending with August 1981, the last month before I went off to college, which ended my dabbling in that particular hobby. (My bent has always been to try understand the physics and mathematics behind things, not just collect observational data. Both are valuable, but were I to ever resume variable-star observing I would be doing it via the high-tech CCD/PC route, which is great because it eliminates both the tedium and much of the inaccuracy of the old-style eyeball observing, while also emabling a much greater data volume per scope.)

Anyway, the copy of my submitted monthly AAVSO observations from that month 8/81 is the only one I still have - tucked away in the back flap of my Norton's Star Atlas. Summary for the month:

Total Number Stars Observed: 175
Total Number Observations: 536

Let's see if any were in Draco: Yes, I see single data points for R, T, V, W, X, RT, SV, U and YZ Dra, but I don't see AB ... ah, here we go:
Code:
DESIGNATION  VARIABLE  JUL.D.&DEC.  MAGN.
===========  ========  ===========  =====
195377       AB Dra     2444,833.7  13.9
"           "             834.6  14.0
"           "             835.6  14.2
"           "             836.7  (14
"           "             837.6  (14
"           "             842.6  (14
"           "             847.6  14.3
For non-variable-star geeks, "(14" means "not visible; fainter than 14th magnitude".