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Trying to Figure Out Problems With a Century-Old Refractor

22 Sunday Sep 2019

Posted by gfbrandenburg in astronomy, History, Optics, Telescope Making, Uncategorized

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Tags

antique, crown, flint, glass, Kiess, refractor, Telescope, testing

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.

 

Gently tapping off the lens cell from the tube
Gently tapping off the lens cell from the tube
Note that the retaining ring holding the front of the first lens merely slides into the cell; it’s held in place by four screws. The threading is on the inside of the ring, and the outside is smooth
Note that the retaining ring holding the front of the first lens merely slides into the cell; it’s held in place by four screws. The threading is on the inside of the ring, and the outside is smooth
You can see the black tape and tan cardboard spacers
You can see the black tape and tan cardboard spacers
Me looking puzzled
Me looking puzzled
The cardboard spacers around the edges
The cardboard spacers around the edges
The two lenses together; note the multiple, small black tape spacers between the pieces of glass
The two lenses together; note the multiple, small black tape spacers between the pieces of glass
The original chips on the second lens element
The original chips on the second lens element
The empty lens cell. Note that they didn't make it black
The empty lens cell. Note that they didn’t make it black

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

20 Friday Sep 2019

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

≈ 3 Comments

Tags

cassegrain, convex, figuring, for sale, primary mirror, richey-chretien, secondary mirror, Telescope

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.

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Cleaning Up a Century-Old Refractor

18 Sunday Aug 2019

Posted by gfbrandenburg in astronomy, History, Hopewell Observatorry, Optics, Telescope Making

≈ 5 Comments

Tags

antique, Carl Kiess, Hopewell, refractor, Telescope

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.

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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:

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IMG_5059

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:

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

Australian TV Bit on Me and the DC ATM Workshop

27 Monday Nov 2017

Posted by gfbrandenburg in astronomy, astrophysics, nature, Safety, science, Telescope Making

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2017, ATM, Australia, eclipse, Stephanie March, Telescope

Some very nice folks from the Australian Broadcasting Corporation came and interviewed me on film for a bit on folks who make their own telescopes to see the great August 2017 eclipse. Here is the link:

( https://www.facebook.com/abcnews.au/videos/10157157152414988/ )

Trying to Test a 50-year-old Cassegran Telescope

07 Thursday Sep 2017

Posted by gfbrandenburg in astronomy, flat, Hopewell Observatorry, Math, science, Telescope Making

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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:

figure xp on the 12 inch cass

my graph of 12 inch cass 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

hindle sphere test

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:

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When was the last time you spent a night under the stars?

29 Wednesday Jun 2016

Posted by gfbrandenburg in astronomy, astrophysics, Hopewell Observatorry, Telescope Making

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Tags

astronomy, Hopewell Observatory, open house, Telescope

If it’s been a while since you spent time looking up at the heavens with your naked eyes, binoculars and telescopes, looking at planets, stars and galaxies, then this Saturday might be your night.

The Hopewell Observatory is having an open house on Saturday, July 2, 2016, and we have a variety of scopes to look through. Some of the scopes will be under our roll-off roof and some will be rolled out onto the small lawn outside the observatory itself.

Mars, Jupiter and Saturn will be very conveniently placed for viewing right at sundown, and if it’s dry and clear enough, we should be able to see the Milky Way. Many nebulae, open and globular clusters, galaxies, and double or triple stars will be visible as well.

You are invited!  And it’s free!

The location is about an hour due west of Washington DC by way of I-66, near the town of Haymarket, VA. For detailed directions, follow this link, which I posted for one of the dates which got canceled because of bad weather. Ignore the date, but do pay attention to the fact that we have no running water! We have bottled water and a composting toilet and hand sanitizer. Plus makings for coffee, tea, and hot chocolate – all gratis.

picture of hopewell

The picture above is of one of our telescope mounts, which carries several telescopes and was set up to take astrophotographs at the time. Below is a picture of the outside of the observatory shortly after a snowstorm.. Notice that there is no dome – instead, the galvanized steel roof rolls back on the rails and columns to the right of the picture when the scopes are in use.

showA13

If you have your own telescope, feel free to bring it. If it needs electricity, we have an outdoor 120VAC outlet, but you should bring your own extension cord and plug strip.  If you want to stay all night, that will be fine, too! If you feel like bringing a cot or a tarpaulin and a sleeping bag, that’s equally OK by us! Show up at or near sunset, and stay until the sun comes up, if you like!

Warning: the area definitely has insects, such as ticks and chiggers, which appear to avoid everybody else and to do their best to attack me. I strongly recommend long pants, shoes/boots, and socks that you can tuck the pants into. Tuck your shirt into your pants as well, and use bug spray, too. I have personally seen plenty of deer, cicadas, moths, wild turkeys, squirrels, and birds, and I have heard from a neighbor that a bear tried to eat his chickens, but other than the insect pests, the wildlife stays out of your way.

Again – for detailed directions, look at this link.

Any Moth Experts Out There?

20 Monday Jun 2016

Posted by gfbrandenburg in nature, science

≈ 1 Comment

Tags

Hopewell Observatory, moth, Telescope, virginia

We found these two beautiful moths that flew into the operations cabin at the Hopewell Observatory a couple of nights ago, and we have no idea what type they are. Never seen them before and can’t find any images identical to them. (One species is similar, though.)

Any suggestions will be welcome.

Ain’t they purty li’l things?

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IMG_5001

And when they opened their wings they were even more spectacular, but I didn’t get a good shot.BTW the yellow-and=red moth is sitting on the struts of a telescope made by Alan Bromborski.

One Way to Make a Telescope Spider

05 Saturday Mar 2016

Posted by gfbrandenburg in astronomy, Telescope Making

≈ 4 Comments

Tags

ATM, diagonal, dobsonian, spider, Telescope

All Newtonian telescopes require a secondary mirror — a flat mirror held at roughly a 45-degree angle to reflect the light from the primary out to the side. Generally this secondary mirror is an ellipsoid, in order to waste as little light as possible.

One major problem is figuring out how to hold this secondary mirror in place securely without interfering with the passage of light from your distant target. The secondary mirror can be held on a stalk, or on crossed arms like a spider’s web.

The images below show how Ramona D made a spider using a piece of extruded aluminum tube with a square cross section, several bolts, a spring, a piece of plastic dowel, some pieces of steel strapping tape, a few thumbscrews, and various small nuts and bolts. She did a very neat job, including threading and tapping several small holes in the aluminum tube.

The idea is not original to me: I got the idea from somebody else on line, but unfortunately, I don’t recall the name of the person to whom I should give credit.

Here are some photos that probably do a better job of explaining how to make it than I could explain in many, many paragraphs.

ramonas spider 1.png

ramonas spider 2

ramonas spider 3

ramonas spider 4

A recently-completed telescope

27 Saturday Feb 2016

Posted by gfbrandenburg in astronomy, Telescope Making

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Tags

CCCC, dobsonian, Telescope

Steve S recently finished a telescope with help from the DC-area amateur telescope making (ATM) workshop that I’ve been running at the Chevy Chase Community Center (CCCC) for several years (I took over from the late Jerry Schnall around the turn of the century) with help from several local ATMers and under the auspices of the National Capital Astronomers (NCA).

Steve had made the mirror quite a long time ago (not here in DC). The optics are quite good according to my tests, and if you look at the photos, I think you will agree that the body of the telescope looks excellent as well.

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steve suranovic's scope 5
steve's scope 6

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As you can see, he used more-or-less dimensional wood rather than the more conventional plywood. Or should I say, clear pine that had been glued into boards at the lumber factory. He made the cradle with a bolt that allows one to loosen or tighten the grip on the tube so that one can rotate it or shift it forward or back to take care of any changes in balance.

It may not be obvious, but the wood is in fact coated with varnish.

The rocker box is held onto the azimuth bearing with sturdy wingnuts so that it can be more easily transported. The two circular sections of the azimuth bearing were table tops purchased at Lowe’s (IIRC).

 

Telescope Making in Cuba?

04 Friday Dec 2015

Posted by gfbrandenburg in Telescope Making

≈ Leave a comment

Tags

ATM, cuba, dobsonian, Grit, Mirror, Optics, Polishing, Telescope

Inspired by a Canadian amateur astronomer who visited the place, I’ve been in recent contact by email with some potential amateur telescope makers in Cuba. 
 .
I proposed bringing the optics for some completed 4″ to 8″ Newtonian telescopes in my luggage (ie parabolized & aluminized mirrors, diagonals, and eyepieces) and then giving them ideas and assistance on making the rest of the scopes. I have a number of already-completed primaries and diagonals at our DC telescope making workshop, but would have to scrounge around for eyepieces. 
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(If mechanics in Cuba can keep 1958-model cars running for over 50 years, I bet that they can probably improvise other stuff a la John Dobson, if they have any raw materials at all, which I am not sure about). I am also not sure whether I should bring focusers and spiders, or whether they should make them there themselves…
 .
I understand from the Cubans that there are almost no telescopes in the entire country except for one no-longer-operational telescope at the University of Havana’s observatory, and certainly no Dobsonians. They sound quite interested in the idea, and also were suggesting that I might stay long enough to demonstrate how to grind and polish and figure a mirror.  If I follow up on that idea, it would probably require me bringing in abrasives and pitch in addition to the finished mirrors, which might cause further luggage problems. Explaining finished mirrors carefully wrapped up is one thing, but containers of, say, 15-micron WAO microgrit? They might cut open the bag and test to see if it’s really cocaine…. thus contaminating it…
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Both the Canadian and the Cubans said that bringing in materials officially labeled as ‘gifts’ would entail lots of red tape and delays.
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For me, the payback would be the chance to practice my crappy Spanish in an exotic place that I’ve never visited, and to observe from Tropical skies that suffer relatively low light pollution, as well as doing some good in a country that seems to have a low violent crime rate…. I was planning on flying to Mexico or the Bahamas and then getting a flight to Havana, which seems cheaper than an official direct flight. I suspect that since this would be a scientific exchange, I might even be able to get both governments to sign off and issue an official visa or whatever.
 .
Any thoughts? Anybody ever been there?
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