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Open House at Hopewell Observatory: April 22, 2023

04 Tuesday Oct 2022

Posted by in astronomy, Hopewell Observatorry, Telescope Making

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Hopewell Observatory is holding our second free, public, night-time open house since COVID on April 22.

You are invited!

You and your friends and family can get good looks at Venus and Mars, as well as the famous Pleiades, Beehive and Orion star clusters — and a gaggle of galaxies and double stars.

We have a variety of permanently-mounted and portable telescopes of different designs, some commercial and some made by us, some side-by-side, enabling several people to view the same object in the sky with different magnifications.

You will be capturing those photons with your own eyes instead of looking at someone’s super-processed, super-long-exposure photograph (as beautiful as they in may be).

The date is Saturday, April 22, 2023 with a rain & cloud date of April 29. We suggest arriving near sundown, which will occur at a few minutes before 8 pm on the 22nd and a few minutes after 8 pm on the 29th. It will get truly dark about an hour later.

There are no street lights near our observatory, other than some dimly illuminated temporary signs we put along the path, so you will probably want to bring a flashlight of some sort.

If you own a scope or binoculars, feel free to bring them!

Hopewell is about 45 minutes by car from where I-66 intersects the DC beltway. The last two miles of road are dirt and gravel, and you will need to walk about 200 meters/yards from where you park. We do have electricity, and a heated cabin, but since we have no running water, we have an outhouse and hand sanitizer instead.

We are located about 30 miles west of the Beltway on Bull Run Mountain – a ridge that overlooks Haymarket VA from an elevation of 1100 feet, near the intersection of I-66 and US-15. Detailed directions are below.

Assuming good weather, you’ll also get to see the Milky Way itself, although not as well as in years past, because of ever-increasing light pollution.

If you like, you can bring a picnic dinner and a blanket or folding chairs, and/or your own telescope binoculars, if you own one and feel like bringing them. We have outside 120VAC power, if you need it for your telescope drive, but you will need your own extension cord and plug strip. If you want to camp out or otherwise stay until dawn, feel free!

If it gets cold, our Operations Building, about 40 meters north of the Observatory itself, is heated, and we will have the makings for tea, cocoa, and coffee.

Warning: While we do have bottled drinking water and electricity and we do have hand sanitizer, we do not have running water; and, our “toilet” is an outhouse of the composting variety. At this time of year, the nasty insects haven’t really taken off but feel free to use your favorite bug repellent, (we have some) and check yourself for ticks after you get home.

The road up here is partly paved, and partly gravel or dirt. It’s suitable for any car except those with really low clearance, so leave your fancy sports car (if any) at home. Consider car-pooling, because we don’t have huge parking lots.

Two of our telescope mounts are permanently installed in the observatory under a roll-off roof. One is a high-end Astro-Physics mount with a 14” Schmidt-Cassegrain and a 5” triplet refractor. The other was manufactured about 50 years ago by a firm called Ealing, but the motors and guidance system were recently completely re-done by us with modern electronics using a system called OnStep. We didn’t spend much cash on it, but it took us almost a year to solve a bunch of mysteries of involving integrated circuits, soldering, torque, gearing, currents, voltages, resistors, transistors, stepper drivers, and much else.

We could not have completed this build without a lot of help from Prasad Agrahar, Ken Hunter, the online “OnStep” community, and especially Arlen Raasch. Thanks again!

OnStep is an Arduino-based stepper-motor control system for astronomical telescopes. For this niche application, OnStep uses very inexpensive, off-the-shelf components such as stepper motors and their controller chips — which were developed previously for the very widespread 3-D printing and CNC machining industry.

Getting this project to completion took us nearly a full year of hard work!!! The original, highly accurate Byers gears are still in place, but now we can control the mount from a smart phone!

We also have two alt-az telescopes, both home-made (10” and 14”) that we roll out onto our lawn, and a pair of BIG binoculars on a parallelogram mount.

The drive is about an hour from DC. After parking at a cell-phone tower installation, you will need to hike south about 200 meters/yards to our observatory.

Physically handicapped people, and any telescopes, can be dropped off at the observatory itself, and then the vehicle will need to go back to park near that tower. To look through some of the various telescopes you will need to climb some stairs or ladders, so keep that in mind when making your plans.

Our location is nowhere near the inky dark of the Chilean Atacama or the Rockies, but Hopewell Observatory is mostly surrounded by nature preserves maintained by the Bull Run Mountain Conservancy and other such agencies. Also, our Prince William and Fauquier neighbors and officials have done a fair job of insisting on smart lighting in the new developments around Haymarket and Gainesville, which benefits everybody. So, while there is a bright eastern horizon because of DC and its VA suburbs, we can still see the Milky Way whenever it’s clear and moonless. “Clear Outside” says our site is Bortle 4 when looking to our west and Bortle 6 to our east.

DIRECTIONS TO HOPEWELL OBSERVATORY:

[Note: if you have a GPS navigation app, then you can simply ask it to take you to 3804 Bull Run Mountain Road, The Plains, VA. That will get you very close to step 6, below.]

(1) From the Beltway, take I-66 west about 25 miles to US 15 (Exit 40) at Haymarket. At the light at the end of the ramp, turn left (south) onto US 15.

(2) Go 0.25 mi; at the second light turn right (west) onto VA Rt. 55. There is a Sheetz gas station & convenience store at this intersection, along with a CVS and a McDonald’s. After you turn right, you will pass a Walmart-anchored shopping center on your right that includes a number of fast- and slow-food restaurants. After that you will pass a Home Depot on the right.

(3) After 0.7 mi on Va 55, turn right (north) onto Antioch Rd., Rt. 681, opposite a brand-new housing development. You will pass entrances for Boy Scouts’ Camp Snyder and the Winery at La Grange.

(4) Follow Antioch Rd. to its end (3.2 mi), then turn left (west) onto Waterfall Rd. (Rt. 601), which will become Hopewell Rd after you cross the county line.

(5) After 1.0 mi, bear right (north) onto Bull Run Mountain Rd., Rt. 629. This will be the third road on the right, after Mountain Rd. and Donna Marie Ct. (Do NOT turn onto Mountain Road, and note that some apps show a non-existent road, actually a power line, in between Donna Marie Ct. and Bull Run Mtn. Rd.) Bull Run Mtn Rd starts out paved but then becomes gravel, and rises steadily.

(6) In 0.9 mi, on BRMtn Road, you will see a locked stone gate and metal gate, labeled 3804. That is not us! Instead, note the poorly-paved driveway on the right, with the orange pipe gate swung open and a sign stating that this is an American Tower property. We use their road. Drive through both orange gates, avoiding potholes keeping at least one tire on the high spots. We’ll have some signs up. This is a very sharp right hand turn.

(7) Follow the narrow, poorly-paved road up the ridge to the cell phone tower station.

(8) Park your vehicle in any available spot near that tower or in the grassy area before the wooden sawhorse barrier. Then follow the signs and walk, on foot, the remaining 300 yards along the grassy dirt road, due south, to the observatory. Be sure NOT to block the right-of-way for any vehicles.

(9) If you are dropping off a scope or a handicapped person, move the wooden barrier out of the way temporarily, and drive along the grassy track to the right of the station, into the woods, continuing south, through (or around) a white metal bar gate. (The very few parking places among the trees near our operations cabin, are reserved for Observatory members and handicapped drivers.) If you are dropping off a handicapped person or a telescope, afterwards drive your car back and park near the cell phone tower.

Please watch out for pedestrians, especially children!

In the operations cabin we have a supply of red translucent plastic film and tape and rubber bands so that you can filter out everything but red wavelengths on your flashlight. This will help preserve everybody’s night vision.

The cabin also have holds a visitor sign-in book; a first aid kit; a supply of hot water; the makings of hot cocoa, tea, and instant coffee; hand sanitizer; as well as paper towels, plastic cups and spoons.

The location of the observatory is approximately latitude 38°52’12″N, longitude 77°41’54″W. The drive takes about 45 minutes from the Beltway. A map to the site follows. If you get lost, you can call me on my cell phone at 202 dash 262 dash 4274.

hopewell map revised

First Time Installation of OnStep Board with NEMA23 Stepper Motors in Ealing Mount at Hopewell

03 Monday Jan 2022

Posted by in astronomy, Hopewell Observatorry, Optics, science, Telescope Making

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A 3-minute video of the results of our first-time installation of something called an OnStep conversion. We are replacing the telescope drive of a venerable but beautifully machined telescope mount, located at a small group-owned observatory called Hopewell, atop a ridge called Bull Run Mountain*.

It’s alive!

Sorry, it’s not the greatest or clearest video. Also, I mistakenly state at about 0:25 in the video that the right ascension axis was turning at 12 RPM, but it’s not: I should have said 5 RPM, or one revolution in 12 seconds.

You can hear some stuttering of one of the motors. You are right, that is not a good sound. We were able to get it to stop and start making that noise and motion by adjusting the precise positioning of some of the gears. It will take some time and experimentation to get that perfect.

Later on (not captured in this video), when I was trying to slew in the declination axis at the highest speed possible, the stepper motor once again screamed and halted. I’m hopeful that all of those problems can be fixed by doing one or more of these things:

  • 1) adjusting the fit of all those gears;
  • (2) changing certain parameters of microstepping and current to the stepper motors in software; and/or
  • (3) increasing the voltage to the board from 18 VDC to 24 VDC.

I’ll need to test things out on my desk at home, using the same OnStep board, but without the gears and timing belt. (That stuff was a royal PITA to remove screw back into place, and none of us have any desire to take them back out again!) I have some identical extra stepper motors that I can test out, with gloved hands, to see if it is possible to stop the motors from turning. Right now, I still don’t think they are putting out the amount of torque needed.

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*Yes, that famous Bull Run of Civil War fame is not far away. However, our observatory is named after a different geological feature, namely the Hopewell Gap that cuts through the hard rock of Bull Run Mountain right about where where the creek called Little Bull Run begins.

If you are reading this, you probably know that serious amateur, and all professional, astronomical telescopes (except for Dobs) are generally driven by ‘clock drives’ so that the object one is viewing or photographing stays properly centered as the earth rotates imperceptibly beneath us. The original Ealing motor drive at Hopewell, while turning excellent Ed Byers gears, had been an intermittent problem ever since it was delivered to the University of Maryland about 50 years ago. It was in fact not operational when they sold it to us for a pittance about 30 years ago. (If you go to the University of Maryland Observatory site I linked to, the scope we have now is the one in the center of the 1970s – era photo labeled ‘Figure 4’.)

Bob Bolster, one of the founding members of Hopewell observatory, disassembled the drive, modified it considerably, and got it working again, several years before I joined the group. The scope worked, off and on, with a very complex clutch system for ‘fast’ and ‘slow’ movement of the scope, for most of the rest of the last 25 or so years, except for occasional motor burnouts and clutch replacements. Also unfortunately, the optics on the original 12″ Ritchey-Chretien telescope, were not very good, so we removed them, had them in an attic for many years, re-tested them, and finally sold the glass and the holders, for a pittance, to someone in Italy who wanted to try to re-figure them.

This was originally a ‘push-to’ telescope, meaning that one loosened up two Byers clutches (one for each axis), located the desired target in the sky, tightened the two clutches, did some fine tuning with an electric hand paddle to center the target more precisely, and then allowed the telescope drive to then keep the object in the center of the eyepiece or camera field of view as long as one wanted. It originally came with metal setting circles (basically, finely-made protractors that showed where the scope is pointing vis-a-vis the polar and declination axes), which made finding targets possible, though not trivial!

About 15 years ago, Bolster (with some help from me) installed Digital Setting Circles, which used a rotary encoder on each axis, along with a small hand-held computer and screen display, to allow one to select a given target; the DSC hand paddle’s display then would indicate how far one should rotate the scope along those axes to find the desired celestial object; when it was in the field of your widest eyepiece, one used the hand paddle to center it more precisely.

Converting this scope to an OnStep drive will, I hope, make this a Go-To scope in which one can command the telescope to aim at whatever target one desires.

Unfortunately, right now, the fastest it seems to rotate in Declination, with no load whatsoever (all scopes have been removed, so no balance or inertia problems) is about one degree per second. So doing a 180-degree turn in a North-South direction would take a full three minutes. A 30-degree turn would take 30 seconds. Can we make this a bit faster? I hope so.

I wasn’t able to really slew in right ascension (East-West) because the counterweight box, even though empty, seems to require too much torque to rotate right now.

Bolster passed away a few years ago, and this summer, the moment I had been dreading finally arrived: the drive on the Ealing died again, and his amazing skills and tenacity in fixing such problems was gone with him. What’s more, in his final years, his incurable, chronic idiopathic neuropathy made it literally impossible for him to speak, and even typing email responses to the rest of us took a very long time. So most of his wealth of knowledge and experience died with him.

As indicated in my earlier posts (here, here, here, and here), with help from others, I was able to take the two motor setups for the two axes out from the mount and get them working again on my workbench in their original format. I was even able to order and install material for the clutches. However, I discovered that one needed to adjust the clutches very, very precisely, or else they wouldn’t work at all.

I couldn’t figure out how to do that.

And nobody else who belongs to our observatory volunteered to help out, except for removing the scopes and drives from their former positions inside the mount.

So I decided to convert to a totally different type of telescope drive, one inspired by the Arduino boards and 3-D printers. A group of really smart and resourceful hobbyists (engineers?) designed a system around the Arduino environment that uses inexpensive off-the-shelf printed circuits and complex sub-boards and components, used originally mostly in the 3-D printers that have become so popular, to drive at telescope just the way astronomers want them to be driven.

Apparently, there have been many, many OnStep successes, but what we are doing may be the largest and most massive mount to date that has done such a conversion.

I was warned that the entire process would take some months. Those warnings were correct. But that’s OK. I’m retired, I have time, and I have access to tools and people who are interested in helping. What’s more, I have learned a whole lot about modern electronics, and my soldering skills are much better than they ever were.

I’d again like to thank Alan Tarica (who’s physically helped a **tremendous** amount), Prasad Agrahar (who first showed me the OnStep conversions he had done on a much smaller equatorial mount), Howard Dutton (who first conceived and implemented OnStep), Ken Hunter (who made and **donated** to us a complete, functional OnStep board together with all sorts of accessories and walked me by phone and video through many of my fumbling first steps), Khalid Bahayeldin, George Cushing, and many others.

Clay Davies’ Links for Telescope Makers

24 Thursday Dec 2020

Posted by in astronomy, flat, optical flat, Telescope Making

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I am copying and pasting Clay Davies’ recent article published on a Facebook page for amateur telescope makers, where he gives links to extremely useful sources as well as commentary. I think he did a great job, and want to make this available to more people.

================================= here goes! ================================================

Amateur Telescope Making Resources & Fast Commercial Newtonian Telescopes

  • Observer’s Handbook, Royal Astronomical Society of Canada. Every amateur astronomer should have at least one copy of this book. Every “newby” should read it cover to cover. Old hands should keep it as a reference. Avid astronomers get it every year, because it’s updated annually.
  • How to Make a Telescope, Jean Texereau. A classic book by a superb optician. The author taught many people how to make their own telescopes, including grinding, polishing and figuring their own mirrors. This book offers unique and practical telescope and mount designs I have never seen anywhere else.
  • The Dobsonian Telescope, David Kriege & Richard Berry. Want to knock off an Obsession telescope? Here is your bible, written by the creators of Obsession Telescopes. Here you will find well thought out and time proven designs for truss Dobsonian telescopes from 12.5” to 25” and more. If you are handy, if you use one of these designs and follow step-by-step instructions, you can build a fine truss dobsonian. But use free PLOP software (below) to design your mirror cell.
  • PLOP Automated Mirror Cell Optimization. This free windows software can help you design a “perfect” mirror cell. Just plug in the numbers, and in seconds, you have a mirror cell design. https://www.davidlewistoronto.com/plop/
  • Engineering, Design and Construction of Portable Newtonian Telescopes, Albert Highe. Do you want your next telescope to truly satisfy you? This book dedicates an entire chapter that asks you questions that help you design and build (or buy!) a telescope that will do just that. And it has beautifully engineered contemporary designs for large truss telescopes.
  • Engineering, Design and Construction of String Telescopes, Albert Highe. Beautifully engineered, yet challenging, ultra-light, air transportable newtonian telescope designs.
  • Newt for the Web (Stellafane). This is a simple, yet effective tool for newtonian telescope design. You can design an excellent telescope with just this free tool, plus old school drafting tools like ruler, protractor, pencil and compass. https://stellafane.org/tm/newt-web/newt-web.html
  • Reflecting Telescope Optimizer Suite. Mel Bartels created this wonderful free online newtonian telescope design tool: https://www.bbastrodesigns.com/telescopeCriteriaCalc.html If you explore Mel’s website you will find innovative, ultra-fast dobsonian telescopes, beautiful deep sky sketches, and mind expanding ideas that will probably make you a better observer. https://www.bbastrodesigns.com/The%20New%20Sub-F3%20Richest%20Field%20Telescopes.html
  • Right Angled Triangles Calculator, Cleve Books. Are you building a truss telescope but can’t remember trigonometry? This site makes it easy: http://www.cleavebooks.co.uk/scol/calrtri.htm
  • Stargazer Steve 6” Truss Telescope. A very portable, ultra-light commercial truss telescope. Moderately priced, too! http://stargazer.isys.ca/6inch.html
  • Explore Scientific 8”f3.9 Want a fast scope but don’t want to build it? This fast astrograph optical tube assembly has a carbon fibre tube and weighs 18.3 pounds / 8.3kg. It’s remarkably affordable, too! https://explorescientificusa.com/products/208mm-newtonian-f-3-9-with-carbon-fiber-tube
  • Orion 8” f/3.9 You can save a lot of work by buying a telescope off the shelf, like this one. Similar to the Explore Scientific, but with a steel tube at an irresistable price. And this OTA is under 18 pounds / 8kg! https://www.telescope.com/Orion-8-f39-Newtonian-Astrograph-Reflector-Telescope/p/101450.uts
  • R. F. Royce Telescope Building Projects. Simple newtonian telescope designs by one of the finest opticians on planet Earth. The first telescope I built, a 10”f6, and the second telescope I built, a 6”f8, were both based on Royce’s designs. Both performed far beyond my expectations. In fact, the surrier-trusses for my 8”f4 design were based on the Royce design. http://www.rfroyce.com/Telescope%20Bulding%20Projects.htm Want to build your ultimate lunar and planetary telescope? Click the third link. And… considering how much you can learn from one of the world’s greatest opticians, shouldn’t you click every link? http://www.rfroyce.com/thoughts.htm
  • Reiner Vogel Travel Dobs. If you are interested in designing and building your own telescope, have a look at this website. You will find easy and effective construction techniques and ultralight, ultra-portable telescopes here. And big ones. You’ll find equatorial mounts and observing notes, too! http://www.reinervogel.net/index_e.html?/links_e.html
  • Here is my talk at the RASC, Toronto, (Royal Astronomical Society of Canada) entitled, “Designing and Building a Newtonian Telescope for Wide Field Visual and Air Travel”. You can scroll the video to 38:20 if you want to go directly to my presentation. https://www.youtube.com/watch?v=Gz7TVQkTGCM

A Weekend at Almost Heaven

06 Friday Sep 2019

Posted by in astronomy, Optics, science, Telescope Making, Uncategorized

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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:

Exploring the geology of Spruce Knob Mountain Center: Lyle Mars in blue shirt and white hat is in front of the entrance to a cave carved in limestone
Selfie with me in front of three others on the geology hike
This lovely sunset did not portend clear skies

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

Alan Goldberg teaching someone how to use a sextant
Me studying my charts, in front of parallelogram binocular mount
Oscar Olmedo and me at our campsite
Mike Laugherty and me
Mike Laugherty and me
Me fiddling with my 12.5″ home-made dob in the daytime
Me fiddling with the parallelogram binocular mount in the daytime
Mike Laugherty and me fiddling with binocular mount
Left to right: Mike Laugherty, Oscar Olmedo, me
The lottery drawing for a whole bunch of neat prizes. None of us 3 won anything.

One Way to Make a Telescope Spider

05 Saturday Mar 2016

Posted by in astronomy, Telescope Making

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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

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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.

IMG_4184

IMG_4183

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

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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…
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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?

Great Long Weekend of Observing Near Spruce Knob, WVa at 11th Almost Heaven Star Party

18 Tuesday Aug 2015

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I put all of my tickets (over 30!) into the raffle for a 100-degree-apparent-field-of-view eyepiece at AHSP but didn’t win it. I probably should have put a few tickets in some other raffles. They had a whole lot of different stuff being raffled off. The eyepiece I wanted was donated by Hands On Optics. At AHSP they give you ten tickets as part of your registration, and then you can buy more of them. Elizabeth Warner and her husband left the morning of the raffle (Sunday) and gave me theirs, which was very nice of them.

At Stellafane, they used to have just ONE humongous item in the raffle, like a full set of really expensive eyepieces from Al Nagler. So it used to be in fact all-or-nothing. Don’t know if it was like that this year?
I discovered that the things I really needed were:
* an inexpensive laser collimator so I can get collimated in a minute or two all by myself, accurately, instead of fumbling around for an hour and needing an assistant… (now on order)
* an inexpensive electronic timer controller for my Canon TSi so it doesn’t need any cables to a computer (also now on order)
* a way to get rid of dew. The last night was fantastic except for the dew, which even defeated the chemical hand warmer packets that I wrapped around my finder and Telrad. I bet it got to the secondary as well. I will study up on the physics of heat production by resistors or heating wire wrapped around those and devise something.
BTW, I had to use a borrowed hack saw and masking tape to cut each of my truss tubes by exactly an inch on the second day so that I could come to a focus with all my eyepieces. I used some local rocks to deburr the cuts.
They had some great presentations on astrophotography, including how to do it simply and effectively. I was much encouraged.

One Way to Build an Alt-Az Newtonian Telescope

16 Tuesday Dec 2014

Posted by in Telescope Making

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One Way to Build a 6-inch Dobsonian-Newtonian Alt-Az Sonotube Telescope

  

Instructions written by Guy Brandenburg

February 2007

Acknowledgements and thanks to Mel Bartels, Richard Berry, Bill Blackmore, Jack Booth, John Dobson, David Kriege, Jerry Schnall, Jean Texereau, and many others whom I can’t remember at the moment. I have modified their ideas somewhat, hopefully for the better. However, the typos and errors are all mine.
  1. Cradle and altitude trunnions
    1. The purpose of the cradle is to hold the tube steady but also allow for changing the altitude angle and changing the balance point when additional items are added to the telescope, and to allow the user to put the focuser at a convenient angle by rotating the tube.
    2. The cradle consists of four rectangular pieces of plywood that are glued and screwed to form a box that the tube fits in, and two altitude trunnions that each consist of a plywood disk and a ring cut from a PVC sewer pipe. The pipe fits onto the plywood disk and is held there by friction. See diagram below.                           make a dob pic 1
    3. Use 2-inch deck screws, and glue, to attach the 9+3/4” by 12” pieces of plywood to the 8+1/4” by 12” pieces of plywood. Three or four screws per edge should be enough. Use plenty of glue, and use a damp paper towel to wipe up the glue that oozes out. It is not necessary to counter-sink the screws. Make sure everything is lined up properly before inserting the first screw. If you want, you can nail in a couple of small nails (say, 2” long) to hold things in place before you put in any screws.
    4. Before attaching the plywood disks that hold the PVC plastic trunnions, it is advisable to draw the diagonals as shown. This will help you make sure that the disk is centered correctly.
    5. When the glue is somewhat dry, this all can be painted, inside and out.
    6. When the paint is dry, then you can attach the handle and fit the PVC trunnions onto the plywood disks. It is supposed to go on with difficulty, so that it won’t come off easily. It should not need to be nailed, glued, or screwed.
  2. Optical Tube
    1. The tube
      1. The tube itself is made of thick cardboard designed for casting cylindrical concrete columns. One brand name for these is Sonotube. When painted, it is strong, relatively rigid and light, and reasonably waterproof, and it’s quite inexpensive: $5 for a 4-foot length of tube. (Carbon fiber composite tubes, which are lighter, stronger, more rigid and much more waterproof, would cost nearly 100 times as much.) The purpose of the tube is to hold the optical components rigidly, in the proper alignment so that the user can look at things.
      2. A 1+1/2” diameter hole will need to be drilled near the front end of the tube for the focuser. Use an ordinary hole saw attached to an electric drill for this. If you are using a 3- or 4-vane spider, measure and drill the holes for this now.            make a dob pic 2
  • It is very important to paint the inside of the tube using flat black paint. I strongly recommend using latex paint so that any drips can be washed off with water, likewise the brush, and so that the fumes are not so bad. Two or more coats are desirable. However, the tube is quite long, and you don’t want to get paint all over your arm. How to reach the middle? One way is to tape your brush to the end of a dowel rod (or other scrap piece of wood) with duct tape, and use that to extend your reach. When you have finished painting the inside, leave the tube to dry in a horizontal position so that air can circulate; or else, if you want to let it dry in a vertical position, make sure that you put it on some little scrap pieces of wood so that air can circulate up and down the tube. Be sure to wash the brush as soon as you are finished painting. Use lots of water and even some hand soap and rinse it very thoroughly.
  1. The outside of the tube also needs to be painted. Any color will do – the wilder the better, in my opinion. See the previous remarks about latex paint. It is a good idea to let the tube dry in a vertical position, propped up on little scraps of wood. Be sure to paint the edges where holes have been cut and the ends of the tube; when the loose cardboard is impregnated with paint, it becomes much harder.
  1. Primary Mirror Holder
    1. The purpose of this item is to hold the mirror rigidly in the correct alignment, at the proper distance from the secondary mirror and the focuser, so that whoever is looking through the telescope can actually see correctly-focused and clear images of whatever it is they are trying to observe. It is not necessary to paint this part, but it won’t hurt, either. However, the parts where the silicone caulk will hold the mirror to the wood must NOT be painted, because the caulk does not stick well at all to painted surfaces.
    2. The mount consists of two plywood disks separated by springs and held together by machine screws, washers, and wingnuts. The upper plywood disk has an outer diameter about one-half inch less than the inner diameter of the tube; so that’s about 7+1/2”. The lower plywood disk has an outer diameter that is about the same as the inner diameter of the tube, so that’s just shy of 8”. Each disk should have a good-size hole drilled in the center for ventilation. The size of the hole is not critical – any size that you have a hole saw for will work fine.
  • The two disks also need to have 3 holes drilled in them at 120 degrees from each other so that the machine screws can go through. The hole in the upper disk should be countersunk so that the machine screw head will not protrude. The hole in the lower disk should be considerably larger than the screw itself, so that the entire assembly can be adjusted easily. Be sure to line the two disks up together before drilling the holes, and mark how they are supposed to go.
  1. A small piece of steel (or any other convenient metal) should be fabricated into an L shape, with a hole in both parts. The hole in the horizontal part will hold a single, small screw to attach the L to the upper plywood disk, and the hole in the vertical part will be used to squirt the silicone glue through, to hold the mirror in place.
  2. When the entire setup is assembled, except for the glueing of the mirror, then lay three ordinary nails underneath where the mirror will go, as temporary spacers. Squirt nice thick blobs of glue in the three locations previously chosen, then carefully place the mirror in the correct location. Then squirt three more globs of glue, one at each of the three L-shaped edge holders. Then put the entire assembly away somewhere level, safe, and dry, with ventilation and protection from dust, so that it will not be disturbed for a couple of days while the glue sets.
  3. See the diagrams for construction details, and below that for an example by a local ATMer, JCN.                                                                    make a dob pic 3Make a dob pic 4
  • The primary mirror cell is about the last thing to be placed in the telescope. It goes in AFTER the focuser and secondary are put in place, and after everything is assembled, painted, and dried. In my experience, it is not possible to calculate exactly where the mirror should go in the telescope; none of the formulas I’ve seen work exactly. The best one can do is to estimate it, but then the exact location will depend on your eyepieces, the height of your focuser, and your eyes. So, once the rest of the scope is together, you carefully slide the primary mirror cell – with its mirror in place – up into the tube to where you think it should go, and do a very rough alignment of the mirror by looking down the tube. Use shims to get it to stay in place temporarily, and try to focus on as distant an object as possible by daylight. To get it to come into focus will probably require pushing or pulling the entire cell forward or back. Then try the entire process at night, when you can see the moon or a very bright star such as Vega or Sirius. It will not focus the same way, I guarantee! When you have the mirror in such a location that stellar objects come into focus, THEN you can use deck or drywall screws to screw it permanently into place in the tube.

make a dob pic 5cell holder variant by JCN

  1. Focuser
    1. The purpose of the focuser is to hold an eyepiece at the very end of the voyage of all of those photons from outer space so that your eye can detect an image.
    2. Unless you are doing astrophotography, a simple rack-and-pinion focuser with a knob will do just fine. The standard size focuser and eyepiece for decent, smaller telescopes (under 12” diameter) is 1/25”. Plastic-and-metal ones work fine unless seriously abused. There are ones designed for refractors, and ones designed for reflectors. Get the latter type, unless you are building a refractor!
  • Use small nuts and bolts to attach them to the telescope tube.
  1. Unfortunately, some focusers will require a shim to be fabricated underneath so that there will not be a light gap between the focuser and the telescope tube. This all depends on the focuser and the tube you are getting.
  2. As mentioned earlier, you will need to cut a hole in your tube large enough to fit your focuser.
  1. End Ring
    1. The purpose of this is to strengthen the end of the telescope tube that points towards the sky.
    2. Use a decent-quality plywood and a router to cut a ring with a thickness of roughly an inch that will fit tightly onto the end of the tube. Our tubes are 8+1/4” OD, so that should be the ID of the ring. After routing it, sand it to remove burrs and splinters, fit it onto the tube, apply some glue and small nails, and let it dry.
  • Only make an upper end ring; not a bottom end ring, or else you won’t be able to view objects near the zenith.
  1. Mid Ring
    1. The purpose of this is to allow the telescope to stay in place at the chosen balance position, and to allow the user to rotate the tube and to adjust the balance if needed.
    2. The instructions for cutting this are very much like the end ring. The difference is that there should be a small amount of ‘play’ or room for the ring to move over the tube, so that with some force and perhaps a mallet it can be moved when desired. Thus its interior diameter should perhaps be 8+5/16” when cut. Paint will reduce that slightly.
  • The mid ring is NOT glued or nailed to the tube. It should be able to slide, with difficulty.

 

  1. Secondary Diagonal Mirror, holder, and spider
    1. The purpose of the secondary diagonal mirror is to re-direct the light from the object of interest out the side of the telescope, so that you can see the object without your head getting in the way.
    2. You can build this or make all of these parts. Unless you are very good with your hands and with making small metal parts, I suggest you buy them. The mirror itself is a true optical flat, and is easier to make on a machine than by hand.
  • Before putting this in place, it is necessary to find the exact center of the secondary mirror, and to measure the distance from this to the level of the screws that hold the spider in place against the wall of the tube. Measure and re-measure, then drill the appropriate holes, and fasten it all in place. (Remove the mirror itself while installing the spider vanes, so that dust and fingerprints don’t get on the surface of the mirror.)
  1. Finder Scope
    1. The purpose of the finder scope is to allow the user of the telescope to aim it at an object of interest. It gives the user a wider field of view than the telescope itself, which can have a field of view as small as hundredths of a degree.
    2. I strongly suggest a Telrad or other similar 1-power non-magnifying heads-up finder. These devices allow you to aim your telescope intuitively at familiar objects and then to star-hop to other objects. Then, as your budget permits, you can upgrade to other finders – and there is a wide variety to choose from!

 

  1. Rocker Box
    1. The purpose of the rocker box is to hold the telescope rigidly upright and to provide a place for the altitude trunnions to fit into, so that the telescope can be aimed up and down, and left and right, just like a cannon.
    2. The rocker box is made out of 4 pieces of ¾-inch plywood. It has a front, two sides, and a bottom, but no back and no top. The front is shorter than the sides, so that the telescope can be aimed at the horizon if desired. The pieces will be glued and screwed together. The two sides will have nearly semi-circular holes cut out of the tops to hold the trunnions bearings.
    3. Need to cut out the following:
  1. Bottom: one piece 11+1/4” by 10+1/2” plywood. A one-half inch diameter hole should be drilled at its exact center.
  2. Sides: Two pieces that are 11+1/4” by 33” plywood (can be a little longer if desired). Make it so that they are left-right symmetrical, so that the good side of the plywood will be facing out. Cut an arc at the top of each side to fit the trunnions.
  • Front: One piece 11+1/4” by 25” (this height can be changed, but should be about 8 inches less than the sides)
    1. Five or six deck screws, 2” long, on each vertical edge, should be enough. Be sure to use glue as well. The bottom edges could use about 3 screws per edge. Clamp the edges for about 30 minutes after glueing, and remove excess glue with a damp rag. Except for the underneath portion, this can be painted. After drying, this will be attached to the base plate.
    2. Two pieces of Teflon will make sliding contact with the PVC trunnions. They will be glued or nailed into place at the tops of the cut-out arcs at the tops of the side pieces.

make a dob pic 6

make a dob pic 7

  1. Base Plate, or Azimuth Disk
    1. The purpose of the base plate is to allow the rocker box to swivel left and right, so that the user can aim the telescope at anything he or she wants to look at, in any part of the sky.
    2. This base plate is the invention of John Dobson, a master scrounger, all-round eccentric amateur astronomer, and former monk who lives in California. It consists of a downward-facing surface of Formica, Wilsonart, or some similar type of counter-top laminate, resting on three Teflon pads attached to an upward-facing surface. Unlike telescopes made from metal parts, this mounting rotates very smoothly and does not have any backlash at all. That is, if you move the telescope to a given position and let it go, it will stay where you leave it.
    3. To cut out:
  1. two 15-inch diameter circular disks from the same ¾” plywood. The precise size is not critical. Using the jig that we have in our shop, you need to drill a ½-inch hole at the exact center of the disks first, and then cut them out, only cutting through half of the plywood at a time, flipping them over for the second cut.
  2. One piece of countertop laminate – any type will do – a square 16” by 16” is fine.
    1. Choose which will be the upper disk, and which part of that disk will be facing up. Use paintbrush to coat the other side of the wooden disk with a smooth layer of latex-based contact cement. Also paint the back side of the countertop laminate. Let the two surfaces dry until no longer gooey to the touch. Then place the laminate on top of the wood and press hard all over (there are special rollers for this, or you can use heavy glass bottles or heavy metal cylinders) so that they make good contact.
    2. Use a trimming router to trim off the excess laminate.
    3. Then use relatively short screws to attach the top disk to the bottom of the rocker box. Make sure the hole in the bottom of the rocker box lines up with the hole in the center of the top disk. Feel free to finish drilling the center hole through the laminate.
    4. The bottom disk will receive three pieces of Teflon pad around the edges at 120-degree intervals.
  1. Legs
    1. The purpose of the legs is to give the entire telescope a bit more stability and to raise the base of the telescope up off the ground by a few inches.
    2. I suggest using a 2×3 or 2×4 and cutting three pieces about 9 to 10 inches long, then cutting them into an L-shape or a shape a bit like a hockey stick, as shown here:    make a dob pic 8
    3. File or sand the legs so that folks won’t get splinters. Make sure there is clearance for the central pivot bolt, the aluminum plate, the nuts, and the lockwasher. Position the 3 legs 120 degrees apart, then screw and glue them into place. Some of the screws will need to be rather long.
    4. Attach the Teflon pads right over the legs themselves, as close to the edge of the disks.
  1. Now paint everything and let it all dry.
  2. Align your optics.
  3. Then go observe!

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