Design and Construction History

September 8, 2012: First light

See the main page.

July 1, 2012: First assembly

The telescope is now nearly finished, everything has been adapted to the new configuration with solid disk f/3.6 mirror. All the wooden parts have been finished with 3 coats of high-quality stain, and we're now preparing for putting the optics in the telescope.

Yesterday we made the first assembly of the final telescope without optics, below some pictures of the occasion. During assembly the telescope is kept in a horizontal position with a winch, so that you can keep your feet safely on the ground. First a light-weight ring is attached to the truss tubes, then the complete upper cage.

May 9, 2012: The 1100 mm Mirror Completed

It's been a challenging ride for the project. After keeping us waiting for 3 years, the supplier of the 1100 mm cellular mirror blank Wangsness went out of business in 2010. As no other supplier of cellular mirrors was available, we finally decided to abandon the ambitious plan of using a cellular mirror, and instead go for a classical solid mirror blank. Mike Lockwood procured a 1100x58 mm Supremax blank in mid-2011 and proceeded with the production of the finished mirror beginning of 2012. The mirror was coated by OMI and finally delivered in May 2012.

After 4 years of interruption the construction of the telescope can now continue. The focal length of the primary mirror was finally set at f/3.6, about 440 mm shorter than the initial f/4 design from 2007. With the smaller thickness of the primary mirror (58 mm instead of 185 mm), the eyepiece height will be 560 mm lower than the original plan. The change in the mirror design requires a number of changes in the mirror cell, and also influences the balancing and the height of the telescope. I hope to be able to reuse most of the hardware that was fabricated nearly 5 years ago...

A Big Thank You to Mike Lockwood There is no doubt that without Mike this project would not have continued. His professional attitude and solution-seeking approach has pulled me through the several years during which we were waiting in vain for Wangsness to produce something. After Wangsness' failure, Mike readily accepted the financial responsibility and went on to find alternative solutions. Thank you, Mike!

June 16, 2010: Update from Wangsness

Most of the CAD work is done and we are now machining the core bases with integral posts. The posts are an extra 10 mm long to imbed into the sockets of the back plate. There is also a 1/2 inch hole in the center for additional anchoring. The core top will be machined separately and glued on to the top to finish the height and curve. Taking a little longer than expected, but looking very good.

May 12, 2010: Work on the cellular mirror blank started!

After 30 months of standstill, finally the production of the 1100 mm cellular mirror blank has started. Below a picture of the base plate for the 1100 mm blank (courtesy Peter Wangsness), see also the Cellular Mirror Design page. More to come soon...


In the meanwhile we have decided to reduce the focal length from 4400 mm (f/4) to 4125 mm (f/3.75), lowering the maximum eyepiece height by 275 mm (11"). Some of the elements that influenced this decision:

• As explained on the Secondary Mirror Design page, the secondary was oversized to be able to use a binoviewer without barlow element. Actual observing with binoviewers over the past years has shown that the binoviewer will primarily be used for planetary observing at high power so this requirement can be dropped.
• The new Televue Paracorr optimized for sub f/4 telescopes
• The recent 100° eyepiece lines favorable for fast telescopes

Back in 2007 f/4 seemed like the best compromise, whereas with today's equipment f/3.75 appears to be the better choice.

March 16, 2008: Waiting for the 1100 mm cellular mirror blank...

First scheduled for October 2007, then moved to January 2008, the casting of the primary mirror blank seems to take quite a bit longer than orginally planned. Currently there is no further information from Wangsness about a likely date for its availability. I do not really mind the waiting, but find the lack of information and the unpredictability of the situation quite annoying. Let's wait and see...

February 26, 2008: Coated secondary mirror arrives!

A package from Mike Lockwood arrived this morning - containing the coated 200 mm secondary mirror and a new 16"x1.4" f/4.9 mirror for the 16" motorized alt-az telescope. The coating of both mirrors was done by Spectrum Coatings and looks absolutely perfect.

February 17, 2008: NEW Mirror Cooling tool

Another spinoff of the design work on the 1100 mm telescope, the Mirror Cooling Calculator for telescope mirrors is made available.

January 22, 2008: A telescope in the nursery room

The structure finally begins to look like a telescope! More information in the following pages:

• the completely revised Mirror Cell page featuring the 54-point floating point cell and the double cable sling edge support
• the Mirror Box page featuring the design of the mirror box, the mirror cell, the truss tube connections, the 7-fan ventilation system with air distribution headers.
• the updated Truss Frame page with details about the truss tube selection, truss tube attachments, and deformation plots of the telescope in horizontal position

With the mirror cell, ventilation system and truss tubes finished, all the pieces of the puzzle are falling nicely into place!

January 8, 2008: Mirror box under construction

Happy 2008! With the design of the primary mirror finalized, the actual construction of the telescope structure can now continue. The first pictures of the mirror box under construction are available. The inside of the mirror box is 1160 mm square leaving 25 mm around the mirror.

November 26, 2007: NEW Mirror edge support calculator

A spinoff of the design work on the 1100 mm cellular mirror, a mirror edge support calculator for Dobsonian (alt-azimuth) telescopes is made available.

November 5, 2007: Mike Lockwood finishes the secondary mirror

The surface of the mirror is fully polished, scratch and sleek free and quite smooth. It is very flat along the major axis. Fringe images show that the over-all accuracy is better than 1/8th wave at 550 nm. A real beauty!

September 27, 2007: MirrorMesh3D

More information about MirrorMesh3D, the automatic mirror model generation for 3D analysis of telescope mirrors.

September 24, 2007: Secondary mirror blank has been cast

During the summer months, there has been considerable progress behind the scenes on the design and validation of the 1100 mm primary and 206 mm secondary cellular mirrors.
The 206 mm (8") minor axis elliptical mirror has been cast last week and is now in the anneal phase in the furnace at Wangsness. Weighing about 1.9 kg (4.3 lbs), it is more than two times lighter than a full thickness solid secondary of the same size.
The mirror has been validated by finite element analysis to hold its surface to better than 1/50 wave RMS over the entire 206x291 mm (8x11.5") elliptical surface when suspended on the 3 support points that are cast in the structure.
See the Secondary Mirror Design page for more details on the cellular design, the validation analysis and the secondary mirror support.

July 9, 2007: A portrait of the largest portable amateur telescope in the world.

A portrait of Erhard Hänssgen's 42 inch (107 cm) Dobsonian telescope.

June 8, 2007: Sneak preview of the Upper Cage

A first sneak preview of the secondary cage.

May 29, 2007: The alt-azimuth mount finished!

The mount with the drive system is now completely finished. Some finishing touches have been applied to make the mount fully operational:

• Four side-bearings for keeping the OTA centered
• Clutch for altitude drive
• Aluminum contact surfaces for the azimuth rim drive and track rollers
• Lifting mechanism for effortless transition between observing and transporting

To preserve the beautiful structure of the wood, a three layer clear finish was applied to the wood. The steel parts are painted in "old copper" finish. Details are shown on the new Alt-azimuth Mount design and construction (Part 2) page.

April 27, 2007: The alt-azimuth mount under construction

The telescope will be built from the ground up. The first element is the 1000 mm (40") diameter ground board, followed by the drive board on which all the motors and bearings are mounted. Click here to view the Alt-azimuth Mount design and construction (Part 1) page.

April 13, 2007: The construction of the telescope has started!

The mounting ring of the Upper Cage is 121 cm (48") wide and weighs about 1.5 kg (3.3 lbs). This piece is light enough to be carried on the ladder and to be mounted on top of the truss tubes at 4.3 m / 14 ft height. After that the telescope can be lowered so that the rest of the upper cage (8 kg/18 lbs) can be mounted while standing on the ground. Click here to view the Upper Cage construction page.

April 7, 2007: The design of the Upper Cage is completed.

A three-ring design featuring a mounting ring to enable fixing the bulk of the upper cage while standing on the ground, an offset spider, the dimensioning of the secondary mirror, a detailed estimate of the weight of the upper cage, and a finite element analysis of the upper cage and truss tubes. All this and more can be found on the Upper Cage design page.

April 1, 2007: A new page with some pictures of my 635 mm (25 inch) Dobsonian is available.

The transport system used on the 1100 mm will be pretty much the same as used on the 635. Click here for some pictures.

March 11, 2007: Mike Lockwood will produce the optics for the 110 cm Cruxis Telescope.

Mike Lockwood will take on the challenge to produce the optics for the Cruxis telescope project. Mike is an experienced and well known amateur telescope maker who has transited into custom, high-quality professional optical work and is currently building a grinding machine to work on mirrors up to 50" (127 cm).
Click here for some details on Mike Lockwood's mirror and telescope making.
More information about the cellular design of the 120 kg (260 lbs) primary mirror and 1.7 kg (4 lbs) secondary flat will follow later. Delivery of the optics is scheduled for April 2008.

March 3, 2007: SiTech Servo system arrived.

The first hardware of the telescope, the SiTech Servo system has arrived. This really is a very well designed and elegantly executed system, with plenty of cool features. The wireless handpad with buttons that glow in the dark works like a charm. Click here to see some pictures of the servo motors.

February 26, 2007: Preliminary mirror cell design.

Second topic of the mechanical design: the mirror cell. It's designed to compensate the flexibility of the truss frame.

February 17, 2007: Selection of the truss tubes

A new "Mechanical Design" section has been added to this page. The first topic is a discussion of the selection of the truss tubes for the telescope. Includes an analysis of the stiffness of the truss frame and of the buckling of the system.

February 2, 2007: Transaction with Optical Structures cancelled

A set-back for the project. We received several kind warnings from the ATM community in response to our previous announcement. After a number of e-mail exchanges with Optical Structures, there indeed appears to be a substantial gap between the claims on the supplier's web site and their actually proven capabilities. Optical Structures cannot provide a single positive reference on optics larger than 20". Despite a generous proposal from OS to finance the project from their pocket if we would put the funds in escrow, we decided not to continue with this supplier. Read the full story here.
Back to square one for the optics...

January 15, 2007: Optical Structures has the best proposal for the optics

After due consideration Optical Structures has the most interesting proposal. It's technically very sound: a cast cellular mirror structure, making the mirror lighter, stiffer and cooling faster than traditional designs. The cellular structure will be specifically designed for this project. This solution overcomes the major problem of obtaining a large mirror blank (seems to be exceedingly difficult nowadays).

January 11, 2007: Servo-motor controller: Sidereal Technologies

A good solution for my needs appears to be Dan Gray's SiTech controller. Servo-motors with plenty of power, a controller that does not require a laptop, and finally the wireless handpad. Nice! The standard gearing is too high for the 110 cm, but Dan kindly suggested swapping some of the gears, in his own words "I'll do this for free, 'cuz you've a cool project". I tend to agree :-)

October-December 2006: Survey of Optical suppliers

It appears that not too many suppliers produce mirrors in the size range 100-115 cm (40-45") at a reasonable price. Some are clearly targeted at the professional market, with budget more than double of what I had in mind. From some preliminary price estimates from the few that did fit the bill, I decided on a 110 cm diameter.

September 2006

Initial design for a 42" scope.

March 2005

A first inquiry with OMI gives an idea about the pricing range. I learn that OMI is limited in size to 42" (106 cm) [P.S. In February 2007 this is no longer the case].