The 1100 mm Cruxis Telescope
Cruxis Home > 1100 mm Telescope

Welcome to the home page of the 1100 mm Cruxis Telescope.
This telescope is designed and constructed by the Belgian amateur astronomer Robert Houdart and should see its first light in 2008-2010-2011 2012. On this page you can follow the progress of this ambitious telescope project.
The actual construction of the telescope started in April 2007, but was interrupted for nearly 4 years because of problems with the production of the cellular mirror blank. After Wangsness went out of business in 2010, it was decided to replace the original cellular blank with a classical, solid Supremax mirror blank.
Mike Lockwood delivered the finished 1100 mm mirror in May 2012, and the construction can now continue.
Latest News September 8, 2012: First light for the 1100 mm telescope

It's been a long journey, 4 years longer than planned, but finally we have landed.

Installation of the primary mirror
Two weeks ago we installed the 1100 mm primary mirror in its cell; three strong family members helped to lift the 105 kg chunk of glass in the mirror box. The mirror had spent about 3 months in its crate outside in all kinds of weather (mostly rain as we've had the wettest month of June in over 30 years), so I wasn't sure what we would find under the cardboard cover taped to the mirror. To my relief the surface was impeccable. It's quite a sight to see a brand new 1100 mm mirror with absolutely spotless coating, without even a speck of dust.
Friday: Quick first light
The first quick star light was Friday 7th September. The main goal of the session was to locate the position of the focal plane, to establish accurately the length of the truss tubes. I had cut the truss tubes about 60 mm too long, so that the focal point was located inside the focuser. A 24 mm Panoptic fits inside a 2" focuser, and by sliding the eyepiece down you can focus the image. The difference between measured and computed position was less than 10 mm. Nice!
While doing this test, Arcturus was the first identified star seen in the telescope. I also looked at M27, officially the first deep-sky object observed with the new telescope.
Saturday: Real first light
The next day I cut the truss tubes to the correct length, set-up the telescope and performed the first collimation. A nice finishing touch by Mike Lockwood is that he etches small circles at the exact center of the mirror - it avoids those awkward moments hanging over the mirror with a template or a ruler trying to locate the exact center where you're going to attach a marking sticker. Collimation was straightforward and remained stable at different altitudes. From 15° to 75° altitude the laser dot on the primary hardly moved - maybe at most a mm. Equally the movement of the Cheshire reflection was very small, again maybe of the order of a mm. That's not too bad for a 3,950 mm focal length mobile telescope!

My wife and myself observed a couple of hours from our sub-urban backyard (SQM 19.5). Only a hint of the Milky Way was visible, but still we had great views of the Veil, NGC 604 (the emission region in M33) and especially M27 with the UHC filter was very beautiful.

The moderate seeing allowed us to magnify up to 350X - remember that the lowest power for this telescope is 150X with a 31 mm Nagler in the Paracorr. For this first test we were hand-tracking. In fact the azimuth movement is way too smooth for that - if hand tracking would be the goal, a braking mechanism would be required. (there is a reason why the teflon/formica pair works so well on Dobsons). Tests of the motorization will follow later.

It's too early to say anything about the optical quality, stars came nicely into focus - but at 350X we're working at "low power" for this telescope. I noticed some astigmatism which is most probably coming from the edge support - in the new arrangement with a single 6 mm cable the edge support may be too flexible. I'll need to find a 10 mm cable or go for a solid 90° edge support.

At f/3.6 the Paracorr II makes a world of difference. Comparing the views in the 31 mm Nagler with and without the Paracorr is shocking. I doubt the Paracorr will ever leave the focuser during the observations!

On the brightest stars (Arcturus, Vega) the double vanes produce an interesting effect, but otherwise the vanes are hardly noticeable, as expected.

With the very stable "Solide" ladder observing at 3+ meter height is no problem whatsoever. Even in this short first session I felt the need for a half-step like I made for the 25" scope. It reduces the ladder rung distance to 12 cm (5") which makes for much more comfortable observing. I'll certainly add this ASAP.

The telescope assembly is easy for a single person. Everything is a bit slower because of the bulk and the mass, but very straightforward. The winch makes the assembly very comfortable, the upper cage can be attached with your feet on the ground. Before taking the telescope to a darker location, I still need to work a bit on the transportation mechanism. The van is ready :).

As expected the telescope is bottom-heavy, about 2 kg counterweight has to be added to the upper cage to balance the tube. That's what happens when you design for a focal length of 4400 mm to end up with a focal length of 3950 mm... By using 30x2 mm square truss tubes instead of the current 30x1.5 mm the balance issue would be solved in a useful way, adding some structural stiffness.

The Megrez 110 telescope acts as a finder, with a 28 mm UWAN eyepiece delivering 23X and a 3.5° field that is tack-sharp to the edge. It's exactly 10 times smaller than the main telescope, and is the same size as the 4.5" Newtonian I used for nearly a decade in the 1980s. I'm very happy with the 30 years of progress :).

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.

Tourism of the Deep Sky

I'm an avid deep-sky observer and spend nearly all my holidays observing at my favorite site in the French Alpes de Hautes Provence not far from the famous Mont Ventoux. My home town Leuven in Belgium (about 25 km east of Brussels) has way too much light pollution to do any serious deep-sky observing.

After having constructed a 406 mm (16") scope in 1990 and a 635 mm (25") in 2000-2002, I naturally wanted to move on. Since 2005 I've been contemplating the "next" telescope. I finally decided to go for the biggest scope I could fund and manage by myself, which turned out to be 1100 mm (43"). The Cruxis Telescope will become one of the largest portable amateur telescopes in the world, together with Erhard Hänssgen's 107 cm Dobson (42") and Dan Bakken's 41.2" Hercules (1046 mm).

What drives me in this venture is the extra-ordinary splendor of the universe one can experience at the eyepiece. Objects like the Orion Nebula, the Whirlpool Galaxy or the Veil Nebula are beautiful beyond description in large telescopes. Photographs simply do not do justice to the visual thrill at the telescope eyepiece (although they may show a lot more details than you could ever see visually). The best description of my favorite hobby would be "tourism of the deep sky".

Design Criteria

Single person transportation
I will quite often have to transport the 270 kg (600 lbs) scope by myself. By no means can or do I want to lift something heavier than about 25 kg. This is basically not very different from my 100 kg (220 lbs) 635 mm scope. I'll either have to use a trailer or buy a van that is big enough to roll the scope in.
Single person set-up in 30 minutes
Assembly should be possible by a single person in about 30 minutes. The upper cage will be too big and too heavy to be mounted on top of the ladder, so a system must be provided to mount the upper cage while standing on the ground.
Alt-azimuth motor drive system
This will not be a Dobsonian telescope! There won't be any teflon bearings, nor will the scope be hand-moved. At a total weight of about 270 kg (600 lbs), I felt it was probably not a good idea to rely on manual pushing and pulling around.
To control the scope movements, I do not want to use a laptop; computer screens hamper the dark adaptation required for the best visual deep-sky observing. Furthermore, with the eyepiece up to 4 meter from the ground, a wireless handpad is a must.
Visual observing only
Initially this scope is intended for visual observations only. This said, the secondary mirror has been designed to give full illumination of a 38 mm (1.5") field, so later photography will be possible.
To enable long-exposure photography, a field derotation unit will be required.
Focal length (height of the telescope)
15 years ago large Dobsonians where typically at f/5, nowadays amateurs are frequently using telescopes as fast as f/3.
Taking into account considerations like eyepiece height, size and weight of secondary (a crucial factor in the total weight of the upper cage) and Newtonian coma I decided to go for f/3.6. At f/3.6 a 31 mm Nagler with Paracorr (equivalent to a 27 mm eyepiece), will give an exit pupil of just above 7 mm and provide a 150X magnification for a 32' field of view.
As for the height, I've never felt uncomfortable observing with a ladder. The 635 mm (25") f/5 requires a 6 step ladder to reach zenith, and in my experience you simply tend to forget that you're observing on a ladder. The 3.75 m zenith height of the Cruxis Telescope will require a 2.4 m (8 ft) ladder with 10 steps. The most annoying aspect will be the weight of the ladder, around 20 kg (45 lbs). Observing will become a good workout!


[The information in this section is not up-to-date. The cellular mirror has been replaced with a classical solid Supremax mirror. This information will be updated soon.]

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 for mirrors up to 50" (127 cm). The picture on the right shows Mike holding the secondary mirror blank at its arrival in October 2007.
To reduce weight and improve the cooling time of the optics, both the primary and secondary mirrors will be cellular. The cellular mirror blanks are designed and cast by Wangsness Optics. I will validate the cellular designs of the mirrors by finite element analysis.
The optical surface of the primary mirror will be approx. 15 mm (5/8") thick, its total thickness about 185 mm (7.3"). More information about the cellular design of the 120 kg (260 lbs) primary mirror will follow later.
The Secondary Mirror Design page contains more information about the 1.9 kg (4.3 lbs) secondary mirror.
Delivery of the finished optics is scheduled for April 2008.


Some Facts and Figures

  • Primary Mirror Diameter: 1100 mm (43")
  • Primary Mirror Focal ratio: f/3.6
  • Primary Mirror Focal distance: 3960 mm (13 ft)
  • Primary Mirror Weight: about 105 kg (230 lbs)
  • Secondary Mirror Minor axis: 200 mm (8")
  • Central Obstruction: 19 %
  • Length of truss tubes: 310 cm (10'2")
  • Diameter of altitude trunions: 156 cm (61")
  • Eyepiece height at zenith: 3.75 m (12')
  • Total weight of the telescope: about 270 kg (600 lbs)
  • Lowest usable magnification: 150x with 32' field of view

Mechanical Design

Design and Construction History

The construction of the 1100 mm telescope has been a 6 year voyage with some very good and some very bad moments.
The key events:
  • September 2006: Initial design for a 42" scope.
  • March 2007: Mike Lockwood will produce the optics for the 110 cm Cruxis Telescope.
  • April-June 2007: Construction of the alt-azimuth mount and Upper Cage.
  • September 2007: Cellular secondary mirror cast by Wangsness.
  • November 2007: Mike finishes the secondary mirror.
  • January 2008: Mirror box construction.
  • February 2008: Secondary mirror arrives.
  • 2010: Wangsness goes bust.
  • July 2011: Mike procures a 1100x58 mm Supremax blank.
  • April 2012: Mike sends the finished primary mirror to the coater.
  • May 2012: Primary mirror arrives.
  • July 2012: First assembly of the new structure.
  • September 2012: First light.

You can click here to view the full history of the Cruxis telescope.

Personal Telescope History

1981: 114 mm f/7.8
I started my observing career as a 13-year old in 1981 with a 114 mm Newtonian (4.5"), a standard newbie telescope at the time.
1990: 406 mm f/5
My first home-made scope saw the light in 1990; it was a 406 mm (16") f/5. I was inspired by the Sky & Telescope article "Hawaii-bound pocket scope" by Tom Clark from Tectron Telescopes. The optics were purchased from Galaxy Optics.
2000-2002: 635 mm f/5
Around 2000 I started designing and constructing a 635 mm (25") f/5 telescope. Finished in May 2002, it's a very fine telescope, with great optics from OMI.
2004-2008: Complete rebuild of the 406
Since 2004 I have completely rebuilt the 406 mm, using some ideas from Mel Bartels. The result can be seen here.
The telescope has since then seen another rebuild, with a new, high-quality 16" f/5 Lockwood mirror. The 35 mm thickness of the new mirror makes it more suitable for short sessions focused on visual planetary observations mostly of Mars, Jupiter and Saturn.
2006-2012: 1100 mm f/3.6
The current project.

Professional Background

I am a mechanical engineer with a soft spot for mathematics and software development.
From 1991 to 1999 I was employed by Tractebel Engineering to analyze earthquake and impact resistance of nuclear piping systems.
Since 1999 my main activity has been the design and development of two web-based CRM applications: first eLink, and since 2005 Efficy. The latter application has been selected by the IT magazine Data News as one of the "Top 10 Products for 2006".
Since 2009 I've also created the Chess Engine Houdini, currently the strongest chess engine in the world. If you're interested, see the Houdini Chess Engine page. Houdini got created during the long and fruitless wait for the cellular mirror of the 1100 mm telescope :).