23Sep2012

Building The 1100 mm Cruxis Telescope.

This telescope is designed and constructed by the Belgian amateur astronomer Robert Houdart . 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. 

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 30×2 mm square truss tubes instead of the current 30×1.5 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.

 

 

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