CG-5 hard wood tripod

By Paweł Łańcucki, Pawel.Lancucki@pl.ibm.com

I use CG-5 mount from Celestron for about one year. After fixing initial problems with poor mechanical performance due to stiff grease and bad surfaces finish (see details on the AstronomyBoy web page), the mount proved quite useful with a range of telescopes, including 90mm f/11 achromatic refractor, 170mm f/7 Newtonian and 100mm f/10 Maksutov.

However, there was still a number of things which remained annoying with the mount, the worst being the poor aluminium tripod, which – while adequate for my small Mak, did not support properly the heavier Newtonian.

After some considerations and planning, I decided to adapt a heavy-duty hardwood tripod for my mount.

Ready made hard wood tripod – a bargain

My initial idea was quite similar to many others – just replace the aluminium legs with ones made from hard wood – preferably high quality oak. Then, while dropping to a local photo exchange, I noticed an old theodolite tripod. Theodolites are devices used by geologists, geodesists and civil engineers to accurately measure distances and angles in the field. Although the theodolite head (the measuring device) weights only around 10 kg, the tripod must be very heavy and stable, because it must support device used to measure minute angles, in order of single arc seconds. Sounds familiar? The tripod I found was made from aluminium and hardwood, with three heavy, full thickness and regulated length legs connected by a triangular central hub. It looked solid. I set it up just by spreading the legs and tried to put all my weight on the tripod (100 kg). I could not detect any flexure or wobble. Well, this was a bargain. It was mine for just below $50.

At home, I inspected the tripod thoroughly. It had no technical flaws, just a lot of traces of wear and tear and a lot of dirt, so I decided to dismantle and clean it. Old paint was removed from all wooden parts using chemical solvent and a lot of sand paper, than they were treated with a polyurethane primer and two layers of clear marine-grade finish. All steel and aluminium parts were also cleaned and painted with a primer and two layers of anti-corrosive black paint (Hamerite made in UK). Well, it started to look good:

Adapting for CG-5

Having the tripod done, I had to decide how to attach the mount head. I have purchased a pier adapter for GP mounts from Orion Optics UK (www.orionoptics.co.uk). GP and CG-5 mount heads fit this adapter in the same way they fit the central hub on the original aluminium tripod. The first adapter get lost in the mail, but Orion Optics promptly issued another one, which I received two weeks later. To mount adapter to the tripod head, I used two triangles, each made of two layers of 15mm plywood glued together with water-proof carpenter’s glue. Upper triangle is slightly larger than the tripod head – just large enough to securely catch metal L brackets holding the GP adapter. Lower triangle must be smaller to fit under the tripod head, between the legs. Both triangles are coupled together using three 80mm M8 bolts, with the tripod head / central hub squeezed between them.

GP adapter is attached to the upper triangle using six 40mm M6 bolts with heads locking in the wood. Using carpenter’s 20mm bore, I made six holes 4 mm deep to capture bolt heads within the thickness of the upper triangle. Three aluminium L brackets were cut from 30 x 30 angle stock with 3 mm walls. Top corners were cut-off and all corners and cuts were sanded and polished with a high-number sand paper.

Because the L brackets are angled to each other, they form a very stiff coupling between the GP adapter and the upper triangle. Obviously, thicker walls would be even better. In my case there was a 3mm gap between the adapter and upper triangle due to the size of available L stock, so I have put a large O-ring made from hard rubber in it to even better couple the two elements and to better transfer vibrations down to the ground. Finally, I attached a hand knob to a 120mm M10 bolt, which holds the mount head in the adapter. All bolts, nuts and washers are stainless steel to avoid corrosion.

In total, it took some 6 evenings of work, of course the duration of the entire project was longer (some 4 weeks) because I had to wait for many layers of paint to dry.

After a quick test in the field I noticed that I need to hold the legs together – especially if setting the tripod on a smooth surface like tiles on concrete – the solution was to connect the legs with 3 meters of a strong string.

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Some more pictures from the field tests are shown below.

CG-5 equatorial head placed on the tripod, counterweight shaft attached, standard 5 kg (11 lbs) and two custom-made 2.5 kg counterweights to balance my heavy 7 inch f/7 Newtonian scope:

My Newtonian OTA attached to the mount using standard dovetail adapter and custom made “tube rings” – rather a length of 2mm thick 25mm wide aluminium tape and some pieces of oak wood:

When assembled, the telescope draw attention of neighbouring children – quite eager to have a look at the sky even in daylight (see the front cover) and then at Mars late in the night:

Final results

How to evaluate final result? Well, try to set your scope up on jelly. And then move to concrete. I still need to eliminate some play in the mount head, but the difference is impressive already. Any vibrations at the eyepiece of my (rather heavy – 8 kg) Newtonian are dampened within 1 – 1.5 seconds!

Drawbacks

There are two. First, the hardwood tripod is substantially heavier than the aluminium one, it weights around 15 kg in total. Second – at its lowers position, it is still about 25 cm higher than the original tripod, so some people may have trouble to reach the eyepiece of a Newtonian telescope in some positions if the tube is longer than 90 cm – and this is true in my case. A one-step ladder however cures this problem, and overall improvement in stability is definitely worthwhile. Furthermore, new, higher tripod is even more convenient for refractors!

I have a future plan to add additional brace between the legs – mostly to prevent them for spreading too far if the scope is set on a slippery surface. It will also support an accessory tray, which will hold eyepieces, filters and power supply box (see further description).

And what about the original tripod? Well, it is not entirely hopeless. I can still use it if I only plan to use the Mak scope, in particular to make it more portable for observing in the field. Also, I plan to use some rainy weekend during this summer or autumn to build a parallelogram for my 16 x 80 binoculars, and this would be a great tripod to put it mount on.