Making a telescope pier for the observatory

This page is about the creation of a permanent telescope pier for the observatory. The reasons for making a pier were:

• My existing pier was shorter than I needed

• My existing pier had greater vibration at its full height than I could live with.

• I wanted something heavier

• I wanted something very sturdy that would ignore my occasional stumblings in the dark which knocked the lighter pier out of alignment.

• It had to take the largest mount I knew I might own. That's the AE Class C mount currently.

• The pier should be removable for when I move house next.

• I had the means to make it

I did look around and have a few chats with local stockists and fabrication workshops as to the expected price. This worked out around £400 in quotes. This seemed a bit steep. So I had a mooch around on AstroBuySell. After some digging I found some steel in Coventry from another astronomer. It consisted of a 18 inch by 3/4 inch thick steel plate and a round 13inch by 1/2 inch plate which he generously cut to size from the steel that was available.

Design

The design of the pier was a straightforward tube of steel pipe with a square base and circular top plate. The base would have 4 mounting holes, one at each corner since it is easy to align. The base sits on 4 nuts mounted on four studs, the studs are 10" long and embedded for 6" in concrete. The pier is raised and levelled by rotating the supporting nuts on the studs, using washers to allow easy rotation of the nuts against the base plate. The alignment of the tube is locked in place with locknuts on the top side of the plate.

The pipe tube chosen was 12 inches in diameter and 1/4 inch wall thickness, in mild steel. This drove the thickness of the base plate to be thick enough not to bend with such a long heavy lever. The length of the tube is 76 inches to handle the 9 inch raised observatory floor and the height of the observatory walls. It's meant to carry a refractor or telescope viewed from the end to give the user easy viewing access. It would not be suitable to support a Newtonian in this configuration for instance. The eyepiece height would be enormous. On the other hand - I could cut it down. The top plate needed to accept both a Vixen clone mounting bolt (M10) and the mounting bolts from my AE systems large mount. This implies internal tube access or a removable top plate. I went for access to the underside of the fixed plate.

The key design problem I needed to address was to ensure that the holes I drilled in the concrete base plinth were perfectly aligned in position with the base plate holes and not skewed and that I could install such a pier aligned to North within adjustment accuracy.

Fabrication

I made a thick drill template for the base holes from multiple thicknesses of wood screwed together and marked and drilled. This was used as a template to drill the base plate and to drill the holes in the concrete. At 25 mm diameter these holes needed special bits to drill the concrete with a SDS drill. The SDS also came in useful for opening out the holes in the base plate once piloted with a bench drill up to half inch diameter. I needed access to the bolts in the top plate that would hold the mounts attached, so a large 4 inch diameter hole was drilled in the pipe 4 inches from the top and bottom ends using a standard hole cutting saw bit, along the alignment line.

This allows hand and spanner access and also allow ducting of cables out of the way back to the PC. I don't run cables under the floor due to the risk of mice eating holes in the cables. Maybe I should just to get them completely out of the way.

I also welded on some angle-iron pieces to mount an accessory board onto. This holds my telescope controller, the focuser handsets, some eyepieces etc on a plywood board with suitable hgoles, screws, tages etc to get everything out of the way. Finally the tube was sanded down to remove burrs and sharp edges and painted with black Hammerite for a long lasting finish.

Alignment

Alignment of the pier can be addressed two ways - make some part of the pier configurable so it can be adjusted in situ or keep it all very stiff and unforgiving and make sure you align the entire pier at time of fixing. I opted for the latter. Levelling of the top of the pier is obtained through adjustment of the nuts running on the mount studs. Alignment of the pier and mounted telescopes is obtained through drilling the holes in the right places and careful assembly of the welded components to make sure they continue to line up. The drilling template was marked with the alignment line and the pipe and top and bottom plates marked with an angle grinder to indicate correct alignment to maintain through welding.

The alignment to the north pole in the observatory was achieved by using the mounting template in its position in the observatory to gauge the meridian line by mounting a large stick vertically though the centre and drawing a line along its shadow at GMT noon, allowing for equation of time adjustments. I rotated the template until its alignment marks lined up with those on the floor and then drilled the holes through it into the concrete for the studs. The studs were then mortared in place and allowed to set while bolted through the template to make sure that they didn't set skew.

Assembly

The pier was assembled by tack-welding it all and then going back after checking alignments and welding it all properly. Welding of the top plate when the pipe was stood vertically was found to be too hard since I was coming at it from underneath, so I tacked it into place with big welding magnets and laid it down to weld. Registration of the top plate and the bottom plate concentrically to the centre of the pipe was done by aligning circles drawn on the plates and by gauging the offset around the outside, tapping with a hammer until all was even. I still managed to weld the top plate on upside down.

Installation

Installation of the pier to the observatory required total observatory dis-assembly! this was because the original mount was designed to fit in the pier recess but the pier recess somehow turned out to be too small. When I originally erected the observatory I got around this by lifting the floor in one piece and dropping it over the original mount. To remove it I had to reverse this. So I took advantage of upheaval and refurbished all the wood and cleaned and repainted all the fibreglass and seals. That took most of a weekend each to pull apart and then rebuild. Fortunately I had designed it to do just that.

The pier assembled weighs over 75Kg sO I couldn't carry the thing. In the end I had one end on a sack trolley and pulled it across the garden that way. I took one wall off the octagonal observatory building and walked the pier in. The floors were reinforced with props for this operation since there were now effectively 3 people on it and it wasn't built for that sort of weight. The mount was 'walked' over the studs and the nuts on the studs were adjusted in sequence to gradually lower the pier. A brief level with a spirit bubble and orientation check with the alignment marks and all was complete.

The first mount I have had on there is the Vixen GPDX. This uses an adapter I designed and had machined years ago to take the Polar axis adjuster screws.

Conclusion

The mount is very steady, knocks aren't even noticed now and the polar alignment is pretty close for the pier itself. The cables are out of the way completely within the pier ands I have lots of space on the pier top to place all my bits and bobs. And you can't notice their use while observing since there is so little vibration; previously operating the SS2K on the pier would cause the image to move. Now there is too little to mention. The tracking of the telescope has also improved since any interaction between the raised floor and the pier legs of the old mount have been removed. Before there was supposed to be none but the close clearances meant any movement caused movement in the pier. All in all, very happy.

All in cost - about £175 for steel, of which £50 was for the plates and £125 for the tube from UK tube Ltd. Welding supplies like welding rods are cheap and i have lots anyway but a proportional cost is about £10. So the total looks like £185 compared to £400 to £600 from Astronomy websites.

Changes and Issues

The mount is 12 inches too tall !. While estimating the tube height I took note of the height of the mount deployed on it but I could still happily take off another 12 inches. On the other hand, while observing at zenith I can walk underneath and use the eyepiece directly and comfortably. So I bought a step-ladder. At least I have clear access down to the horizon now. The accessories box should be altered to cover the top wiring hole - from a functional and cabling point of view to hide and cover the emerging cables and also to raise the controllers attached there to a more handy height.

After 3 months of using this pier with the GP/DX mount I have now taken this mount off and replaced with the AE Luton Class C mount. It's a bit of a beast.
I broke it into three segments - the altitude base and RA head, the Dec head and the telescope mount plate. It was a bit of a lean to get the dec head on the RA axle, being quite a long extension with a heavy weight in my arms, on the top of a ladder. Finally the mount plates went on and after that the scopes.

The first isue was that the scopes were much lighter than the counterweight. I had to find a new counterweight. I made do with a larger 2517 axle hub I had lying around that uses a taper to clamp onto the axle. The other problem was in adjusting the altitude of the polar axis of this thing. I had done it with a protractor during the day before assembly but now I wanted to do it in anger during the night, with fine control, in the dark. This mount just isn't built for that! I found that out when I had completed 3-point alignment, east-west alignment and was tapping the north-south axis into place, when the whole thing tipped and crashed on its buffers. Clearly the central bolt had not been tight enough, and getting it back upright was a hard job, before I start all over again.

So now I have used metal banding from crate strapping, bolted underneath the mount to pull it up on a screw thread. The screw pulls against a plate bolted to the North base of the mount and rotating on a thrust washer for easy movement. I can get down to about 10arc-minutes control now.

The last thing is the azimuth adjustment for the mount. In the end I have resorted to a rubber mallet and a stock of wood being used to move the mount around the sourth pivor bolt during drigt alignment at x400 to get the sensitivity. It has taken about 10 hours or so to get near the pole. My key lesson learnt is to do the altitude first in order to get the azimuth in the ball park. I was about to panic when I realised that I was moving outside the range of movement I had planned into the mount on the fixed pier.