Homemade DIY dovetail bar for LXD55 telescope

My telescope setup consists of an LXD55 equatorial head mount on a homemade concrete pier. On this mount is my Helios 200mm F5 newtonian reflector with a piggy back 80mm F5 refractor for guiding. See this image of my setup taken in the summer.

The tube rings of the Newtonian attach to a dovetail bar which is held onto the saddle plate of the mount. The supplied dovetail bar looks like some kind of extruded bar. I've noticed that it will flex by several millimetres if you wobble the mount, and is by far the most unstable part of the setup.

Replacement LXD55 dovetail bar

So I've made a seriously over-engineered replacement dovetail bar for my setup. It's simply a 1" by 2" aluminium bar which I've milled a dovetail in the middle, and a couple of flats on each end. The main points are as follows:

• Must have approaching zero flexure under the 20lb to 30lb load of scope, guidescope, artemis, filterwheel etc etc.
• Don't care how heavy it is
• Made from aluminium bar, not steel to stop rust.
• Don't care what it looks like - its dark outside and it doesn't have to be a jewel.
• Don't want adjustments for trimming the orthogonality of the OTA to the RA axis - a bit of misalignment is easily countered by doing some syncs in Autostar, and 90% of the time I image on only one side of the meridian. Such adjustments reduce rigidity.
• A small (0.5") clearance between dovetail and OTA for somewhere to pass wires. I've actually got the OTA about 10mm closer to the fulcrum of the RA axis, which will slightly reduce the moments on the system.

The setup is extremely rigid now. My main worry is that the flexure in the original bar was acting as a spring and spreading out the force of any wobble on the cheap cast saddle plate. Hopefully it won't break. Strengthening that saddle plate is my next job!

The flats on the bottom of the rings where they attach to the dovetail were not flat. These have been lapped to make them flat, and therefore less prone to wobble. The original cheap bolts that attached the scope to the dovetail (by threading into tapped holes in the cast rings, eeeek!) have been thrown away and replaced with M8 coach nuts+bolts that can stand a little more spanner torque.

The flats on the top of the bar where the guidescope mount attaches have also been lapped flat and the guidescope is now bolted in one position until I get some new tube rings for that. The computer controlled guidescope mount system worked extremely well with my SC3 modified toucam, but had too much flexure for the longer exposures used by my artemis. I am working on the tube rings.

I don't think this part is going to get the home anodising treatment for a while - I need a bigger anodising tank!

I have also strengthed the method of attaching the dovetail to the end of the DEC axis. Normally the dovetail slots into a saddle plate. This is a very poor solution, because you get flexure and there is a strong danger that the sides of the saddle plate will snap because it is cheap cast aluminium.

To solve this I have drilled a hole down the middle of the dec axis which communicates with the polar alignment hole. I have drill a matching hole in the saddle plate, and the new dovetail bar. A length of threaded rod passes through all these holes and is bolted tightly at both ends. Inside the DEC axis I have glued a thread plug to add additional strength.

This solution creates a strong link beteen all the parts - the DEC shaft, the saddle plate and the dovetail bar are all clamped together and do not move.

This is one of the best changes I've made to my LXD55 mount. I've seen modifications that involve bolting the dovetail to the saddle plate, but this is a very thin bit of metal - this approach gives a stronger mechanical link between the dovetail and the DEC axis. I might add this setup is not exactly portable...

Some pictures of the Homemade DIY dovetail bar for LXD55