Like many smaller equatorial mounts, the EQ-6 uses the “Vixen GP” dovetail standard. This is perfectly adequate for small scopes but it’s less satisfactory with my 9¼” SCT. The narrow dovetail (44mm at its widest) is inclined to lean to one side or the other, especially when the dovetail clamp is loosened to balance the scope. This is compounded by the poor quality of the EQ-6’s dovetail clamp — mine does not properly fit any dovetail I’ve tried in it. The solution is to upgrade to the 3″ wide “Losmandy” standard.
There is a wide choice of 3″ dovetail clamps available from companies such as Losmandy, Astro-Physics and William Optics. The best value for money here in the UK was the William Optics saddle clamp and dovetail plate combo, so that’s what I chose. The dovetail plate is just the right size for my 6″ f/5 refractor.
Attaching a new dovetail clamp to the EQ-6 can be done by milling away the existing clamp to leave a flat surface, but a better way is to replace the clamp with an adapter. Fortunately a Cloudy Nights member called Kerry Hanna is a dab hand at machining telescope accessories and recently offered a few adapters for sale. I got in just in time to buy his last one.
The first job is to remove the existing dovetail clamp. There are three small set screws just below the clamp that fix it to the main body of the dec. axis. After removing these screws it still needed a few gentle taps with a hammer to loosen the clamp.
Before doing any irreversible machining on my new adapter I made sure it fitted my mount. There’s always the possibility that my fairly old EQ-6 might be a different size from the one Kerry used when designing his adapter. My worries were unfounded — it’s a perfect fit.
Now comes the part I was dreading: drilling and tapping holes to mount the new dovetail clamp. “Measure twice, cut once” is the rule here, yet still I all too often get things wrong. The first challenge is to choose which of the clamp’s many holes to use. I decided to use four 6mm screws rather than two 8mm. Unfortunately, the most suitable group of 6mm holes is about 50mm offset from the centre of the saddle. I offset my holes in the adapter by about 18mm so that the dovetail clamp would be offset by just over 30mm when fitted. I didn’t get the hole positions exactly right, but the holes in the clamp have enough clearance that this was not a problem.
This last picture shows the mount with the new dovetail clamp installed. Now all I need is a clear night to try it out…
Update Sept 2006: Since writing the above I’ve discovered that the William Optics dovetail and saddle clamp are not compatible with other manufacturers’ “Losmandy standard” equipment. The dovetail I bought is 76.9mm wide at the base of the dovetail, compared to 74 to 76mm for dovetails made by others. Later WO dovetails are 78.0mm wide, as shown on their web-site. I have fitted a Baader 3″ rail to my SCT, and my WO clamp only just grips it. If I were to do this again I would pay the extra and buy a more standard saddle clamp.
Later on, I bought one of the newer WO combo plates so I could mount my new 80ED refractor. I have modified the saddle clamp to make it compatible with the older WO dovetail, the Baader dovetail and the Celestron CGE dovetail I’ve recently fitted to my C9¼.