Dovetail upgrade after earthquake

[ChrisM]

The Gippsland M5.3 earthquake last June found a weakness in the dovetail clamp holding my SCT and refractor. In retrospect, it was lucky that the whole lot didn't hit the deck. At that time, the scope had the Losmandy 24" dovetail plate and the mount had the 8.5" dovetail saddle plate (both D-style). The saddle plate, although well made, has only 2 small brass chamfered pins that push up against the dovetail plate.

Whilst nothing actually broke or became obviously bent, a 10 thou gap opened up between the two dovetail halves when the scopes were in certain orientations, allowing a vibration to occur with the slightest of bumps.

To more securely hold the not inconsiderable load (~40 kg), I've upgraded to the 16" Easy-balance dovetail saddle plate, which has 3 large block clamps that provide much more clamping surface. I also fitted an extra section of aluminium bar (540 x 100 x 10 mm) to the 24" dovetail plate, which is heavily milled out on its underside, to act as a stiffener. The two are clamped together by ten socket-head countersunk screws.

The end result appears very solid, but is yet to be tested on a clear sky.

The photos show:
1. The old 8.5" saddle plate with two brass pin-type clamps
2. The new 16" saddle plate with three block-type clamps
3. The 24" dovetail plate with the add-on aluminium stiffening bar
4. Ditto - assembled
5. Ditto - assembled on telescope
6. Both scopes mounted

Chris

[DavidTrap]

Noice. Few dollars in counterweights (and shipping them across the Pacific!)

How do you find the SCT mounted like that? Rigid enough with only those small bolts biting into the OTA? I'm thinking about a C-11 for photometry/spectroscopy in the years ahead.

DT

[Capricorn1(Tom)]

David i have those small bolts holding a C14 on a celestron plate attached to a standard losmandy plate(G11)......i worry about the shear stress failure on those bolts.....i do not want to add rings or a saddle due to the weight constraints on the G11 mount. But in the process of doing my own mod with aluminum strapping to eliminate the shear stress on the the hex bolts.....when completed subjected to work constrainsts will post on line this year for comments. Cheers :D

[ChrisM]

David, the connection to the OTA is very rigid. The curved mounting blocks are a very good fit, and there was no sign of any issue there after the earthquake.

What is a bit surprising though is the relatively small PCD and small size of the four bolts that hold the AP1200 mount adapter to the top of the pier. If the mount is balanced well, there will be no resultant shear force on them.

Chris

[DavidTrap]

Thanks for the feedback Gents.

DT