|
To fix flexure in the focuser, we attached a very rigid bar, 25x25 mm square hollow, and half a metre long, and bolted very securely to the dovetail, to run from the dovetail backward to a point level with the camera, and about 250 mm out from the side of the camera body. That acts as a rigid reference point. We then ran a stainless steel strut (250 mm long, obviously), hinged at both ends, to run between the camera body and this rigid anchor point. Since we only ever need to move the focuser a couple millimeters at the very most, this system provides great lateral stability to the focuser (in the direction it most naturally wants to fall) without putting any appreciable strain on it.
In principle, one would need two such struts, one to take flop in one direction, and another to take flop at 90 degrees, but in practice only one direction was important.
The hinge pins on the strut, and the attachments and all the other components have to be extremely rigid, or it won't work. There is an adjustment screw to set the length of the strut very precisely, so there is no strain on the focuser in the neutral position.
Also, the strut needs to be very long compared with the amount of focuser travel, or moving the focuser in and out a long way will put strain on the focuser. The lateral stress on the focuser is given approximately by square(h)/2R where h is the focuser movement and R is the length of the strut, so for us, a 2 mm focuser movement and a 250 mm strut, the lateral stress on the focuser is only 8 microns. (It gets very rapidly worse after that.)
The strut not only helps prevent elongation of the star during an exposure; it also helps prevent change in focal length by preventing the tilt and flop.
Hope that helps a tiny bit.
Mike
|