TripleSpec Mass and Balance

The total weight of TripleSpec plus its aluminum cart was measured using a load cell at multiple points around the cart. At any point the load cell carried about 1/2 the weight of the system and a little bit of physics and algebra was used to derive the total mass and location of the center of gravity.

The figure below shows the geometry for the cart measurements while the table summarizes the measurements made at each of the circled positions. A single measurement was made at each of points 1 and 2. Multiple measurements were made at positions 3 and 4 as the dewar rotated through 360 degrees. Rotation is reference CW and CCW looking at the dewar window (i.e. from the Nasmyth fold mirror). The front ring of the dewar is nearly coincident with the front of the cart, thus the mechanical centroid of the dewar is displaced about 2 inches toward the front of the cart. Centroid offsets listed below are with respect to the center of the cart not the center of the dewar.

Weight (lbs)CG offset
1/20618 / 4374.1"
toward window
3/4135CCW490 / 5140.36" left
3/490CCW493 / 5120.28" left
3/445CCW496 / 5100.21" left
3/40498 / 5090.16" left
3/445CW496 / 5090.19" left
3/490CW494 / 5120.27" left
3/4135CW491 / 5160.37" left

The system total weight (aluminum cart plus TripleSpec) is about 1050 lbs with an uncertainty of about 5% based on uncertainty about the position of the load cell and orientation of the cart wheels). The aluminum cart has a weight of 165 pounds, so the dry weight of TripleSpec, loaded up with onboard electronics, pump fittings, etc., but empty of cryogen) is 885 pounds.

The axial mass centroid is shifted 2" toward the front plate of the dewar, but in practice the rear spring support will pick up substantial weight moving the centroid toward the rotator and reducing the total vertical load on the rotator.

The lateral mass centroid shifts by about 1/4" as the system rotates (the zeropoint is offset due to uncertainty in the placement of the load cell and orientation of the cart wheels). In practice this will change as counterweighting is available that mounts just behind the rotator flange.