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The 390 variable wedge is an important tool for calibration of optical instruments.  This item is made to fit over the front flanges of the horizontal or down-pointing instrument mount tubes on our model 270BN Calibration Test Stand. Two thumbscrews securely lock the 390 to the tube flanges.  It can then be used to measure angular deviation from optimal during optical instrument calibration.  An integrated glass block is rotated manually to produce a measurable angular deviation in any line of sight which is roughly normal to the glass.  The "zero point" on the graduated scale may be rotated to either a horizontal or vertical position (index marks may be seen in the picture at the 12 o'clock and 3 o'clock positions), allowing either horizontal or vertical measurements to be made without physically relocating the entire unit.

You may wish to see our other optical wedge, the model 290.