Sorry for the delayed response. The field of vision of the VL53L1X sensor is 27 degrees, so it is unclear exactly what the sensor is measuring the distance to in your system (it could be seeing a blurred response from the walls of the tube, for instance). We do not have any specific advice for using that sensor in such a configuration, but you might try to do a calibration to see if you can develop a repeatable relationship between the value the sensor returns and the distance of the target that is in the tube. Also, in general, highly reflective surfaces like mirrors can be less than ideal measurement targets for optical sensors like this. Although the surface might return a high percentage of light directly to the sensor if it is oriented correctly, you should also keep in mind that the outbound light might also be reflected off the mirror to another surface and the sensor might try to measure that other surface through the mirror.