Measuring the top angle of a triangular prism by reflection method requires that the telescope,collimator and triangular prism all meet the test requirements.The requirements for spectrometer adjustment aremore comprehensive.The method of coarse adjustment assisted by the reflected image outside the telescope isoften used when the coarse adjustment of the spectrometer is not ideal.Considering that this method is not easy to master, by analyzing the imaging law of the reflected image of telescope in a plane mirror,it is determinedthat the landmark phenomenon of a good coarse adjustment(i.e.reaching the best tolerant position) is that thegreen cross light source observed from outside the telescope and the horizontal diameter on the telescope'sdividing plate is located on the same level.The placement of the prism is an important factor in the observationof the slit reflection image.The best placement of the prism is to place the top angle A to be measured at thecenter of the stage to ensure that the slit image reflected from both the AB and AC optical surfaces can beobserved.