Why Measure Angles to the Side of the Yellow Line in Spectra Experiment?

[NitroNinja]

Homework Statement

Okay, I'll start off with a summary of the practical. And then the question.
The aim was to measure the wavelength of yellow light from a sodium vapour lamp, using a telescope and a diffraction grating of 300 lines per mm. The instructions tell us to prepare the apparatus, so that the telescope [If you want to see an image, search 'spectrometer telescope' into google images] aligns with the centre yellow wavelength.
We then move this telescope, to the first order on the left hand side of the original position. To measure the angle, we move the telescope to the right of the yellow line on the first order spectrum, then measure the angle. We repeat for the first order spectrum on the other side, and subtract, divide by two, to find the position of the centre position. This is compared to true results.

Now the question is, why did they make us go to the right of the yellow line, rather than on top of the yellow line (when measuring angles)?

Homework Equations

- N/A

The Attempt at a Solution

I attempted to look at this from different aspects, but keep wondering... 'wouldn't this just decrease accuracy?'.

 

[Simon Bridge]

The line has a thickness - which part of the top of the line did you have in mind?

Note: you are not "comparing to true results" - there is no such thing. You are comparing with accepted results.
You conclusion should state whether your results are consistent with the accepted results (cite the source) and to what uncertainty.