<p>If you are at zenith, it doesn't matter - the light coming through the atmosphere is not differentially refracted.</p>
<p>If you are away from zenith, then the position of the star in the sky depends on wavelength due to differential refraction. This differential refraction is along a line from the star to the zenith - at the so-called <a href="https://en.m.wikipedia.org/wiki/Parallactic_angle#:%7E:text=In%20spherical%20astronomy%2C%20the%20parallactic,zenith%20at%20the%20celestial%20object." rel="nofollow noreferrer">parallactic angle</a> to a N-S line. If you are doing spectrophotometry or spectroscopy over a large wavelength range, it's usual to rotate the slit so it is parallel to this line . If you don't, then the danger is that some wavelengths of light won't fall in the slit at all. The size of the effect increases with angle from the zenith.</p>

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2022-04-17T14:17:26.410Z
2022-04-17T14:24:08.017Z
2022-04-17T14:24:08.017Z
ProfRob
<p>If you were to measure the spectrum of an object while it's exactly at the zenith, how should you orient the slit of the spectrograph to minimise atmospheric dispersion? Is North-South or East-West better? Or does it not make a difference either way?</p>

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2022-04-16T20:57:14.410Z
You can also ask which way to orient it *away* from the zenith; that will also have an interesting answer. I'm just curious; asking which is better, N-S or E-W has the premise that one is better. Is it known that "neither" can't be an answer?
uhoh
<observational-astronomy><spectroscopy><spectra><atmospheric-effects><dispersion>
When measuring the spectrum of an object at the zenith, is it better to orient the spectroscopic slit N-S of E-W?
Soph
# When measuring the spectrum of an object at the zenith, is it better to orient the spectroscopic slit N-S of E-W?

(Asked by: username_0 on 2022-04-16)

If you were to measure the spectrum of an object while it's exactly at the zenith, how should you orient the slit of the spectrograph to minimise atmospheric dispersion? Is North-South or East-West better? Or does it not make a difference either way?

**Tags:** observational-astronomy, spectroscopy, spectra, atmospheric-effects, dispersion

> **Comments:**
> **username_1**: You can also ask which way to orient it *away* from the zenith; that will also have an interesting answer. I'm just curious; asking which is better, N-S or E-W has the premise that one is better. Is it known that "neither" can't be an answer?  

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## 1 answers

### Answer by username_2 on 2022-04-17. Score: 2 | Accepted answer

If you are at zenith, it doesn't matter - the light coming through the atmosphere is not differentially refracted.

If you are away from zenith, then the position of the star in the sky depends on wavelength due to differential refraction. This differential refraction is along a line from the star to the zenith - at the so-called [parallactic angle](https://en.m.wikipedia.org/wiki/Parallactic_angle#:%7E:text=In%20spherical%20astronomy%2C%20the%20parallactic,zenith%20at%20the%20celestial%20object.) to a N-S line. If you are doing spectrophotometry or spectroscopy over a large wavelength range, it's usual to rotate the slit so it is parallel to this line . If you don't, then the danger is that some wavelengths of light won't fall in the slit at all. The size of the effect increases with angle from the zenith.


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