<p>If you want bins that have an equal step in km/s then you will have to rebin in steps of equal log wavelength. On this scale the rest wavelength of H alpha is at zero km/s and then each bin will represent a increment in velocity with respect to that.</p>

<p>See <a href="https://astronomy.stackexchange.com/questions/16266/how-do-i-apply-a-velocity-shift-to-a-wavelength-array-with-uniform-logarithmic-s?rq=1">How do I apply a velocity shift to a wavelength array with uniform logarithmic spacing?</a></p>

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2016-07-05T03:08:07.283Z
2016-07-05T03:08:07.283Z
2017-04-13T12:59:06.910Z
ProfRob
<p>I have a spectral cube (in FITS format) whose spectral dimension is in ångströms. The sampling along the spectral dimension is 0.28A (CDELT=0.28). The observation in the cube is Ha emission of a galaxy at redshift 0.1. </p>

<p>How can I convert the sampling to km/s?</p>

<spectroscopy><spectra><units>
Converting ångström spectral dimension to galaxy speed (km/s)
AstrOne
# Converting ångström spectral dimension to galaxy speed (km/s)

(Asked by: username_0 on 2016-07-05)

I have a spectral cube (in FITS format) whose spectral dimension is in ångströms. The sampling along the spectral dimension is 0.28A (CDELT=0.28). The observation in the cube is Ha emission of a galaxy at redshift 0.1.

How can I convert the sampling to km/s?

**Tags:** spectroscopy, spectra, units

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## 1 answers

### Answer by username_1 on 2016-07-05. Score: 3 | Accepted answer

If you want bins that have an equal step in km/s then you will have to rebin in steps of equal log wavelength. On this scale the rest wavelength of H alpha is at zero km/s and then each bin will represent a increment in velocity with respect to that.

See [How do I apply a velocity shift to a wavelength array with uniform logarithmic spacing?](https://astronomy.stackexchange.com/questions/16266/how-do-i-apply-a-velocity-shift-to-a-wavelength-array-with-uniform-logarithmic-s?rq=1)


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