<p>The variance is the square of the standard deviation, so you can calculate 1$\sigma$ errors of the spectrum by taking the square root of one over the inverse variance of HDU1 (<a href="https://data.sdss.org/datamodel/files/SPECTRO_REDUX/RUN2D/PLATE4/spPlate.html#hdu1head" rel="nofollow">https://data.sdss.org/datamodel/files/SPECTRO_REDUX/RUN2D/PLATE4/spPlate.html#hdu1head</a>). </p>

<p>(You will get infinity for bad pixels since their inverse variance is set to zero.)</p>

CC BY-SA 3.0
2016-07-16T21:47:21.937Z
Is there anyway to increase the level of sigma error?
Syed Ali Mohsin Bukhari
CC BY-SA 3.0
2016-07-16T21:58:26.750Z
Absolutely right. The inverse variance is used because that is directly what you use to weight any fit to a model.
ProfRob
CC BY-SA 3.0
2016-07-18T01:04:15.760Z
@syedali, like the 3sigma error? typically you assume a normal distribution and just multiply  1sigma  by 3 (for example). see: https://en.wikipedia.org/wiki/Normal_distribution#Standard_deviation_and_tolerance_intervals
E. Douglas
CC BY-SA 3.0
2016-07-16T21:41:53.193Z
2016-07-16T21:41:53.193Z
E. Douglas
<p>I need to know does SDSS spectrum contains error spectrum as well?</p>

<p><a href="http://www.sdss.org/dr12/spectro/spectro_basics/#AbouttheSpectra" rel="nofollow">http://www.sdss.org/dr12/spectro/spectro_basics/#AbouttheSpectra</a></p>

<p>The above link says, further HDU's contain error spectrum and than tells about the inverse variance. Does inverse variance contains error spectrum? If yes, how can I extract that, if not where can I find error spectrum?</p>

<spectroscopy><spectra><quasars>
Extraction of error spectrum in Sloan Digital Sky Survey
Syed Ali Mohsin Bukhari
# Extraction of error spectrum in Sloan Digital Sky Survey

(Asked by: username_0 on 2016-07-16)

I need to know does SDSS spectrum contains error spectrum as well?

<http://www.sdss.org/dr12/spectro/spectro_basics/#AbouttheSpectra>

The above link says, further HDU's contain error spectrum and than tells about the inverse variance. Does inverse variance contains error spectrum? If yes, how can I extract that, if not where can I find error spectrum?

**Tags:** spectroscopy, spectra, quasars

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## 1 answers

### Answer by username_1 on 2016-07-16. Score: 1 | Accepted answer

The variance is the square of the standard deviation, so you can calculate 1$\sigma$ errors of the spectrum by taking the square root of one over the inverse variance of HDU1 (<https://data.sdss.org/datamodel/files/SPECTRO_REDUX/RUN2D/PLATE4/spPlate.html#hdu1head>).

(You will get infinity for bad pixels since their inverse variance is set to zero.)

> **Comments:**
> **username_0**: Is there anyway to increase the level of sigma error?  
> **username_2**: Absolutely right. The inverse variance is used because that is directly what you use to weight any fit to a model.  
> **username_1**: @syedali, like the 3sigma error? typically you assume a normal distribution and just multiply  1sigma  by 3 (for example). see: https://en.wikipedia.org/wiki/Normal_distribution#Standard_deviation_and_tolerance_intervals  

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