<p>The quote appears to be from the <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...873..111I/abstract" rel="nofollow noreferrer">Ivezić et al. 2019</a> paper about <a href="https://www.lsst.org/" rel="nofollow noreferrer">LSST</a>.
In this context, <em>σ</em> quantifies the noise level in the images, and <em>r</em> is a <a href="https://en.wikipedia.org/wiki/Photometric_system" rel="nofollow noreferrer">photometric</a> passband of ~620±70 nm wavelength.
In single <em>r</em>-band images, they expect stars of <a href="https://en.wikipedia.org/wiki/AB_magnitude" rel="nofollow noreferrer">AB</a> magnitude 24.5 to stand out 5σ above the background level.</p>
<p><em>δ</em> refers to <a href="https://en.wikipedia.org/wiki/Equatorial_coordinate_system" rel="nofollow noreferrer">declination</a>; they plan to cover the sky from the south celestial pole to 34.5° north of the equator.</p>

CC BY-SA 4.0
2020-09-28T13:50:12.450Z
2020-09-28T13:50:12.450Z
Mike G
<blockquote>
<p>The typical 5σ point-source depth in a single visit in r will be ∼24.5 (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg2 with δ&lt;+34.5∘, and will be imaged multiple times in six bands, ugrizy, covering the wavelength range 320--1050 nm. About 90% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg2 region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to r∼27.5.</p>
</blockquote>
<p>I believe 'five-sigma' refers to standard deviations in a Gaussian distribution (statistics), but am unclear what it has to do with a 'point-source depth'.</p>
<p>And I am even more confused by 'r' and 'lowercase-delta'....</p>

<observational-astronomy><telescope>
What do 5-sigma, r, and lowercase-delta mean in the physical descriptions of major telescopes?
Kurt Hikes
# What do 5-sigma, r, and lowercase-delta mean in the physical descriptions of major telescopes?

(Asked by: username_0 on 2020-09-28)

> The typical 5σ point-source depth in a single visit in r will be ∼24.5 (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg2 with δ<+34.5∘, and will be imaged multiple times in six bands, ugrizy, covering the wavelength range 320--1050 nm. About 90% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg2 region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to r∼27.5.

I believe 'five-sigma' refers to standard deviations in a Gaussian distribution (statistics), but am unclear what it has to do with a 'point-source depth'.

And I am even more confused by 'r' and 'lowercase-delta'....

**Tags:** observational-astronomy, telescope


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## 1 answers

### Answer by username_1 on 2020-09-28. Score: 2 | Accepted answer

The quote appears to be from the [Ivezić et al. 2019](https://ui.adsabs.harvard.edu/abs/2019ApJ...873..111I/abstract) paper about [LSST](https://www.lsst.org/).
In this context, *σ* quantifies the noise level in the images, and *r* is a [photometric](https://en.wikipedia.org/wiki/Photometric_system) passband of ~620±70 nm wavelength.
In single *r*-band images, they expect stars of [AB](https://en.wikipedia.org/wiki/AB_magnitude) magnitude 24.5 to stand out 5σ above the background level.

*δ* refers to [declination](https://en.wikipedia.org/wiki/Equatorial_coordinate_system); they plan to cover the sky from the south celestial pole to 34.5° north of the equator.


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