<p>It's not entirely clear to me what you want to know -- you say &quot;spatial resolution&quot; in the title, but then say, &quot;I need to check if its beam coincides with coordinates I've collected for objects in GALEX&quot;, which implies you're more interested in <em>positional accuracy</em>.</p>
<p>In either case, <a href="https://noirlab.edu/public/media/archives/brochures/pdf/brochure007.pdf" rel="nofollow noreferrer">this link</a> probably provides what you want to know. The fiber positioning accuracy is given as 5 microns, which translates to <span class="math-container">$\approx 0.07$</span> arcsec -- that's the positional accuracy -- while the <em>fiber diameter</em> is 1.5 arcsec in angular terms. The latter is effectively the spatial (or angular) resolution: everything within the diameter of the fiber is mixed together to form the spectrum.</p>
<p>The numbers you quote from &quot;Flaughera &amp; Bebekb (2014)&quot; are <a href="https://en.wikipedia.org/wiki/Spectral_resolution" rel="nofollow noreferrer"><em>spectral</em> resolutions</a> (the Wiki page I linked to calls this the &quot;resolving power&quot;). They have no units because they are defined as <span class="math-container">$\lambda / \Delta \lambda$</span>, which has units of wavelength / wavelength, which reduces to &quot;no units&quot;.</p>

CC BY-SA 4.0
2024-07-10T00:46:21.983Z
Can you please clarify your phrase "5 microns, which translates to ≈0.07
 arcsec"? How does this translate?
Jim421616
CC BY-SA 4.0
2024-07-12T08:10:46.233Z
@Jim421616 Page 4 of the PDF file I linked to lists the "focal-plane scale" of DESI as "70.8 microns/arcsec (average over FOV)"; this inverts to give a [plate scale](https://en.wikipedia.org/wiki/Plate_scale) of $\approx 0.014$ arcsec/micron. The same page gives the positioning accuracy as "5 microns RMS". So $5 \times 0.014 = 0.07$ arcsec.
Peter Erwin
CC BY-SA 4.0
2024-06-18T11:48:47.947Z
2024-06-18T11:48:47.947Z
Peter Erwin
<p>I'm using data from the Dark Energy Spectroscopic Instrument (DESI), and I need to check if its beam coincides with coordinates I've collected for objects in GALEX.</p>
<p>From the Fermilab conference proceeding Flaughera &amp; Bebekb (2014) &amp; DESI collaboration <a href="https://lss.fnal.gov/archive/2014/conf/fermilab-conf-14-235-ae.pdf" rel="nofollow noreferrer">The Dark Energy Spectroscopic Instrument (DESI)</a> (Section3):</p>
<blockquote>
<p>The DESI instrument is designed to meet the key science goals within the operational requirements. The primary
drivers of the design are:</p>
<ul>
<li>[...]</li>
<li>Spectroscopic resolution for redshift error (precision and accuracy) &lt; 0.0005(1 + z): &gt;1500 for λ &gt;360 nm and longer, &gt;3000 for λ &gt;555 nm, and &gt;4000 from λ &gt;656 nm.</li>
<li>The three arms cover wavelength range from 360 nm to 980 nm with resolution R = λ/∆λ = 2000–5500 depending on wavelength.</li>
<li>[...]</li>
</ul>
</blockquote>
<p>There are no units on the 1500, 2000, 4000 or 5500, so I'm not sure what all this means. Can anyone help me parse these statements?</p>
<p>Where else can I find information on DESI's spatial resolution?</p>

CC BY-SA 4.0
2024-06-16T22:25:31.073Z
I think someone needs to answer about exactly how to parse the line about redshift. But I think that "Fiber density ∼700 per square degree." gives some spatial resolution information as well (if we consider space having three dimensions)
uhoh
<spectroscopy><space-telescope><angular-resolution>
What is DESI's spatial resolution?
Jim421616
# What is DESI's spatial resolution?

(Asked by: username_0 on 2024-06-16)

I'm using data from the Dark Energy Spectroscopic Instrument (DESI), and I need to check if its beam coincides with coordinates I've collected for objects in GALEX.

From the Fermilab conference proceeding Flaughera & Bebekb (2014) & DESI collaboration [The Dark Energy Spectroscopic Instrument (DESI)](https://lss.fnal.gov/archive/2014/conf/fermilab-conf-14-235-ae.pdf) (Section3):

> The DESI instrument is designed to meet the key science goals within the operational requirements. The primary
> drivers of the design are:
>
> * [...]
> * Spectroscopic resolution for redshift error (precision and accuracy) < 0.0005(1 + z): >1500 for λ >360 nm and longer, >3000 for λ >555 nm, and >4000 from λ >656 nm.
> * The three arms cover wavelength range from 360 nm to 980 nm with resolution R = λ/∆λ = 2000–5500 depending on wavelength.
> * [...]

There are no units on the 1500, 2000, 4000 or 5500, so I'm not sure what all this means. Can anyone help me parse these statements?

Where else can I find information on DESI's spatial resolution?

**Tags:** spectroscopy, space-telescope, angular-resolution
> **Comments:**
> **username_1**: I think someone needs to answer about exactly how to parse the line about redshift. But I think that "Fiber density ∼700 per square degree." gives some spatial resolution information as well (if we consider space having three dimensions)  

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## 1 answers

### Answer by username_2 on 2024-06-18. Score: 3 | Accepted answer

It's not entirely clear to me what you want to know -- you say "spatial resolution" in the title, but then say, "I need to check if its beam coincides with coordinates I've collected for objects in GALEX", which implies you're more interested in *positional accuracy*.

In either case, [this link](https://noirlab.edu/public/media/archives/brochures/pdf/brochure007.pdf) probably provides what you want to know. The fiber positioning accuracy is given as 5 microns, which translates to $\approx 0.07$ arcsec -- that's the positional accuracy -- while the *fiber diameter* is 1.5 arcsec in angular terms. The latter is effectively the spatial (or angular) resolution: everything within the diameter of the fiber is mixed together to form the spectrum.

The numbers you quote from "Flaughera & Bebekb (2014)" are [*spectral* resolutions](https://en.wikipedia.org/wiki/Spectral_resolution) (the Wiki page I linked to calls this the "resolving power"). They have no units because they are defined as $\lambda / \Delta \lambda$, which has units of wavelength / wavelength, which reduces to "no units".

> **Comments:**
> **username_0**: Can you please clarify your phrase "5 microns, which translates to ≈0.07  arcsec"? How does this translate?  
> **username_2**: @Jim421616 Page 4 of the PDF file I linked to lists the "focal-plane scale" of DESI as "70.8 microns/arcsec (average over FOV)"; this inverts to give a [plate scale](https://en.wikipedia.org/wiki/Plate_scale) of $\approx 0.014$ arcsec/micron. The same page gives the positioning accuracy as "5 microns RMS". So $5 \times 0.014 = 0.07$ arcsec.  

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