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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) (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?

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    $\begingroup$ 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) $\endgroup$
    – uhoh
    Commented Jun 16 at 22:25

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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 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 (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".

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  • $\begingroup$ Can you please clarify your phrase "5 microns, which translates to ≈0.07 arcsec"? How does this translate? $\endgroup$
    – Jim421616
    Commented Jul 10 at 0:46
  • $\begingroup$ @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 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. $\endgroup$
    – Peter Erwin
    Commented Jul 12 at 8:10

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