blog

By: Aaron Meisner

It’s remarkable to think that our DESI Legacy Surveys team completed on order a thousand nights of ground-based observing from Kitt Peak and Cerro Tololo. All the while in low-Earth orbit, NASA’s WISE satellite has been steadily and reliably amassing nearly a decade of full-sky data at infrared wavelengths of 3-5 microns. WISE continuously obtains a new pair of degree-sized images every ~10 seconds, observing around the clock.

Selection of DESI’s luminous red galaxy and quasar targets requires not only optical data from telescopes like the Mayall and Blanco, but also infrared fluxes from WISE. It’s therefore crucial that DESI target selection make full use of the entire WISE data set. Once each year, we download millions of recently acquired raw WISE images to NERSC and use these to update DESI’s custom, coadded WISE maps. As of DR9, the raw WISE data set assembled at NERSC has grown to a quarter petabyte in size! Each year, upon completion of our latest WISE map-making efforts, we can once again declare that DESI has created the deepest ever full-sky maps and catalogs at mid-infrared wavelengths. DR9 incorporates seven years of WISE observations, versus five years for DR8 and just one year for DR1.

WISE has scanned the entire sky more than a dozen times, lending a strong time-domain component to the Legacy Surveys data products. Our Legacy Surveys WISE light curves for ~2 billion sources represent a totally unprecedented and as-yet little explored data set. Mining DR9’s infrared data products, especially in combination with optical Legacy Surveys photometry and DESI spectroscopy, will provide a diverse array of scientific opportunities throughout the coming years.

 

By: Adam Myers

The Dark Energy Spectroscopic Instrument will conduct spectroscopy of truly vast numbers of cosmological and astrophysical sources. These include Bright Galaxies, Emission Line Galaxies (ELGs), Luminous Red Galaxies (LRGs), Quasars, and objects in our own Milky Way Galaxy. DESI spectra are obtained by aligning optical fibers with locations on the sky, to collect light to be analyzed by dedicated spectrographs. But, how do DESI scientists know where to place those optical fibers in the first place?

Sources for the DESI key projects are targeted using images of the sky from the DESI Legacy Imaging Surveys. The Legacy Surveys include optical photometry from dedicated campaigns with the Mayall and Bok telescopes at Kitt Peak National Observatory, near Tucson, and the Blanco telescope at Cerro Tololo Inter-American Observatory near La Serena in Chile. The Legacy Surveys also incorporate infrared imaging from the WISE and NEOWISE missions, and source detections from the Gaia survey.

When envisioning the process of finding distinct objects in the sky, it is tempting to picture a bright, extended galaxy, such as this one:

But, in truth, the vast majority of DESI targets are far less spectacular to the eye, and are selected based on properties such as their color in addition to their shape. Below is the same image from the Legacy Survey Viewer above with the targets identified with circles.  You’ll see that there are many more DESI targets in this field than you might have naively expected!

To determine which of the one-and-a-half-billion or so sources in the Legacy Surveys will be the lucky few tens-of-millions targeted by DESI requires sophisticated computer algorithms to sift through sources and target objects with specific photometric properties. The publicly available software that DESI uses, which is called desitarget, comprises tens-of-thousands of lines of code and has received contributions from dozens of DESI scientists.

The DESI collaboration recently released a series of research notes detailing the currently expected targeting algorithms for the DESI five-year survey:

• Bright Galaxies (Ruiz-Macias et al.)
• Luminous Red Galaxies (Zhou et al.)
• Emission Line Galaxies (Raichoor et al.)
• Quasars (Yèche et al.)
• Milky Way Sources (Allende Prieto et al.)

The target catalogs that correspond to these notes, which are drawn from Data Release 8 of the DESI Legacy Imaging Surveys, are publicly available here in a format described here.

Although it is a significant milestone to have the first official DESI target catalogs in-hand, the dedicated effort of the collaboration continues. The next data release of the Legacy Surveys (Data Release 9) will soon be used to optimize, refine and finalize the target catalogs for the DESI five-year survey, during a phase of the project known as Survey Validation.

By: Daniel Eisenstein

DESI commissioning has raced forward this winter, and we have now demonstrated the key performance parameters of the instrument! Since installation, refinement of the performance of the 8 square degree corrector, high-precision (10 micron) positioning of the fibers under active feedback, accurate calibration of the spectrographs, and on-sky commissioning of the whole user interface have been demonstrated.

All of this progress culminated in the successful demonstration in March of spectroscopy with the full DESI system of many tens of thousands of survey targets.  We have observed spectra of faint galaxies and quasars with redshift distributions and spectroscopic signal-to-noise that match well to what we expected.

Unfortunately, as it is with so many around the world, the COVID-19 outbreak is forcing us to adjust our plans.  We’re taking a break from on-sky observing until it is easier for our collaboration members to travel safely to Arizona.  But we’re fortunate that this winter’s commissioning produced so much data that we can work on it, in the meantime.  DESI will be back, we hope soon, with continued momentum toward our next goal of validating the survey design.