| --- |
| license: cc-by-4.0 |
| task_categories: |
| - image-to-image |
| tags: |
| - astronomy |
| - supernovae |
| - desi |
| - legacy-surveys |
| - cosmos |
| size_categories: |
| - 10K<n<100K |
| --- |
| |
| ## Description |
|
|
| Spectroscopic galaxy sample of the **COSMOS Deep Drilling Field**, the field the |
| Vera C. Rubin Observatory will observe with roughly twice the cadence of the |
| other LSST DDFs. Redshifts come from |
| [DESI Data Release 1](https://data.desi.lbl.gov/doc/releases/dr1/) and images are |
| multi-resolution cutouts of the **DESI Legacy Imaging Surveys DR10** served |
| through [hips2fits](https://alasky.cds.unistra.fr/hips-image-services/hips2fits) |
| by CDS. |
|
|
| It contains **66,373 galaxies**, one row each. Every cutout is centered on the |
| **galaxy itself**, so there is no augmentation and no `host_pos` label: the |
| galaxy sits at the center of every image by construction. This makes it the DESI |
| counterpart of |
| [h2f_ps1_pasquet](https://huggingface.co/datasets/PRISM-Astro/h2f_ps1_pasquet), |
| built with the same pipeline and the same conventions, but on a different field, |
| a different survey and a redshift range almost five times wider. |
|
|
| ## Sample selection |
|
|
| Galaxies were taken from `desi_dr1.zpix` inside a cone of radius **1.6 deg** |
| centred on RA 150.1060, Dec +2.2260 — the median pointing of the 41 DESI |
| tiles dedicated to COSMOS. That radius is the field of view of the Mayall with |
| DESI's corrector: 3.2 deg across, 8.04 deg², which is what a single DESI |
| tile covers. |
|
|
| An earlier version of this dataset used the 1.7481 deg circle matching LSSTCam's |
| 9.6 deg² field of view instead. That is slightly wider than DESI's own |
| pointing, and the outer annulus showed it: source density fell from ~9,000 to |
| ~2,900 per deg² beyond 1.5 deg, and what remained there was mostly filled in |
| by the wide main survey rather than the dedicated COSMOS observations. Cutting at |
| the DESI field of view removes 5,374 galaxies and leaves a spatially complete |
| sample. Only primary spectra with a reliable |
| redshift were kept (`zcat_primary`, `zwarn = 0`, `spectype = 'GALAXY'`), and each |
| was crossmatched to its nearest source in `ls_dr10.tractor_s` within 1.5 arcsec. |
|
|
| From the 83,620 rows that survived the crossmatch, three cuts were applied before |
| downloading any image: |
|
|
| - **4,498** galaxies whose `ls_id` appeared more than once, since several DESI |
| fibers can land on the same imaging source. The closest match was kept. |
| - **4,259** sources of Tractor type `PSF` or `DUP`, which carry `shape_r = 0` and |
| have no measurable morphology. |
| - **121** galaxies with a non-physical redshift, `z <= 0`. |
|
|
| A further **5,374** fall outside DESI's own field of view, as described above. |
|
|
| A further **2,995** were dropped after download because at least one pixel came |
| back as `0.0` in some band, marking a gap in HiPS coverage. Those gaps are real |
| holes in the survey rather than transient failures: re-requesting the same cutout |
| recovered none of the 447 cases that had a whole `(level, band)` plane empty. No |
| image in this dataset contains a single zero pixel. |
|
|
| ## Cross-validation |
|
|
| The `fold` column holds a 5-fold split **stratified by spectroscopic redshift**. |
| Redshift is discretized into 764 uniform bins using the same bin width as |
| `h2f_ps1_pasquet` (`0.4 / 180 = 0.00222`), extended to cover this field's much |
| wider redshift range. The 40 galaxies falling in bins with fewer than 5 members |
| carry `fold = 0` and belong to **training only**, since they cannot be |
| stratified. They sit almost entirely at the high-redshift tail, so validation |
| metrics say nothing about the `z > 1.5` regime even though the model sees those |
| examples. |
|
|
| ## Image format |
|
|
| `image` has shape `(66373, 5, 4, 30, 30)`, which maps to |
| `(examples, resolution levels, photometric bands, height, width)`. Bands are |
| **griz** in that order — four bands, not the five of the PanSTARRS |
| counterpart. Level `L` has a pixel scale of `0.262 * 2^L` arcsec/px, so level 0 |
| spans 7.86 arcsec and level 4 spans 125.76 arcsec around the galaxy. |
|
|
| ## Target group |
|
|
| `group` labels each galaxy as **BGS**, **LRG**, **ELG** or **NON**, following the |
| categorisation of Zhou et al. 2024 [3]: DESI's main-survey target selection is |
| re-applied to the photometry rather than read back from the target bitmasks. The |
| official `desitarget` implementations (`isBGS`, `isLRG`, `isELG`) are used with |
| `south=True`, since this field lies entirely in DECaLS; BGS is the union of its |
| `bright`, `faint` and `wise` sub-selections, and the 628 galaxies satisfying more |
| than one cut are assigned in the order BGS > LRG > ELG. |
|
|
| The distinction matters. The bitmasks record *why DESI observed a galaxy*, which |
| is undefined for the `special` programmes that make up roughly half of this field |
| — they leave `desi_target` at zero, so almost half the sample would be |
| unclassifiable. Re-applying the cuts asks instead *what the galaxy's colours make |
| it*, which is defined for every source. |
|
|
| | group | galaxies | median z | median r | |
| |---|---|---|---| |
| | BGS | 10,274 | 0.220 | 19.33 | |
| | LRG | 3,798 | 0.810 | 21.88 | |
| | ELG | 10,420 | 1.098 | 22.96 | |
| | NON | 41,881 | 0.505 | 21.52 | |
|
|
| `survey` is kept alongside it. Depth is far from uniform across this field and it |
| varies by campaign, not by position: the SV1 tiles reach a median exposure of |
| 13,955 s, against ~1,800–2,050 s for `sv3`, `special` and `main`. Cut on |
| `survey` rather than on sky position if a uniform-depth subsample is needed. |
|
|
| ## Ellipse convention |
|
|
| `ellipse_phi` is **not** the raw catalog angle. Legacy Surveys stores ellipticity |
| as a complex number, `e = (a-b)/(a+b) * exp(2i*phi)`, from which the position |
| angle is `0.5 * atan2(shape_e2, shape_e1)`. That angle is defined against the sky |
| axes, and drawing it on a pixel array requires a further 90 degree rotation, |
| verified against second-moment measurements of the cutouts themselves. This |
| column already carries that rotation, wrapped back into `[-90, 90)`, so it can be |
| fed directly to a profile generator in the same way `rSerPhi` is used in |
| `h2f_ps1_pasquet`. The raw components are kept in `shape_e1` and `shape_e2` for |
| anyone who needs the catalog convention. |
|
|
| Galaxies of type `REX` are round by construction: their ellipticity is fixed at |
| zero, so `ellipse_ab = 1` and `ellipse_phi = 0`. That zero is a convention, not a |
| measurement — a circle has no orientation. Use `type` to mask them. |
|
|
| ## Columns |
|
|
| <div style="font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,sans-serif;max-width:1000px;margin:0 auto;padding:16px 0"> |
| <table style="width:100%;border-collapse:collapse;font-size:13px"> |
| <thead><tr> |
| <th style="padding:10px 7px;text-align:left;font-weight:600;border-bottom:2px solid #4a6fb5;color:#4a6fb5;font-size:14px;width:18%">Column</th> |
| <th style="padding:10px 7px;text-align:left;font-weight:500;border-bottom:2px solid #4a6fb5;color:#4a6fb5;font-size:14px;width:20%">Type</th> |
| <th style="padding:10px 7px;text-align:left;font-weight:500;border-bottom:2px solid #4a6fb5;color:#4a6fb5;font-size:14px">Description</th> |
| </tr></thead> |
| <tbody> |
|
|
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Image</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>image</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>(5,4,30,30)</code> float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Multi-resolution Legacy Surveys DR10 <em>griz</em> cutout, centered on the galaxy.</td> |
| </tr> |
|
|
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Identification</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>ls_id</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">int64</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Legacy Surveys DR10 identifier of the galaxy, unique in this dataset. Packs <code>RELEASE</code>, <code>BRICKID</code> and <code>OBJID</code> into one integer.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>targetid</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">int64</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">DESI identifier of the spectrum the redshift comes from. The key back to <code>desi_dr1</code> tables and to SPARCL.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>sep_arcsec</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Angular separation of the DESI–Legacy Surveys crossmatch, in arcsec. Median 0.06, at most 1.496.</td> |
| </tr> |
| |
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Sky position</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>ra</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Right ascension of the galaxy, in degrees. This is the center of the cutout.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>dec</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Declination of the galaxy, in degrees. This is the center of the cutout.</td> |
| </tr> |
| |
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Redshift</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>z</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Spectroscopic redshift from DESI DR1, the regression target. Ranges from 0 to 1.697, with a median of 0.534.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>z_error</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Uncertainty reported by the DESI spectroscopic pipeline.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>z_class</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">int64</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Redshift bin, 0 to 763, of width 0.00222. Used to stratify the folds and as the classification target.</td> |
| </tr> |
| |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>group</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">string</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Target category from the photometric selection: <code>BGS</code>, <code>LRG</code>, <code>ELG</code> or <code>NON</code>. See the section above.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>survey</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">string</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">DESI campaign the spectrum comes from: <code>sv1</code>, <code>sv3</code>, <code>special</code> or <code>main</code>. Proxy for exposure depth.</td> |
| </tr> |
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Photometry</td></tr><tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>mag_g</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">AB magnitude in the g band, as observed.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>mag_r, mag_i, mag_z</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">The same for r, i and z.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>dered_mag_g</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">AB magnitude in g corrected for Galactic extinction.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>dered_mag_r, <code>_i</code>, <code>_z</code></code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">The same for r, i and z. A handful of sources have no magnitude where the Tractor flux is negative.</td> |
| </tr> |
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Extinction</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>ebv</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Reddening E(B−V) from the SFD dust map.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>mw_transmission_g</code>, <code>_r</code>, <code>_i</code>, <code>_z</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Galactic transmission per band, in linear units between 0 and 1. Multiply the pixels by the inverse to correct for extinction.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>extinction_g</code>, <code>_r</code>, <code>_i</code>, <code>_z</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">The same extinction expressed in magnitudes, <code>-2.5*log10(mw_transmission)</code>.</td> |
| </tr> |
| |
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Morphology — Tractor fit, all bands jointly</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>type</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">string</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Model the Tractor selected: <code>REX</code> round exponential, <code>EXP</code> exponential, <code>DEV</code> de Vaucouleurs, <code>SER</code> free Sérsic. All are Sérsic profiles; the label says how the index was treated.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>shape_r</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Half-light radius of the fitted model, in arcsec. Not band-specific: the Tractor fits one morphology across all bands.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>shape_e1</code>, <code>shape_e2</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Raw ellipticity components as the catalog stores them. Both exactly 0 for <code>REX</code>.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>ellipse_radius</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Semi-major axis of the fitted ellipse, in arcsec. Equal to <code>shape_r</code>; the minor axis is <code>ellipse_radius * ellipse_ab</code>.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>ellipse_ab</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Axis ratio b/a, between 0 and 1. Derived as <code>(1-|e|)/(1+|e|)</code> with <code>|e| = hypot(shape_e1, shape_e2)</code>.</td> |
| </tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>ellipse_phi</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">float32</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Position angle of the fitted ellipse, in degrees over <code>[-90, 90)</code>, already rotated to the pixel frame. See the ellipse convention section above.</td> |
| </tr> |
| |
| <tr><td colspan="3" style="padding:8px 12px;font-weight:600;color:#4a6fb5;border-bottom:1px solid rgba(74,111,181,0.25);background:rgba(74,111,181,0.12)">Cross-validation</td></tr> |
| <tr> |
| <td style="padding:7px 7px;padding-left:20px;border-bottom:1px solid rgba(128,128,128,0.15)"><code>fold</code></td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">int64</td> |
| <td style="padding:7px 7px;border-bottom:1px solid rgba(128,128,128,0.15)">Validation fold, 1 to 5, stratified by <code>z_class</code>. A value of 0 means the galaxy is used for training in every split.</td> |
| </tr> |
|
|
| </tbody> |
| </table> |
| </div> |
|
|
| ## References |
|
|
| [1] DESI Collaboration 2025, *Data Release 1 of the Dark Energy Spectroscopic |
| Instrument*. [arXiv:2503.14745](https://arxiv.org/abs/2503.14745) |
|
|
| [2] Dey, A., Schlegel, D. J., Lang, D., et al. 2019, *Overview of the DESI Legacy |
| Imaging Surveys*, The Astronomical Journal, 157, 168. |
| [doi:10.3847/1538-3881/ab089d](https://doi.org/10.3847/1538-3881/ab089d) |
|
|
| [3] Zhou, X., Li, N., Zou, H., et al. 2024, *Estimating Photometric Redshifts for |
| Galaxies from the DESI Legacy Imaging Surveys with Bayesian Neural Networks |
| Trained by DESI EDR*. [arXiv:2412.02390](https://arxiv.org/abs/2412.02390) |
|
|
| ```bibtex |
| @article{Dey_2019, |
| doi = {10.3847/1538-3881/ab089d}, |
| url = {https://doi.org/10.3847/1538-3881/ab089d}, |
| year = {2019}, |
| month = {apr}, |
| publisher = {American Astronomical Society}, |
| volume = {157}, |
| number = {5}, |
| pages = {168}, |
| author = {Dey, Arjun and Schlegel, David J. and Lang, Dustin and others}, |
| title = {Overview of the DESI Legacy Imaging Surveys}, |
| journal = {The Astronomical Journal}, |
| } |
| ``` |
|
|
