John Photometry Tools

Gaia DR3 lookups for small-telescope observers: comparison stars with synthetic Johnson-Cousins UBVRI magnitudes, bright stars, 2MASS, the asteroids in your field, star-cluster ages, and the dust reddening towards any position. Free for non-commercial use, from this page or from your own programs.

An independent service; not affiliated with ESA or the Gaia collaboration.

What this service computes

Most columns are catalogue values passed through unchanged. These are calculated here, by the methods below, and are not part of the source catalogues:

• Asteroid positions are computed for your time and site: the Minor Planet Center's orbits are integrated with the gravity of the Sun, planets, Moon and Pluto (JPL DE440) and the Sun's relativity, then light time is applied. Checked against JPL Horizons for 13 asteroids (main belt, near-Earth including Phaethon and Apophis, a Trojan, Chiron) from 15 months before to 23 months after the orbit epoch: all within 0.1″. Predicted V uses the H, G system and is good to a few tenths of a magnitude at best.
• 2MASS counterparts of Gaia stars: each star is moved back to the 2MASS observation date with its proper motion, then the nearest 2MASS source within 1″ is taken.
• Johnson V and B−V of bright stars from Tycho BT, VT (ESA 1997 transformation, valid for −0.2 < BT−VT < 1.8); Hipparcos stars missing from Tycho-2 are moved from epoch 1991.25 to J2000.
• E(B−V) is interpolated from the SFD dust map; the S&F 2011 value is 0.86 × SFD.
• Star-cluster age, metallicity, distance and reddening are fitted here. Members are picked from Gaia DR3 proper motions and parallaxes (a cluster-plus-field mixture model that allows for each star's errors). MIST v1.2 isochrones (Gaia EDR3 bands and 2MASS Ks, used with the permission of A. Dotter) are then fitted to the members' BP−RP, G−Ks and G. The fit allows for unresolved binaries and stray stars, takes the distance from the cluster parallax as a prior, and uses the dust map as a prior or an upper limit on the reddening. [Fe/H] has a prior of ±0.15 dex around the Galactic open-cluster gradient at the cluster's distance from the Galactic centre. When your centre is on a cluster in the Hunt & Reffert (2023) catalogue, its proper motion, parallax and radius guide the membership and its own age and extinction are shown for comparison; elsewhere, a group of stars is accepted as a cluster only if it moves together to within 4 km/s, so a field without a cluster gets "no star cluster found". Tested on six well-studied clusters (M67, NGC 188, the Pleiades, Praesepe, NGC 752, NGC 6791): ages within about 20% for all but NGC 6791 (fitted too old), [Fe/H] within about 0.15 dex and E(B−V) within about 0.03 of published values. A fit takes up to a minute. Give a cluster name instead of a position to get the fit precomputed for the 4,004 Hunt & Reffert open clusters they class as reliable (3,205 could be fitted; 2,215 pass both checks: [Fe/H] not at the edge of the model grid, distance agreeing with the parallax), with their own values alongside. Against Hunt & Reffert, our distances agree to 2% (scatter 4%); our ages are older by 0.08 dex on average with a scatter of 0.36 dex, most of it in clusters younger than 30 Myr.

Using it from a program

Cone search, returning JSON, CSV or VOTable:

IVOA Simple Cone Search (for TOPCAT, Aladin and other Virtual Observatory tools; RA, DEC, SR in degrees):

Bright stars (Tycho-2 and Hipparcos, to about V = 12; radius up to 4°) and dust reddening towards a position:

2MASS near-infrared sources, and Gaia stars with their 2MASS counterpart:

Known asteroids in a field at a given time (UTC; optional observer site latitude, longitude, height in metres; radius up to 2°; at most 10 searches per minute):

Fit a star cluster (centre, field radius up to 60′ — about three times the cluster's half-member radius — and, in crowded fields, its approximate proper motion; one fit at a time, up to a minute each):

Look up the precomputed fit of a catalogued open cluster by name, or download the whole table:

Limits: radius up to 60 arcmin, 50000 rows per answer (nearest first), 60 requests per minute. Interactive API description: /docs. Coverage: checking…

What is in it

• Astrometry for every Gaia DR3 star brighter than G = 19.5: position, parallax, proper motion, G, BP−RP and RUWE.
• Synthetic photometry in Johnson-Cousins U, B, V, R, I for the ~220 million stars with Gaia XP spectra, with errors; ok_X = 1 marks magnitudes inside the range Gaia validated for that band.
• Positions are ICRS at epoch J2016.0; apply the proper motions for other epochs.
• Variability: Gaia DR3's class for the ~10 million stars it found variable (e.g. ECL eclipsing binary, RR, LPV, DSCT|GDOR|SXPHE), with periods for eclipsing binaries, RR Lyrae, Cepheids and long-period variables — useful for keeping variables out of comparison stars.
• Bright stars: Tycho-2 BT, VT with Johnson V and B−V, plus the Hipparcos stars Tycho-2 lacks (the very brightest), with Hipparcos parallaxes. Positions ICRS at J2000.
• Reddening: total Galactic E(B−V) along the line of sight from the SFD dust map and its 2011 recalibration. Nearby stars have less, and it is unreliable near the Galactic plane.
• 2MASS: J, H and Ks magnitudes with quality flags for the 471 million sources of the 2MASS All-Sky Point Source Catalog (positions at their 1997–2001 observation dates). Added to a Gaia search, each star gets the 2MASS source it matches after its proper motion is wound back.
• Asteroids: every known minor planet from the Minor Planet Center's orbit file, refreshed daily, moved to the requested time with the gravity of the Sun, planets and Moon (JPL DE440). Positions are astrometric (directly comparable with the star catalogues), with predicted V, motion in arcsec per hour and distances. Numbered and multi-opposition asteroids agree with JPL Horizons to well under an arcsecond; positions from one-opposition orbits are rougher. Comets are not included.