From GEMINI: “The Deepest Ground-based Photometry in a Crowded Field”


Gemini Observatory
Gemini Observatory

September 30, 2015

Expecting to resolve stars deep into the crowded field of a globular cluster is a tall order for ground-based telescopes.

The Messier 80 globular cluster in the constellation Scorpius is located about 30,000 light-years from the Sun and contains hundreds of thousands of stars.[1]

However, Paolo Turri (University of Victoria, Canada) and colleagues have used the Gemini Multi-conjugate adaptive optics System (GeMS) with the Gemini South Adaptive Optics Imager (GSAOI) to do just that.
Left: (Ks, F606W-Ks) color–magnitude diagram of NGC 1851; the detail of the double SGB is shown in the inset. Right: same as the left panel with average photometric (random) uncertainties indicated. Overlaid is the fiducial line with the approximate locations of the main sequence turnoff and main sequence knee highlighted by red dots.

Gemini GeMS

Gemini GSAOI
Gemini GSAOI instrument
GSAOI and instrument

Their data present the most accurate and deepest near-infrared photometry from the ground of a crowded field. It also illustrates the remarkable potential of MCAO-equipped Extremely Large Telescopes of the future.

Their Ks measurements of the Galactic globular cluster NGC 1851 are combined with HST photometry and the resulting color-magnitude diagram demonstrates that the ground-based data is of an unprecedented depth and precision for crowded field observations.

A Ultraviolet image of NGC 1851.

NASA Galex telescope

NASA Hubble Telescope

The delivered image quality approaches Gemini’s diffraction limit, with an average measured full-width at half-maximum (FHWM) of 0.09 arcsecond. The work is published in The Astrophysical Journal Letters.


The Extremely Large Telescopes currently under construction have a collecting area that is an order of magnitude larger than the present largest optical telescopes. For seeing-limited observations the performance will scale as the collecting area, but with the successful use of adaptive optics (AO), for many applications it will scale as D4 (where D is the diameter of the primary mirror).

ESO E-ELT Interior
ESO/E-ELT 39 meter telescope

TMT Schematic
UCO/Caltech Thirty Meter Telescope (TMT)

TMT Schematic

Giant Magellan Telescope
Giant Magellan Interior
21 meter Giant Magellan Telescope at Las Campanas, Chile.

Central to the success of the ELTs, therefore, is the successful use of multi-conjugate adaptive optics (MCAO) which applies a high degree of correction over a field of view larger than the few arcseconds that limits classical AO systems. In this Letter, we report on the analysis of crowded field images taken on the central region of the galactic globular cluster NGC 1851 in the Ks band using the Gemini Multi-conjugate Adaptive Optics System (GeMS) at the Gemini South Telescope, the only science-grade MCAO system in operation. We use this cluster as a benchmark to verify the ability to achieve precise near-infrared photometry by presenting the deepest Ks photometry in crowded fields ever obtained from the ground. We construct a color–magnitude diagram in combination with the F606W band from the Hubble Space Telescope/Advanced Camera for Surveys [ACS].


As well as detecting the “knee” in the lower main sequence at Ks ‘20.5, we also detect the double subgiant branch of NGC 1851, which demonstrates the high photometric accuracy of GeMS in crowded fields.

1.The Hubble Heritage team (1999-07-01). Hubble Images a Swarm of Ancient Stars. HubbleSite News Desk (Space Telescope Science Institute). Retrieved 2006-05-26.

Hubble Images a Swarm of Ancient Stars
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 15 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.

See the full article here .

Please help promote STEM in your local schools.


Stem Education Coalition

Gemini North
Gemini North, Hawai’i

Gemini South
Gemini South, Chile

Gemini’s mission is to advance our knowledge of the Universe by providing the international Gemini Community with forefront access to the entire sky.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai’i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in six partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country’s contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, and the Brazilian Ministério da Ciência, Tecnologia e Inovação. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.