The Resolution of the Human Eye

In an age of ever-improving technology, the question of the human eye’s resolution is more interesting than ever. Resolution is the ability to see two sources very close together, and is measured in arcminutes, with 60 arcminutes in a degree. The further away two objects are, the smaller the angle separating them—for example, the Hubble Space Telescope has a resolution of 0.0008 arcminutes, which means that if two stars are closer together than that, they appear as a single object. The resolution limit of the human eye varies from person to person, but if you had 20/20 vision your resolution would be 0.6 arcminutes. Our vision isn’t like an image, though; it’s more like a constant video stream, with our eyes moving rapidly and constantly updating the images we see, so the resolution of the human eye is far greater than any camera—it’s the equivalent of 576 megapixels. Potentially, a 576 megapixel image could fool our eyes into thinking it was the real thing, but only if you covered one eye—as soon as both eyes move, you’d be able to detect that the image was flat, not 3D. Intriguing stuff.

NASA’s SWIFT has made the largest ever ultraviolet image of the Andromeda Galaxy. The image shows a region 200,000 light-years wide and 100,000 light-years high (100 arcminutes by 50 arcminutes). Credit: NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP) [4412x1939]

This galaxy looks quite special. That’s because the bright central region of NGC 289 is only about 3 arcminute in diameter while the extensive and faint outer arms extend the the size of the galaxy out to 14 arcmin across. That’s 300,000 light years across and it’s pretty huge as it’s more than twice the size of our own galaxy Milky Way.

Image copyright: Mike Sidonio


Otherwise known as M31, the Andromeda Galaxy lies 2.5 million light years away in the Andromeda Constellation. It is the nearest spiral galaxy to our own Milky Way galaxy and is the largest galaxy within the Local Group. M31 is more than 220,000 light-years across and contains around one trillion stars.

Between May 25 and July 26 in 2008 the Ultraviolet/Optical Telescope aboard NASA’s Swift spacecraft captured 330 individual images of M31 at wavelengths of 192.8, 224.6, and 260 nanometers, with a total exposure time of 24 hours. The 85 gigabytes of images were merged into this mosaic, the highest-resolution image of M31 ever recorded in the ultraviolet. The region shown in the image is 200,000 light-years wide and 100,000 light-years high (100 arcminutes by 50 arcminutes). About 20,000 sources of light can be seen in the mosaic.

Within the mosaic, there is a clear difference between the galaxy’s spiral arms and its central bulge. The bulge is filled with older and cooler stars, making it appear smoother and redder. The spiral arms contain most of the dust and gas needed for the formation of new stars; clusters of hot, young and blue stars can be seen in these regions. The ring full of star clusters in the image is about 150,000 light years across. 

The intense star formation within Andromeda’s huge ring is thought to be aided by the small galaxies that orbit it. It is thought these galaxies cause tides, which boost the interactions within the gas clouds that cause the formation of new stars. 


Sources: 1, 2

Credit: NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP).
Optical: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

Beautiful barred spiral galaxy M109, 109th entry in Charles Messier’s famous catalog of bright Nebulae and Star Clusters, is found just below the Big Dipper’s bowl in the northern constellation Ursa Major. In telescopic views, its striking central bar gives the galaxy the appearance of the Greek letter “theta”, θ, a common mathematical symbol representing an angle. Of course M109 spans a very small angle in planet Earth’s sky, about 7 arcminutes or 0.12 degrees.

Chandra Deep Field South

Sky field within the wide field imager (WFI) image of the Chandra Deep Field South (CDF-S), reproduced at full (pixel) size to illustrate the exceptional information richness of these data. This subfield measures 6.8 x 7.8 square arcminutes (1717 x 1975 pixels).

Technical information: The very extensive data reduction and colour image processing needed to produce these images were performed by Mischa Schirmer and Thomas Erben at the “Wide Field Expertise Center” of the Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn (IAEF) in Germany. It was done by means of a software pipeline specialised for reduction of multiple CCD wide-field imaging camera data. This pipeline is mainly based on publicly available software modules and algorithms (EIS, FLIPS, LDAC, Terapix, Wifix).

The image was constructed from about 150 exposures in each of the following wavebands: B-band (centred at wavelength 456 nm; here rendered as blue, 15.8 hours total exposure time), V-band (540 nm; green, 15.6 hours) and R-band (652 nm; red, 17.8 hours). Only images taken under sufficiently good observing conditions (defined as seeing less than 1.1 arcsec) were included. In total, 450 images were assembled to produce this colour image, together with about as many calibration images (biases, darks and flats). More than 2 Terabyte (TB) of temporary files were produced during the extensive data reduction. Parallel processing of all data sets took about two weeks on a four-processor Sun Enterprise 450 workstation and a 1.8 GHz dual processor Linux PC. The final colour image was assembled in Adobe Photoshop. The observations were performed by ESO (GOODS, EIS) and the COMBO-17 collaboration in the period 1/1999-10/2002.

Credit: ESO