SPHEREx First Images

First light images from SPHEREx, captured March 27, 2025. These give a tantalizing first look at SPHEREx’s wide, multi-wavelength view of the sky. Each bright spot is a source, typically a star or a galaxy. Each image is expected to contain more than 100,000 detected sources. There are six images in every SPHEREx exposure — one for each detector. The top three images show the same area of sky as the bottom three images, spanning the observatory’s full 3.5 x 11.5 deg field of view. When the SPHEREx observatory begins routine science operations in April, it will take approximately 600 exposures every day.

To make the infrared images shown here, we assigned a visible color to each infrared wavelength captured by the observatory, ranging from 0.75 to 5.0 um, as indicated by the color bar. SPHEREx uses color filters set on top of the detectors, where the wavelengths change continuously from the top of the image to the bottom. The brightness stretch is chosen for visual effect in each of these uncalibrated images.

The bright curved band in the upper left image is 1.083 um line emission from Helium in Earth’s atmosphere. The same feature causes the bright band at the bottom of the upper middle image, due to overlapping wavelength coverage. One can also see a glint just below the Helium line due to a bright star landing on the detector edge. The brightest stars in the images show a black region in the center due to a known instrumental artifact, where pixels saturate before they can be read out. The distinctive crosses often seen in telescope images of bright stars are missing with SPHEREx. The crosses are caused by secondary supports used in conventional on-axis telescopes, but SPHEREx avoids this undesirable effect with its off-axis telescope design. The faint ‘zebra’ striping in the lower right image is due to varying optical response in the instrument. By zooming in on a small region of the image we can see the point spread function of the telescope, which shows the optics are in excellent focus.