Welcome to the STEREO Learning Center

Image artifacts - Telescope and camera defects

Some artifacts are caused by defects in the cameras or optics used to take the images. The most noticeable example of a camera defect is shown below. This is a small fiber on the surface of the Charge-Coupled Device (CCD) camera for the COR2 telescope on STEREO Ahead. The total length of the fiber is just 1 millimeter. It has been there since launch. Most likely this fiber came off of one of the clean room wipes used during the assembly of the camera. Even though the material of the wipes is selected to shed as few particles as possible, the occasional particle does end up inside the instrument.

Close up image of a small fiber on the surface of the CCD camera for COR2 on STEREO Ahead. Click on the image to see the location on the detector.

The position of this fiber on the detector is fixed, and always shows up in the same position in the raw images. However, the processed images on the website include a correction for the spacecraft roll angle so that solar north always points straight up. This causes the position of the fiber to change over the course of the year, or whenever there's a spacecraft roll maneuver.

There are also several much smaller dust particles on the surface of the various detectors. The image below shows a typical dust particle on the EUVI detector on STEREO Behind, in this case just above the Sun's north pole.

Small dust particle on the surface of the EUVI detector on STEREO Behind.

The movie below shows how spacecraft jitter affects dust particle images, this time for the EUVI detector on STEREO Ahead. The left side of the movie shows the images just as they appear on the detector, before any corrections are applied. In these images, the triangular dust particle in the center does not move, because it is adhered to the detector, but the solar images show significant jitter. On the right side of the movie, the images have been corrected for spacecraft jitter, causing the dust particle to appear to be moving around, while the solar images remain steady.

Frame:   Speed: (frames/sec)

Light leaks inside the EUVI telescope optics can also affect the images. EUVI uses thin metal films to filter out direct sunlight and pass through only the wavelengths of interest. Small holes can develop in these filters, letting stray light through the system. To get around this problem, two sets of filters are used, one at the front of the telescope, and another at the back, to reduce the effect of light leaks. However, some faint leaks are still visible in the images, as shown below for the 284 Angstrom bandpass on STEREO Behind.

Streaks caused by light leaks in the thin film filters for the 284 Angstrom bandpass of EUVI on STEREO Behind. The contrast in the image have been stretched to bring out the faint streaks.

See also the discussion about the COR1 field lens artifacts in the background subtraction section.

Back to image artifacts page.