SDO is Almost Four Years Old and Our First Problems Arise

SDO is now approaching four years old and we are seeing things that we have to figure out how to handle. Today I'll write about the AIA 4500 Å images.

The AIA 4500 Å images have been removed from the SDO web page. Several streaks have appeared in the images (you can see some on the solar disk between about 9 and 10 o'clock.) We have decided to stop serving those images and recommend that people use the HMI continuum images.

There are four channels in the 4500 Å telescope, 4500 Å, 1600 Å, 1700 Å, and 171 Å. A series of filters is rotated in front of the CCD to measure the light in each channel. About a year ago a small pinhole appeared in the front filter of the 171 Å side of the filters. This is a thin foil filter that keeps the visible light out of the EUV images. The pinhole allows visible light into the telescope. Although the 171 Å has a second filter close to the CCD to keep out the visible light, the 4500 Å cannot as it is designed to look at the visible light.

There is a selector that allows light going through either the foil filter with the pinhole or the other front window. That selector must be rotated between those windows between exposures. You see strange streaks on the 4500 Å images because the selector and other filters are being moved while the 4500 Å is being read out. The light that comes through the pinhole is enough to create a signal on the CCD.

The 4500 Å images are still available through the JSOC.