3D anaglyph by John Wattie from the NASA STEREO mission original images.
The stereo pairs below show a similar "slinky" structure but again it was only on this 284 Angstrom wavelength and only at this time:
"Hi John,
Like Bob (Evans), I'd say the 'slinky' structures look very much like artefacts to me. That 90° arrangement is a bit of a give away that they'll be diffraction spikes. I see that the 'slinky' had faded by the time I took the (Ha) photo. Looking at the STEREO EUVI physical data on
http://secchi.lmsal.com/EUVI/5171_15.pdf
warning: took 'forever' to load on my dial-up. I see that the light first passes through 150nm thickness of foil to block most of the UV and all of the visual and IR light. See: The mesh supported filter avoids the absorbing polyamide layer, which is a major concern at longer wavelengths, especially for the somewhat weaker line at 28.4 nm. The fine mesh with 0.36 mm line spacing causes a noticeable amount of diffraction, especially in the presence of very bright solar features.
Looking at the primary mirror, it's interesting that it's divided into 4 sectors and each coating is different and optimized for one of the four wavelength bands the camera detects at 17.1, 19.5, 28.4 & 30.4 nm. So each band's image comes from only one of the mirror's sectors.
In summary, it's not surprising that it produces some pretty strange diffraction spikes, after looking at that front end filter arrangement.
andy "