In this document we answer some frequently asked questions about our recent M57 image.
Q. Why are the stars somewhat elongated?A. The LAICA camera has a special arrangement of CCD image sensors. It has four CCDs separated by gaps of nearly the same size as the sensors themselves, as
shown by this image. This yields a completely contiguous field coverage by pointing the telescope at four different locations. In this way the camera can cover one square degree without any gap. The drawback of this configuration is that each CCD is just at one border of the telescope's field of view, where the stars are not perfectly circular.
Q. Why have you left some bloomings uncorrected?A. We leave a blooming in the image when data integrity are compromised. We haven't done any cosmetic repairs that can compromise integrity of visible image structures, or that require guessing unavailable data to the point of arbitrary creation of nonexistent image structures.
Q. Why does the center of the nebula seem sharper than the rest of the image?A. When you have very high signal levels in the image, you can reach a point where you can apply some delicate image restoration algorithms. In this case, deconvolution has been applied to the higher signal areas of the image exclusively (excluding the stars to avoid the Gibbs effect, or
ringing artifacts). Leaving the image without the deconvolution step would limit the documentary value of this photo.
Q. How can I understand the colors of this image?A. In the nebula, each hue represents a different chemical composition. The teal color is indicative of presence of doubly-ionized Oxygen. The violet color reveals the presence of the combined Hydrogen alpha and beta emission lines. Finally, the red color shows the molecular Hydrogen (H2) infrared emission.
Take into account that the inner nebula is extremely bright compared to the outer halo. In this photo, the inner nebula has high luminance values; this means that the redder emission components are perceived as pink. But if you measure chrominance values on these areas, you will see that the red hues are preserved. The whole image wouldn't be coherent at all with low luminance values in the inner nebula.
Q. Comparing this image with the corresponding image by the Hubble Space Telescope (HST), it seems that you have some false details. Are they really artifacts?A. The HST image and our image have been done with different filters and processing techniques. When we processed this image, we took special care at enhancing Hydrogen structures. Part of the Hydrogen data were transferred to the luminance, taking care to preserve pixel hues. If you
compare the red channel from both pictures, you will see there aren't artifacts at all.
