Mars - 2022-10-22 09:07 UTC - Rind Effect in the Red Channel
Almost out of nowhere I recently began to experience a halo on the limb side of my Mars images that I now understand is called the Mars Rind Effect. I had not heard this term until I started reading about the ghost rings that began appearing in my Mars images.
In the red channel image posted here, the rind effect appears as an outer ring and inner ring on the limb, or the left side of Mars' disk. The outer diffraction ring is dimmer than Mars but brighter than space. It is seen around the outside of Mars' disk as a halo that starts at about 12 O'clock and proceeds in a counterclockwise direction and starts to diminish at about 9 0'clock, and is gone by about the 8 O'clock position. As this outer ring begins to diminish in brightness, the inner ring that is dimmer than Mars and traces the inside of the limb begins to appear and it continues to about 6 O'clock on the disk.
The root cause of the effect is the stark intensity difference between the bright surface of the planet against the darkness of space with almost no gradient in between. This sharp and higly-contasted difference interacts with the aperture ring and the secondary mirror of the telescope to create diffraction rings in the image. In short, anything that sharpens the transition from the darkness of space to the brightness of the planet will cause the rind effect to be more noticeable.
An interesting charicteristic of the effect is that it is more noticeable in the IR channel, and less so progressing through the R and G channels to the B channel. It is barely noticeable in unsharpened images coming out of Autostakkert, but wavelet sharpening in Registax really makes it stand out.
There is no diffraction ring on the terminator side, or the right side of Mars. This is because the transition from daylight to darkness on the surface of Mars is a gradient over a larger number of pixels. In a somewhat like manner, it turns out that Jupiter and Saturn are less prone to diffraction rings because of the limb darkening effect that creates a gradient of light reflecting off of cloud tops instead of a distinct surface.
It turns out that I may have started experiencing the rind effect as a result of a comedy of successes as I have worked to improve my planetary imaging techniques lately. These successes are better focus and better collimation. Switching to Chroma filters may also have been an improvement that made the effect more noticeable, and better seeing that favored me on the night of this capture could have also contributed. That I am finally seeing this effect in my images may be a high-quality problem!
Now that I have identified the cause, the question becomes what to do about it. There are various Photoshop and WINJUPOS remedies that can be found in Cloudy Nights, I see images captured near the same time as mine that appear to have successfully removed the rind effect. Some imagers may deem these remedies to be "unfair" alterations of the "true" image, and personally elect to leave the effect in the image.
I have not yet made a personal decision on how to handle the rind effect in my Mars images. More to come!
Mars - 2022-10-22 09:07 UTC - Rind Effect in the Red Channel
Almost out of nowhere I recently began to experience a halo on the limb side of my Mars images that I now understand is called the Mars Rind Effect. I had not heard this term until I started reading about the ghost rings that began appearing in my Mars images.
In the red channel image posted here, the rind effect appears as an outer ring and inner ring on the limb, or the left side of Mars' disk. The outer diffraction ring is dimmer than Mars but brighter than space. It is seen around the outside of Mars' disk as a halo that starts at about 12 O'clock and proceeds in a counterclockwise direction and starts to diminish at about 9 0'clock, and is gone by about the 8 O'clock position. As this outer ring begins to diminish in brightness, the inner ring that is dimmer than Mars and traces the inside of the limb begins to appear and it continues to about 6 O'clock on the disk.
The root cause of the effect is the stark intensity difference between the bright surface of the planet against the darkness of space with almost no gradient in between. This sharp and higly-contasted difference interacts with the aperture ring and the secondary mirror of the telescope to create diffraction rings in the image. In short, anything that sharpens the transition from the darkness of space to the brightness of the planet will cause the rind effect to be more noticeable.
An interesting charicteristic of the effect is that it is more noticeable in the IR channel, and less so progressing through the R and G channels to the B channel. It is barely noticeable in unsharpened images coming out of Autostakkert, but wavelet sharpening in Registax really makes it stand out.
There is no diffraction ring on the terminator side, or the right side of Mars. This is because the transition from daylight to darkness on the surface of Mars is a gradient over a larger number of pixels. In a somewhat like manner, it turns out that Jupiter and Saturn are less prone to diffraction rings because of the limb darkening effect that creates a gradient of light reflecting off of cloud tops instead of a distinct surface.
It turns out that I may have started experiencing the rind effect as a result of a comedy of successes as I have worked to improve my planetary imaging techniques lately. These successes are better focus and better collimation. Switching to Chroma filters may also have been an improvement that made the effect more noticeable, and better seeing that favored me on the night of this capture could have also contributed. That I am finally seeing this effect in my images may be a high-quality problem!
Now that I have identified the cause, the question becomes what to do about it. There are various Photoshop and WINJUPOS remedies that can be found in Cloudy Nights, I see images captured near the same time as mine that appear to have successfully removed the rind effect. Some imagers may deem these remedies to be "unfair" alterations of the "true" image, and personally elect to leave the effect in the image.
I have not yet made a personal decision on how to handle the rind effect in my Mars images. More to come!