M3: Inconstant Star Cluster - Image Anomoly? (15 April 2007)

[FranksHobbies]

Please see the following APOD:

http://antwrp.gsfc.nasa.gov/apod/ap070415.html

I have seen this APOD posted before but never seen anyone comment about the anomoly or image artifact or...?? affecting the star to the left of and below center. As the movie loops there a dark figure appears within the disk of the star and in the next frame it is not here but a smaller distortion appears within the star's limb. I'm not saying that this series of images captures a planet occulting it's star but if it is something else I'd be happy to know what. Sunspot? Has one ever been imaged on another star?

Note that a similar anomoly appears in the lower left corner of the FOV where two dark spots on different stars blink in and out.

Thanks in advance for your responses.

[emc]

Hi FranksHobbies,
I am relatively new to APOD and know little about what I am doing here except to enjoy the incredible images and mysteries I am discovering and try and learn from others that may understand these images. Thanks for bringing this one to light... very interesting!

[FranksHobbies]

I contacted Joel Hartman and with his kind permission I am posting his explanantion:

<<Hi Bill,

Thanks for your interest in the animation. The anomoly that you refer to is in fact an image artifact. It's caused by the way the images are matched before combining them into an animation. Because the images were taken at different times, through different sky conditions, and are also composited from multiple filters, you have to first match the seeing (how much the stars are blurred by the atmosphere) between the images and the light levels. When you match the seeing, stars that are saturated (they're so bright that their light fills the pixel wells on the camera) get little black rings around them and little black spots inside. These rings will change from image to image, but since it's not a physical variation, I tried to mask them out - otherwise they're quite distracting and take away from the real variations in the non-saturated stars that are interesting. Unfortunately, I missed a few of them - and so you see the little spots show up. I suppose that should probably be removed with photoshop or something.>>

[iamlucky13]

Good job contacting him for the explanation, and great eye for catching it in the first place. I figured it must be an image artifact from how sharply defined in the pixel it was.

I also see now one frame with quite a few such spots in the center of the cluster.