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Consider images such as those in https://hubblesite.org/contents/news-releases/2021/news-2021-007, which show bright supernovae embedded in their host galaxies. No matter which telescope and which detector were used to capture the image, the supernova invariably appears as a comparatively large bright disk. Foreground stars also form disks if they are bright enough, but very much smaller ones.

Which effects exactly cause this effect? Is charge bleed-over between pixels really still an issue with modern detectors? Or is this due to imperfections in the optical components?

And, a related question, is the apparent size of the bright object's disk strictly proportional to brightness?

enter image description here

AUTHOR: NASA, ESA, Ryan Foley (UC Santa Cruz)

IMAGE PROCESSING: Joseph DePasquale (STScI)

Public domain

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  • $\begingroup$ Does the disc size in the image differ from the reported size of the supernova? Have you checked whether there's a cloud of gas/dust reflecting the optical-band output of the supernova proper? $\endgroup$ Oct 25, 2021 at 14:45
  • $\begingroup$ @CarlWitthoft This is clearly due to the point-spread function for unresolved sources (the supernova is unresoved, and not nearly enough time has passed for any significant reflection by the local environment). $\endgroup$ Oct 25, 2021 at 15:14
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    $\begingroup$ The image info in the link says (trimming for size) "These images are a composite of separate archival exposures acquired by the WFPC2 instrument on (HST) and more recent ground-based optical data from the Lick Observatory. Several filters were used to sample narrow wavelength ranges. In this case, the assigned colors are: Blue: LCO/B, Green: LCO/V, Orange: LCO/r, Red: LCO/i, Luminosity: HST/F606W". So if the recent ground-based SN image has been resampled onto the higher resolution HST images, its larger pixels and atmosphere-affected image will appear as a larger disk. $\endgroup$ Oct 25, 2021 at 16:40

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