<p>Consider images such as those in <a href="https://hubblesite.org/contents/news-releases/2021/news-2021-007" rel="nofollow noreferrer">https://hubblesite.org/contents/news-releases/2021/news-2021-007</a>, 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.</p>
<p>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?</p>
<p>And, a related question, is the apparent size of the bright object's disk strictly proportional to brightness?</p>
<p><a href="https://i.sstatic.net/vRwPA.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/vRwPA.jpg" alt="enter image description here" /></a></p>
<blockquote>
<p>AUTHOR: NASA, ESA, Ryan Foley (UC Santa Cruz)</p>
<p>IMAGE PROCESSING: Joseph DePasquale (STScI)</p>
<p>Public domain</p>
</blockquote>

CC BY-SA 4.0
2021-10-25T14:45:57.953Z
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?
Carl Witthoft
CC BY-SA 4.0
2021-10-25T15:14:00.753Z
@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).
Peter Erwin
CC BY-SA 4.0
2021-10-25T16:40:08.210Z
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.
astrosnapper
<photography><hubble-telescope><instruments>
What exactly causes very bright sources, such as supernovae, to appear as large circles in photographs?
jstarek
# What exactly causes very bright sources, such as supernovae, to appear as large circles in photographs?

(Asked by: username_0 on 2021-10-25)

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](https://i.sstatic.net/vRwPA.jpg)](https://i.sstatic.net/vRwPA.jpg)

> AUTHOR: NASA, ESA, Ryan Foley (UC Santa Cruz)
>
> IMAGE PROCESSING: Joseph DePasquale (STScI)
>
> Public domain

**Tags:** photography, hubble-telescope, instruments
> **Comments:**
> **username_1**: 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?  
> **username_2**: @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).  
> **username_3**: 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.  

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## 0 answers

