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Originally Posted by RickS
I dropped an email to Richard Crisp with a couple of simple questions. Will let you know if I hear back from him. If anybody knows this stuff it will be him!
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Richard was kind enough to respond with his comments and also wrote a little article (link included below.)
I think this in combination with Ray's thoughts on seeing explain bright star bloat, at least to my satisfaction.
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Originally Posted by Richard Crisp (in email to RickS)
the primary source of the bloat is the fact that there's a Detection Limit of the electronic imaging system.
If you could plot the intensity of individual stars versus the distance from the centroid of the star you get a curve reminiscent of a Gaussian (bell) curve.
At the top of this curve you have a small width of the curve, at the bottom it is very wide
The brighter the star is, then the lower in this curve is the Detection Limit: sort of like the base of a tree trunk versus the tip of the highest straight branch if that analogy makes sense.
Certainly there can be other contributing factors such as light scattering caused by any of a number of different sources: external to the system can be turbulence in the air, dust and so on. Internal to the system dirty optics, internal reflections within the optical path including microlens interactions could in principle contribute to bloat
Finally it is possible to have an image sensor that has poor diffusion MTF at the wavelengths of interest.
A common problem is encountered when one attempts to image at NIR wavelengths with a sensor not designed specifically for NIR such as a Deep Depletion device as made by E2V.
Diffusion MTF is simply a measure of how efficiently all of the charge is collected in the target pixel versus being collected by adjacent pixels. For visible wavelengths and KAF or KAI sensors statistically only a handful of electrons wind up in the wrong pixel. If very many did compared to the total charge in a given pixel, then the image would appear smeared or out of focus. That's a real issue for KAF/KAI and NIR imaging but this shouldn't be any impact on star bloat when we image using NIR blocking filters.
Here's a link to a PDF tutorial on the subject of Star Bloat etc
http://www.narrowbandimaging.com/inc...ters_crisp.pdf |