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Where is the hot pixel filter (salt&pepper) ?
hibou from France  [30 posts]
10 years
CCD's show "dark current" noise with longer exposures when warm, which shows up as isolated bright "hot pixels". In low light conditions, these isolated bright pixels on a dark background must be removed.

Which is the appropriate filter for this? I looked at the "Min" filter, which does help, but according to the documentation, it smears out dark areas, which is not exactly what is needed. A "hot pixel' filter should search for isolated single (or double?) pixels that are significantly brighter than the mean of their surroundings, and set them to this mean. This would not blur the image. A feature like that is called "salt&pepper" filter in Paint-Shop-Pro-X, but I like "hot pixel" filter :-)
Where is the hot pixel filter (salt&pepper) ?
hibou from France  [30 posts]
10 years
Here are a couple of examples of hot pixel background patterns from long exposures with completely different cameras, an EMCCD and an interline CCD. I know that you could address this problem by subtracting a "dark field" image, but a "hot pixel" filter would be useful for a quick in-line fix.


Anonymous 10 years
Alan,

We've got that filter ready to go but are missing an example image. Do you happen to have an image with some larger stars in them that are more visible that the ones you included that have "hot" noise? The reason we ask is that the example needs to clearly show what is a star and what is just noise ... thus, having some larger stars (ie. about 5 pixels wide) will make for a better example on how to use this filter.

Thanks,
STeven.
hibou from France  [30 posts] 10 years
Thanks STeven. Actually I am not looking at stars, but at much more diffuse blobs in a neutron diffraction pattern, so that will make it easier for my case. The previous images were both background only i.e. all of the sparkles are noise.

[image1] is a diffraction pattern (blobs) with lots of dark current noise (speckles).
[image2] is the same salt&pepper filtered on 1 pixel noise by PaintShopPro-X
[image3] is the same salt&pepper filtered on 4 pixel noise

So already a single pixel "hot pixel" filter does a pretty good job, but in my case I could use a larger filter. What would be nice would be to be able to choose the size of the filter as with PSP-X, since people looking at stars, which is common using this type of camera, will have different filter size requirements.

Alan H., Grenoble.
Where is the hot pixel filter (salt&pepper) ?
hibou from France  [30 posts]
10 years
Sorry, I hit the wrong button and posted my "Hot pixel" GIFs under Unknown Title. Here they are again for those following this thread. (Maybe the duplicate post could be removed).


Anonymous 10 years
Alan,

The Salt & Pepper filter was just launched. Check ver 1.8.6.0

Note that it has not yet been announced on the homepage but is available and documented.

Let us know if this suits your needs or suggest any needed tweaks.

Thanks for your help on this!
STeven.
Where is the hot pixel filter (salt&pepper) ?
hibou from France  [30 posts]
10 years
That was quick work STeven. Thanks. Your documentation is also very clear and precise. A couple of comments. First a trivial one - the "down" pixel size control stops at 3 not 1.

I have compared this filter with the PSP-X equivalent, and they seem very similar except that perhaps a little less blurring occurs with larger pixel filters in PSP, which also produces a slightly cleaner result, especially if the "include all lower speck sizes" is checked, and/or the "aggressive" (?) PSP-X option is checked. Looking at the PSP doc. I don't know what "aggressive" corresponds to here, but "include all lower speck sizes" seems like a good option. The PSP-X filter is quite a lot slower too - whatever it is doing it takes a couple of seconds.

PSP also says that speck sizes must be odd numbers, but it does allow even numbers, apparently silently adding 1. (I don't understand the reason for odd numbers). So my "4pix" filter was really "5pix". PSP's sensitivity scale is 1-10, and I used 5 for my examples.

This really is a very useful filter. It makes me wonder whether it is still really necessary to physically reduce noise by Peltier cooling the CCD. Thanks again.

Alan.

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