Select>color range perhaps with a fuzziness of zero?
Or non-contiguos magic wand tolerance 1, no antialias?
Then desaturate? Or fill with black (assuming surrounding color is black)
Hmm…
John…
It sounds like Jack is taking the right tack with the "20-30 second-Lens-Cap-on-exposure-then-create-a-mask" technique. Your suggestion seems a bit too broad, and wouldn’t offer enough fine control to successfully isolate the CCD noise.
Jack…
Astrophotography is something I’m interested in, but wholly unfamiliar with. Please make it a point to visit here when you can to report on the things you learn and the techniques you use.
Pardon my ignorance but what is a lens-cap-on exposure?
Logic would seem to dictate that the lens cap would prevent any exposure at all….
I think you are doing two different things or I’m not quite following this? -The lens cap on will show you "stuck" pixels…white like snow and they do not vary from image to image as it is like a sensor pattern of blank pixels. I think you have to clean something to fix it.
-Hot pixels, such as when a Nikon contains red pixels in a navy sky at high resolution…sometimes caused by low light, sometimes by a hot day or running your camera continuously, etc. I think this noise is picked up by maxing the saturation level by way of Hue/Sat. and then you make a selection.
Which is being discussed here? Pls. clarify for me as it is of interest.
To John Slate & Terrat:
As I understand it, the camera’s CCD, when taking long exposures, gets hot from the current that runs through it. This results in ‘hot spots’ or noise in the resulting file. By taking an exposure with the lens cap on, the resulting file contains ALL the offending noise and no other data in the file. There is a technique that I’m trying to understand, in which this file can create a mask that can be used to subtract the noise from a long exposure of a genuine subject. The Idea is that the ‘hot spots’ will be in the same location in the array for all shots at that exposure time.
I hope this clarifies my request for you.
Jack
Yes I see. Interesting.
So you would take the lens-cap-on image, perhaps increase the contrast so that the lightest pixel is white and the darkest is black, then copy/paste this into an alpha channel of an image you want to correct, then load the alpha channel as a selection.
Once the selection is made, what you do inside the selection depends entirely on what needs to be done… fill with black?… neutralize? What ever it is, it would be global to the selected pixels.
Edit:
here’s an idea: dupe your background layer, load the selection of the alpha channel, and then turn it into a layer mask of your duped layer… then unlink the layer mask from the image in this new layer, select the image portion of the layer, and nudge it to the side. The result being that all the hot pixels get replaced with the pixels that are adjacent to them.
Hi Jack,
you wrote:
"As I understand it, the camera’s CCD, when taking long exposures, gets hot from the current that runs through it. This results in ‘hot spots’ or noise in the resulting file. By taking an exposure with the lens cap on, the resulting file contains ALL the offending noise and no other data in the file. There is a technique that I’m trying to understand, in which this file can create a mask that can be used to subtract the noise from a long exposure of a genuine subject. The Idea is that the ‘hot spots’ will be in the same location in the array for all shots at that exposure time. "
Your understanding is correct, but the temperature of the CCD is crucial. The very best way to do this would be to record two images at the exact same exposure (same ISO, AND same length of exposure…this will necessitate manual, shutter priority or aperture priority modes…as program mode will change the exposure based upon a complete lack of light with the lens cap affixed). The first shot, with the lens cap on would approximate the pattern of "hot pixels", the second, with the cap off, would later benefit from the initial mask that you made. Your lens capped shot would give you a rough "mask" to work with when editing your actual image later (there are a number of different ways that one could do this within PS). Please let me know the Photoshop workflow that you or others employ the mask in, I’d be interested to see what works best.
Thanks,
-Bryan ()
