Colored lines RGB in many images?

[stephen99]

Hi all. For some reason I am getting some RGB lines in my stacked images. I dont see them in the individual ones. Does not happen all the time. Im using the Atik 420 color with the hyperstar on the edge hd8 scope. I took some darks but that did not help. Any ideas how I can get rid of them? Could it be my dew heater? I attached a file to see.

Thanks

Steve

[NickK]

Are you dithering (ie moving the mount between images or have some tracking error)?

This will occur if hot pixels are not removed before stacking if the target is slowly moving across the image. When the alignment is done, it's done at a sub-pixel level so that the target in each image sits directly on top of each other. However noise in the form of hot pixels from the CCD do not, they will appear to move over the image.

Alternative way of removing these is to use Windsored Sigma Stacking. This rejects movement between aligned images being stacked.

[stephen99]

This is a bit strange. On my win7 32 bit computer. I use DSS to stack When I save it and go into PS the weird lines are there. I do this on my 64 bit win7 and nothing is there. I am using PHD for guiding and the movement is minimal. Oh and that Windsored Sigma Stacking. Dont know what that is Could you elaborate a bit?

Thanks
Steve

[NickK]

Hmm the difference between computers could be down to the display, the colour profile being used by the display (set in Windows), or the number of bits per pixel set for the desktop (set in Windows). Most LCD displays can only display 12 bit colour - i.e. 12bits for all colours (red through green through blue) whereas doing RGB with OSC will return more colours than the LCD panel can display. RGB Mono is 16bits for each colour, so LRGB would be 16+16+16+16 = 64 bits and way beyond the LCD! Note - not all colours are used, stretching the image attempts to fit the colours.

The sigma stacking is a way of stacking (after alignment) that analyses each frame in the stack and if there are differences (outside configured thresholds), it will reject the pixels.

I've attached an example of what Sigma stacking can do - below is a low light level photo, with a hand held cannon ixus IS80 that only saves jpegs. 10 images, aligned then sigma stacked (windsored) - though glass. You'll note the lack of reflection and the high signal to noise ratio.

[NickK]

Next up is what the sigma stacking rejected - I deliberately used the movement of the camera to allow the algorithm to detect (a) noise, (b) the reflection of the woman looking through the glass, and (c) the artefacts created by the jpeg compress from the Ixus camera.

The image here is jpeg'd to fit the 600KB limit but it will give you an idea of what the rejection can do:

[NickK]

Lastly, a native resolution crop of one of the coins to show the quality.

[NickK]

The technique I used was exactly the same sort you'd do for astrophotography - in this case capture, align (using features in the images) and stack.

In this case I've used PixInsight, but sigma stacking is becoming quite common in image processing packages.