noise reduction phase

[guggle]

Hi Ivo,

I'm a member of the DSLR Astro Image Processing Yahoo group. If you're not familiar with it, they post unprocessed photos other members have taken, and challenge members to process them as they see fit. It's a great way of learning new processing skills!

The most recent challenge was NGC 6559 and I processed it in StarTools (see attached) and this is my workflow:

Autodev
Cropped: X1=58, X2=4518, Y1=411, Y2=3142
Bin 50%
Wipe - accept defaults
Develop: Digital Development=83.78&, all other defaults
HDR: Optimise Hard, all other defaults
Contrast: Compensate Gamma=Yes
Sharpen: Amount: 462%, Small detail bias:89%, Mask Fuzz: 17.2 (mask set around nebulosity)
Deconvolution: Inverse star mask, Radius=1.6 pixels
Life: Heavy preset, Glow Threshold=4%, Saturation=90%, Strength=100%
Color: Red Bias Reduce=1.03, Green Bias Reduce=1.03
De-Noise: Smoothness=84%, Brightness Detail Loss=22%, Color detail loss=22%

I received the following feedback from a member of the group and I would like to hear your thoughts on it please.

I'm not familiar with many of the processes in Star Tools, but I suspect that your Denoise may have been too aggressive. The nebulosity has an overly smooth look to it, so that's why I suspect the denoise process.

I'm not sure about what kind of masking capabilities StarTools has. However, one of the things that I would do in Photoshop would be to mask the noise reduction process using an inverted luminance mask. This makes it so that the noise reduction is concentrated on the background and faintest nebulosity. At the same time, it'll have minimal effect on the highlights (bright parts).

How would I go about doing what he says about creating an inverse luminance mask?

Any thoughts you could offer on his feedback plus my workflow would be greatly appreciated!

Cheers, Michael.

[admin]

Hi Michael,

Apologies for the tardy reply - I'm travelling with limited Internet availability.
I'd love to have a look at the data when I get back, as it'll inform exactly how much noise reduction is warranted.
From the log, it appears the noise reduction applied was rather heavy. The Life module's "Heavy" preset may have contributed to the softness.

Luminance masking ("Brightness masking" in StarTools) is a very old and (often) suboptimal technique to create a blend from two differently processed images. The blend for each pixel then depends on the brightness of the pixel.
StarTools' denoise algorithm already does this (but in a much more sophisticated way, as brightness of a pixel is not a very good predictor of the 'noisiness' of the pixel - instead Tracking information is used).

That said, if you'd like to apply this technique you are of course free to do so. You can use the Layer module for this. Put the noisy image in the background, put the noise reduced image in the foreground and use the "Where foreground is light, use background" for "Brightness Mask Mode". You can tweak the response with "Brightness Mask Power".

Hope this helps!