Quote:
Originally Posted by alpal
Dear Marcus -
who said anything about losing or erasing stars that are there now in your picture?
You can stack all the frames & even blur the image you create but
use it as a blurred layer mask to increase the intensity of which ever areas are whiter - in that mask by clicking luminosity in Photoshop for that layer
& then increasing the brightness using curves.
This will bring up the faint halo better for the tidal streams.
It has to work.
By the way - it's not cheating or devaluing your sharpness of the picture you already have.
It's using the so called lost information to increase the brightness in the areas you want.
cheers
Allan
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Hmmm ... you're not understanding the processing challenge Allan. You're giving me a beginners lesson on how to stretch an image using masks and curves because, presumably (?), you think there is more faint extent to reveal in my data and that this can be achieved using bad data. Firstly, the image already compares favourably with others on the net in regards to depth so, frankly, I don't actually feel the need to stretch any more, especially with this mediocre dataset. Moreover, I'm telling you that I can't use the bad data to improve my image (noise levels mostly) without losing faint stars, galaxies or even structure in the streams.
The subs I excluded ranged from 3 to 5 arcsec FWHM with most somewhere in the middle. The sky was also somewhat brighter during most of the bad subs - quite common. Consequently, S/N is much worse and the faintest stars in the stacked bad data are dead & gone. They have passed on. They are no more. They have ceased to be. They are at best fuzz balls that are hidden at or near the sky background.
To illustrate, see the two screen captures below of the stacked good and bad data (autoscaled in CCDStack with, no other processing). The capture is from the tip of the right hand tidal stream where the reeeeally faint lobe is. No prizes for guessing which is the crap data (
31 subs worth compared to only 16 good data subs).
Now, you want me to layer in the stretched bad data (ie blurred with poor S/N) where faint stars and structure are not visible, to achieve some kind of miraculous recovery of depth/noise without losing faint star detail. Theoretically I can do that layering, but not without painting into the mask those stars and structures that are very faint so I don't blur them out of existence. Note that no automated mask creation can pick out ALL of the faintest stars & structures I'm talking about unless your S/N is much better than this example (ie: would need much more data).
On top of that, I'd have to deal with this challenge over the entire frame. Even if I could brighten the faint stream extents I would drown out the good faint stars unless I brighten them too. I'd do that by inverting the mask and applying it to a curves layer and brightening every other star in the field - unless I mask out the rest of the field as well. It's quite probable that I'd also add signal into the streams (and elsewhere) that
isn't real because the faint extents are practically invisible above the sky brightness in the bad data.
Now, if you look at that region in my processed image, you'll see the lobe quite clearly (albeit with noise) AND the faintest stars in the screen shots. I'd be chasing my tail for no benefit and probably end up still losing faint detail and/or adding detail that's not real.