[Shiraz (Ray)]

Originally Posted by Lee

I think you can stretch more and increase the contrast before it becomes undesirably clipped.... ??

will have a play around and post a revised version if I can do any better.

Originally Posted by Peter.M

That's what she said

I couldn't help myself. Great image of a great target!

thanks Peter

Originally Posted by PRejto

That's a cracker of an image!! I tried this a year ago but from light polluted Sydney I think it was a lost cause before I started.

I'll be curious to see the Ha blend when and if.

Peter

Thanks Peter. It is pretty hard imaging faint objects in light pollution - can't imagine what it must be like in Sydney

Originally Posted by naskies

I was thinking about this today on my way to/from classes today, and remembered that I'd read about it during my visual system module at uni last year.

The short answer is that it's physiologically wired into our retinas.

Our retinas have separate and different circuits, retinal ganglion cells, for detecting a "white dot on black background" (so-called OFF retinal ganglion cells) versus a "black dot on white background" (ON cells). The photosensitive rods and cones that we all know about feed their output (eventually) to the retinal ganglion cells.

It sounds a bit absurd, but the different types of cells are used to detect object edges - see the red/green computer simulation diagram at the bottom:

http://en.wikipedia.org/wiki/Receptive_field

When viewing low contrast scenes, ON cells can detect both decreasing light ("grey dot on white background" turns into a "black dot on a white background") and increasing light ("black dot on a white background" becomes "grey dot on a white background") changes. However, OFF cells can only detect decreasing changes in contrast.

Therefore, with low-contrast images - looking at faint structures in astro images - our eyes are more sensitive to light decrements (i.e. black detail on a white background as with inverted astro images) because both the ON and OFF pathways are triggered.

Here's an article that describes the technical detail behind the mechanisms (warning: it's not an accessible read for non-physiologists), but receptive fields in general are discussed in most vision texts.

http://www.jneurosci.org/content/23/7/2645.full.pdf

Thanks very much for that Dave - very clear explanation. So there is a physiological basis for our ability to see dark detail against a light background - fascinating. Now for an hypothesis on why. Maybe our eyes have the same structure as those of smaller creatures that had a need to see predatory birds against the bright sky. I was watching Meerkats at the zoo and they maintain a constant watch on the sky responding to birds and aircraft with alarm. Physics dictates that most objects at a distance end up as dark spots against a bright sky and mammalian visual systems have evolved to be very sensitive to that threat - maybe?.

Originally Posted by Paul Haese

Yeah it took me a few years to find my spot.

It was OK over here, but then Viterra decided that the whole town needed to be able to bask in the glow of the floodlighting on their silos and the local council decided that the hospital helipad needed Sodium lighting that burned off the surrounding grass as part of its task.

Originally Posted by strongmanmike

Oi! what's wrong with red splodges ...some people love'em

Don't listen to him Ray dooooon't.. get out there blend, BLEND BLEND I say!

Mike

Two experts with differing opinions - unprecedented . Probably will tread warily until I get a better understanding of what I am trying to do

Originally Posted by Nicola

Excellent details!

thank you Nicola