Finally the cloud cleared lone enough to get started on imaging again. This one is last night's grab of NGC 6357, aka Lobster Nebula, aka War and Peace Nebula, actually looks like a man with someone on his back to me. This image was captured with my ASI1600MM-C camera, through the MN190 Mak-Newt, on a CGX mount. Image consists of 15x300s Ha, 12x300s Oiii, and 12x300s Sii subs, total integration time of just 3hrs and 15 minutes. Processed in DSS, and Photoshop CC. Colour scheme is SHO of course. Some may find this rendition somewhat lurid, which seems to be my habit. It has vast amounts of Ha, and almost no Oiii except for the central pocket, surprisingly there is Sii spread througout the Ha areas but not is great amount, it needs much more time on Sii in my opinion. I may return to this object and put up a revision when I have acquired more data.
This is a good image, and I'm surprised it hasn't received lots of positive support.
The very best way to handle the overwhelming excess of H-alpha over the other channels is to take a much longer set of subs in the weaker channels. With an object like this, that can border on the insanely impractical.
Another is to use 2x2 on-camera (not after the event) binning, preferalby with dithering. However, if you are undersampled to start with, that can lead to loss of resolution in the OIII and SII channels.
But even if none of that is possible, you can still enhance the OIII and SII channels in processing. Most folk do that routinely.
Your black points for OIII and SII look pretty spot on. So all you have to do is to increase the brightness for OIII and SII. For concreteness, a linear increase in OIII of 150% would mean that 0, 1000, and 2000 become 0, 1500, and 3000 counts.
Another trick that can help a bit is to carefully increase the contrast in the H-alpha channel, so long as you don't overdo it and posterize, this will reduce the excessive H-alpha in the weaker parts of the image.
Here I've had a go at doing that: increasing the H-alpha contrast a bit, and then increasing the OIII and SII linearly until the mean counts for OIII and SII exactly match the H-alpha mean count for the whole image.
The resulting image no longer pretends to show which of OIII, SII, and H-alpha is the strongest. But that often isn't very interesting anyway. Think of a map showing the distribution of dirt, coal, and industrial diamonds in Australia. There is overwhelmingly more dirt, but we want to know where the diamonds are. The approach I'm advocating shows where the SII is concentrated, and how it looks, not how much there is.
It's never a good idea to do this to someone's finished image. It would be far better if you did it yourself at an early stage in processing. That will avoid the OIII burning out in the very brightest parts.
I've also zapped the OIII and SII stars, so there are only H-alpha stars, mapped here to white. I've only done that very very roughly, just to give the general idea. Two reasons for doing this. The first was that in this particular image, your channels weren't quite aligned properly in the corners, suggesting your registration software isn't quite right. The proper way to fix that is with better registration and stacking software that can handle rotation, change of scale, and other distortions between frames. The other reason is to reduce the inevitable magenta haloes that the technique I've described lead to.
Hope I'm being useful here, rather than a pain in the potato.
looks like some really good quality data Glen - the stars, SNR and resolution are all excellent.
The colour looks a bit too green to me, but that is what the data gave you. It will be very interested to see where you go from here if you can get more SII - NB processing seems to be a bit of an arcane art.
great capture nice and sharp, I really like your imaging setup.
here some useful and handy tips for getting some more colour variation from green heavy images - (if you felt like playing around with it) - of course NB is dealers choice. http://bf-astro.com/hubblep.htm
Thank you Russell and Ray. The past few months here have been just terrible and even getting one useable night was rare. In the case of NGC 6357, I basically had just one night to work on it before it was lost in cloud again. So I built this image with less than half the data I would normally want to even start with. So the data is very skinny, and as a result there is a penalty in processing, it just cannot be 'pushed' as hard before breaking down and losing its smoothness. A great low noise camera and a fast (f5.3) scope give me some advantage, but time is the big limitation.
I had run across Bob Franke's piece before, and did do some colour balancing on the way through, but I need to review the whole thing.
As Russel say's "NB is dealers choice", and my approach is not going to probbly ever look like the "Standard Model", but I do listen.
