Blend NB with RGB

[Andyfx]

Hey everyone I'm just hoping someone might be able to point me in the right direction. I have a nice new Oiii filter on the way and am looking to image in RGB, and then take some subs using the Oiii filter and blend to the blue channel.

Anyone know of any good tutorials? Am hoping to try this out on M42 hehe

[ollypenrice]

Hi, thanks for the PM.

I think Robert Gendler's site has some info.

Personally I make a copy of the NB image, fully stretched and processed and aligned to the fully processed LRGB or RGB.

I then split the LRGB channels and paste the NB onto the relevant channel in Ps. The Layers palette allows you to select Blend Mode Lighten. This blend mode will have little effect on stars. I look at the effect the NB data is having on the original channel. On some objects where the NB has a clearly defined location (galaxies, perhaps) I just erase it from the starfield for good measure. Otherwise I might feel it wasn't having enough (or any) effect so I would lift it in Curves until it was doing something useful. Flatten the channel and recombine. Now some people add the NB a little at a time. I tend to add it full on because I can then dilute the effect by putting the NB-LRGB on top of the LRGB and choosing the opacity that I like, reducing the NB component that way.

I would add O111 to both green and blue. I'd make one copy with it added only to blue, one with it added only to green, paste one on top of the other and again control the mix that way in Levels, Opacity.

What I rarely do is use Ha as luminance. Too pink, too many blue haloes. However, I do sometimes add it at very low opacity.

Others will come up with alternatives. Nik did an article about this in Astronomy Now very recently. His method is unlike mine and well worth looking into.

My method is very 'try this, try that' and has nothing to do with science and more to do with jazz!

Olly

[Andyfx]

Thanks Olly that confirms what i have seen on a few threads before, especially about Ha data used as luminance coming out a bit pink!

Just a couple of follow up questions though:

Do you need a blue, green or red filter for the appropriate channel or simply blend the monochrome NB info to the colour captured on the RGB image?

How do you split the RGB channels in photoshop?

Advanced stuff + noob = ??????? lol

[Andyfx]

Am just looking at Rob's site now Olly, lots of info by the look of it

[ollypenrice]

Rob is characteristically generous with his knowledge!

RGB splitting? It doesn't matter whether you captured R and G and B or did it via one shot colour. In Photoshop just go to Window, then Channels and find the little 'play' arrow which willl open up a menu including Split Channels. They open up, zip, zip, zip with red on the top. When you've done the mods by adding the NB to a channel of your choice just go to the Play arrow again and choose Merge Channels. Out of the options choose RGB. Then put R in the top box of the drop down, B in the second and G in the third. It will probably offer to put the NB image in the top box by default but don't accept that.

Olly

[Andyfx]

Nice one, thanks very much Olly! As soon as the filter arrives and I catch a clear night I'll get some Oiii data to add to my Pleiades subs just as a test

[ollypenrice]

The general wisdom is that the Pleiades nebulosity is reflection rather than emission so there should be no gain from using emission line filters. However, I've always had a hankering to see if there was a little Ha signal in there. There are certainly some deep reds. I've never followed this up, though. For strong O111 signal at this time of year try Thor's Helmet. It's an O111 classic, one of the very few green emission nebulae to be had.

Olly

[Andyfx]

That sounds very interesting and I've never heard anyone mention that nebula before, I am just about to google it for details and am hoping it's a largish nebula so I can take advantage of my nice wide field

You're always a wealth of information Olly, it's great you're on here to help so many of us starting out.

[ollypenrice]

I get plenty of help on here myself, believe me! Thor's is a little small but really is a nice one if you have a decent southern horizon and don't live in the north of Scotland...

Olly