Rosette nebula in colour - HaR(synthesized G)B

[swag72]

I had previously got some Ha in the Rosette and decided that I'd try for some colour. I only managed red and blue as I had another play with my reducer spacing the following day and so the green wouldn't have matched up. I decided to cheat and synthesize a green channel.

Details

M: HEQ5

T: Pentax 75SDHF with Optec NexGen 0.7x reducer

C: 460EX mono with Baader 7nm Ha, R and B filters

Running at -10, this was taken over 2 nights. There are bias frames for all the channels, but only flats for the red and blue.

Ha 15x1200s

R 25x300s

B 27x300s

The only reason I have 27x blue is that as I left it running over night I took 2 extra in case I had to delete any for any reason. I didn't and so I kept them in the mix.

[opticalpath]

It's looking really good, Sara, but I would say it does need a (real) green channel to get the colour balance in the nebula right, and for more natural star colour. The faint stars within the nebula are also very reddened, which can happen when you blend the Ha image into the red channel, if the faint stars are brighter in the Ha image than in the original red image. If you use Apply Image/ Lighten to blend in the Ha, you can clone or filter out the faint stars in the Ha first to reduce the effect.

Could you not just resize your green image to match the scale of red and blue?

Adrian

[Explorer 190 MN]

Pixinsight will rescale your green channel. If you dont have pixinsight you could use a dropbox and I´ll rescale your subs....

[swag72]

I've not got any green channel and after fiddling with the spacing the stars are out of shape in different places and so I'd never get the green to match the rest of the data. No bother, I am happy to put this one to bed and move on!!

Good tip about the stars in the Ha when added to the red channel. I'll look at that next time. So when I add the Ha to the red I need to add it as lighten?

[opticalpath]

There are various ways to blend Ha into red, Sara. A good choice is to use the command: Image/ Apply Image, because you can vary several parameters and get an immediate preview of the effect before committing.

Start by having 2 or 3 layers containing your Ha image at different stages of processing, and your RGB image layer.

Make the RGB layer visible and all other layers invisible. Highlight the RGB red channel - this sets the target for your Apply and restricts the blend to that channel only. (Keep the 'eye' on RGB so you see the effect in colour.)

Select ' Image/ Apply Image'. The drop-down has a box where you enter the source (file, layer, channel) of what you're going to blend in. Below that is the target channel you selected before and a place to enter the blend mode and % for what you are blending.

'Add' will do what you expect and just make everything too red.

'Lighten' is a good choice as your Ha image should only be lighter than the RGB in the nebulous regions, so will only brighten the red channel there (so keep the sky background darker in your Ha than in RGB for example before you Apply.)

Play with the % opacity, and also try Apply-ing different versions of the Ha image to find the one that works best - often a moderately processed Ha that is a closer match to the red channel that your full-on Ha version.

Try also applying the same Ha to the blue channel at 10%, to simulate the small amount of H-beta present in most Ha nebulas.

Hope this helps. Example:

Adrian

[swag72]

Adrian, that is a brilliant write up. No wonder I've got issues, I've just been splitting the RGB and pasting in the processed Ha image. Is that something akin to using a chain saw to prune the roses?!!

If I get some time tonight I'm going to try this tonight just to see that I understand it and can follow the steps and write it all down. I am such a PS virgin, it's 'virgin' on the ridiculous!!!!

[swag72]

Oh Adrian, I knew I'd not manage to follow this. Please see my screen shot, I don't know what's happening. I'm a little confused on the channel bit - How do I highlight the red channel? I think I've done it right, I show it on the screengrab, but it's not making any difference.

*I hate processing...... I hate processing*

[opticalpath]

Sorry Sara, I was a bit vague on that point. I meant ..... on the *Channels* tab of the RGB image, highlight the red channel - so action will apply to it only - but ensure all the 'eyes' are switched on in all the channels/RGB so that the full colour RGB is visible while the action only applies to red channel. Channel tab should look like this:

Just need to be careful if you go back to the Layers tab not to click on any layers. That has the effect of resetting the channel 'view' to all channels instead of just the red one that you want to apply to.

It's fiddly to get the hang of this at first but once you're used to it Apply is very vesatile. You can try the other lightening blend modes too, e.g. Screen, but Lighten is usually best: it will compare the applied Ha layer to the target red channel - pixel by pixel - and choose the higher brightness value of the two in every case. So anything brighter in the Ha image than in the target red channel image will replace the red channel pixels. Hence watch out for too bright stars in Ha or too bright a sky background - you don't want either of those replacing the balanced RGB image pixels. Process the Ha so that only the nebulosity is brighter in the Ha image and only that part of the image will enhance the RGB red.

Adrian

[opticalpath]

Sara, sorry, I just looked again at your screen grab and noticed the channels window that I missed the first time. You have done it correctly.

If you're not seeing any difference, just check quickly that the layer you're applying (Ha) has exactly the same dimensions as the RGB layer (there can be a not-obvious difference in layers' dimensions if you pasted in layers from different sources, but never cropped the composite image - it only becomes apparent when you pull the individual layers out of the composite image. Any mismatch and Apply won't work. Try the brighter Ha version with opacity set to 100%, or try blend mode Screen - that should certainly have an effect. If still nothing happens, suspect a mismatch in layer dimensions.

You can try the blend another way. Copy the RGB red channel and paste it in as a new *layer. Compare this layer to the source Ha layer that you plan to blend with to make sure that it really is brighter than the RGB(red) in the important places that you want to enhance. You can do the blend right here by placing the Ha layer above the RGB(red) layer and setting the Ha layer blend mode to Lighten (or Screen). Then hide all other layers and Merge Visible the Ha blend and RGB(red) layers. Paste the resulting merged layer directly into the red channel of the RGB image proper.

Let me know what you get. If still problems can you upload a 50% resize (or 8 bit) version of the image as a layered TIF somewhere and I'll take a look?

Adrian

[Coco]

Excellent image Sara..

[swag72]

Adrian, you have been most helpful, thank you. I did try it again, and could see a very small difference, so it was working!! I shall use that way in the future.

Thanks Guy!! You got that little combo of yours up and running yet?

I've played with the red stars in the nebulosity and think I've helped them a little. That is it for this image now!!!