Image Processing, (or how many ways to skin a cat)

[Mac]

How can you process an image that you have taken,
Well in an answer loads, each person has a different way of processing, stacking, sharpening, ect, ect.

These are the methods that i use.
Im not saying these are any better or worse then any other way, but have a look at the results and then decide.

I used to do the following for my image processing.

1) take a red image, using narrow band (ha)
2) Take a blue (hb)
3) take a green (oiii)

sometimes i would take an sii (orange ) as well as lp and or clear.

I would then stack and align them, with bias, flats(if i remember) and darks(if i remember). for each group
these would then be saved as the master greys for each filter.

I would then import them into photo shop and paste them into their individual channels (Red, Green, Blue) to produce an image
and then spend the next hours, tweeking the hue, saturation, curves untill i obtained a result that was to MY liking.

Now is it correct to use the HA as red, HB as blue, Oiii as green,

well yes, if you look at an emission spectrum then you can see that these emission colours, Ha, Hb, Oiii, Sii appear at specific wavelengths,
so when you image you are taking an image withing these colours. You are taking these specific colours.

Looking at the Hydrogen spectrum

you can see that the Ha is in the middle of the red area, and the Hb is in the middle of the blue.

and the oxygen band is in the middle of the green,


So using these filters as my RGB is correct.

Now I have changed how I process my images, I still use the RGB filters as RGB but how they are combined is now different.

I will walk you through step by step and you can see how I build up my image.
I will then show the end result compared to the original.

1) Open your first image into photoshop and rename it to represent your filter colour.
   This is my Red image


opened into photoshop as a layer NOT into a CHANNEL


2) Add the other two layers for the Blue and the Green and rename.

3) Change the mode to RGB as it will default to greyscale

4) For the first layer add a hue adjustment layer by clicking on the two coloured circle.


5) on this adjustment layer


Select colourise and then slide the saturation to 100
Adjust the hue so that the bottom coloured bar represents the colour that you want to assign to the layer.


once you are happy with the colour click ok

6) Now click on the image layer for the colour you have set, in this case its the blue layer.
Change the mode of the layer from Normal to Screen.


7) You now need to link the layer to the colour adjustment that you have made, to do this click on the chain so that the two layers are linked


Goto the menu and create a clipping mask.


9) Now add another hue adjustment layer for the Red, link this to the red layer create a clip mask and then do the same for the green layer.
Your image will look a bit like this


10) Now for each layer double click on the hue adjustment and start to adjust the sliders
changing the hue, brightness and saturation.

11) If you add these layers to a single folder, when you have finished you can then adjust the whole image for saturation
also you can sharpen each individual layer.

12) when you have finished adjusting the levels you will end up with an image like this.


The original image is on the Left, the hue adjusted is on the Right.

The settings i ended up with were
Blue. Hue:239 Sat 100% Bright -52
Red. Hue:0 Sat 100% Bright -51
Green. Hue:128 Sat 100% Bright -51

So will this work on astronomical images.
well here are two that i have quickly added the hue adjustments above and then saved.

The M42 is just single images of Ha, Hb, Ra,


And Jupiter


Original posting on Right
New processing on Left.

Now should you wish to change the colours that you represent all you do is change the hue.
So if i want to add an Sii layer, I would just change the Hue to an orange colour for that layer.

Comments please.

Mac

[mickw]

Admin -Admin, I must complain...........................

This should be moved to the "Never in the field" post ................... in Technical

Mac, excellent stuff, thank you

[Fay]

That looks really helpful, Mac. I will peruse at my leisure, thanks

Fay

[Whitters]

Nice one Mac, I will have a go at this. 

[Carole]

Very interesting Mac.  Since I use a DSLR I do not need to process in this way, so good to get some insight into other methods.  I do however adjust the different colours in curves. 

Yes I agree should go into the appropriate Stickys folder for future reference.

Carole

[Mike]

Nice tutorial Mac.

[Mac]

Since I use a DSLR I do not need to process in this way,

True, but if you take the original colour channels from the DSLR sub, you can then paste them into
their own layers, and then process them as above.

If you look at the final stain glass image, you can see that the original on the left was infact a JPG from a DSLR.
I chose that image as it contained loads of different RGB

All i did was to copy the colour channels (R,G,B) in to their own layers on a different image,

The reason behind that was to show that you can adjust the original greyscale images using the screen
method to get the same colours as the original image,

when you apply these same settings to astronomical images, it will give you a starting point for the adjustments to your colours,

If you look at my Jupiter, the image on the right is processed in much the same way you are processing yours, and the colours are ok, but the image on the left is processed using the above method,

Im not saying which one is the corect colour as again that is down to the indvidual, but the colours on the new image are so much easier to adjust, as each colour is itself separate.

So you can also adjust each individual layer for brightness and contrast, something you cant realy do easily when working on the images as a single coloured layer.

Mac.

[PhilB]

Looks very comprehensive, Mac. Thanks for taking the time to explain your methods, I'll have to try it.

[Carole]

OK Mac, yes will have a go at that.

Carole

[RobertM]

That's very interesting Mac and I can see your logic in wanting to do it that way.  In the end it ends up as the same result but includes the possibility of adding other channels such as for Hb which can't be done as easily using the RGB approach.

The only thing it lacks is a way of setting the wavelength for accurate rendition of NB images.

Thanks for sharing.
Robert

[Mac]

The only thing it lacks is a way of setting the wavelength for accurate rendition of NB images.

This method allows you to set exactly that, This is done via the Hue.

The way this works is as follows, imagine you have a coloured sheet and shine a light through that sheet onto a piece of white paper,
you get a coloured image on that piece of white paper, do this three times and you get the coloured image,
similar to an old slide,

except here you are selecting the colour of the light to shine through, there is no limit to the colour you can set,
if you use the RGB colour channels, then you are limited to exactly that RGB

If you have an sii filter, then to add this layer all you do is change the hue to what ever the colour / wavelength represents,

(Ha)           656   
(Hb)           486   
Oxygen III   496   
Oxygen III   501   
Helium II   469   
Helium I   588   
Nitrogen II   655   
Nitrogen II   658   
Sulfur II   673   

by looking these wavelengths up on a chart you can select the hue accordingly,



thats why you have the brightness at 0 and the saturation at 100 so you can see the exact colour you are using on the bottom slider
so looking at the Hb its wavelength is 486nm which is inbetween Blue and Cyan,
so you would set you hue to exactly the same.

Mac.

[RobertM]

Guess I should have been more specific.  I meant being able to set the wavelength more exactly by value rather than using the eye to colour match (though it can do a reasonable job).  Matching by eye also relies on having an acurate monitor and reference card/image available; on the other hand colour is rather subjective so I suspect it's probably not an issue anyway.

Robert

[MarkS]

That's a very informative tutorial Mac - thanks.

Since I use a DSLR I do not need to process in this way

I have a DSLR and sometimes have to process this way:
http://forum.orpington-astronomy.org.uk/index.php?topic=7692.msg50441#msg50441

Mark