IRIS tutorial
LRGB processing

Carsten Arnholm.
This text is based on an email from Tom How in the UK. Many thanks Tom!

This little tutorial is an attempt to explain how to use IRIS to process deep sky images using the LRGB technique. IRIS is an amazing (and free!) program written by Christian Buil. The program contains many advanced features for image processing, but can be a bit challenging to use, as it is largely command oriented and different in style from other programs.

If you have detailed questions about the program, its theory or implementation or basic use, I suggest you visit the Yahoo group IRIS_software and ask your questions there.

I may add illustrative pictures to this tutorial some day. But until that happens, please refer to my Rosette Nebula (NGC2237) image, processed using this technique.



This tutorial assumes you have captured 4 colour channels separately, for example using a black and white CCD camera and using filters as explained above. This could be 4 single frames straight from the camera. But more likely it will be 4 series of frames, each series aligned and stacked individually, using IRIS, K3CCDTools or whatever you prefer. The main thing is that the resulting 4 channels should be saved as 4 16bit signed FITS files (monochrome).

Start with the 4 FITS files in the same folder


In File | Settings set the working directory to the folder with the FITS files in. Open the IRIS command prompt (button on toolbar), and type
load L 
This loads your L frame on the screen. This step is not vital, but makes sure you are on the right track. Next, type
coregister L R
This registers the R frame to the L frame, and puts the registered R frame on the screen. Next, type
save i1
This saves the aligned red to a file called "". Now register G and B against L.
coregister  L G
save i2
coregister L B
save i3
You now have 3 aligned fits files i1=red, i2=green, i3 = blue... they are aligned to the Let us go further and scale the colour ... load the red one with...
load i1
Using the mouse, draw a square around a non saturated sun-like star... well repeat the scaling command with different stars until you like the balance... and type
scalecolor i j 1 3
This scales the i1,i2,i3 to balance the colours and save it to , , ..... the third parameter is the reference frame and the fourth is the number of frames. Next make the colour picture from the registered R, G and B frames
trichro j1 j2 j3
You now have a colour picture on the screen. I may not be perfect at this point, but save this as TIFF, colour FIT or, what i do, save it in photshop PSD format
savepsd colour
which makes a colour.psd.

Open this in photoshop or whatever program you have available. Adjust the curves as normal. Tweak the colour balance. Then... if you are in photoshop, you can open and copy paste it on top of the colour picture while blending luminosity. To make it even better, you can also consider using Robert Gendlers Multiple Luminance Layering (LLRGB) technique.

Final words

The IRIS stuff may sound complex, but, provided it can match the stars, it works like magic.