How to RGB balance in IRIS

[luka]

Hi,

I was playing with iris for a while and after stacking a bunch of images I get a greenish-looking image. According to the instructions, I am supposed to adjust the RGB balance to correct the image.
The iris webpage gives as example factors 1.96/1.0/1.23 for red/green/blue for the 10D camera. Where do these numbers come from? What should they be for Nikon D80?

Thanks

[[1ponders] (Paul)]

I'm just bumping this for you so that if Tornado33 see it he will reply as he is probably the most proficient user of Iris here.

Also changed you thread title so he'll pick it up.

Also I'd like to know as well

[luka]

Thank you.

[Terry B]

Originally Posted by luka

Hi,

I was playing with iris for a while and after stacking a bunch of images I get a greenish-looking image. According to the instructions, I am supposed to adjust the RGB balance to correct the image.
The iris webpage gives as example factors 1.96/1.0/1.23 for red/green/blue for the 10D camera. Where do these numbers come from? What should they be for Nikon D80?

Thanks

I have been using Iris for a long time but not for colour work until recently so I am still a learner.
The numbers above should have come from the scaling of the colours when you take an image of a G2 star, ie like our sun. you can take an image of a G2 star and measure the coefficients for your camera. There are lists of standard G2 stars available (see the bottom part of this document http://www.ghg.net/akelly/artdraf7.htm )
You need to take 3 images through the standard filters (or extract the colour components of a colour image)
When you have your 3 images do the following

SCALECOLOR [IN] [OUTPUT] [REFERENCE INDEX] [NUMBER]
Scale the input images sequence [i] for color combining (or drizzling algorithm). For each images, a rescaled sky was substracted and a gain ajusted relative to a reference image of the input sequence. The index of the reference image is [REFRENCE INDEX]. The generic name of the output sequence is [OUTPUT].
Before SCALECOLOR command select an unsatured star with the mouse (the images are to be registered before applying SCALECOLOR, see REGISTER or COREGISTER for example). Idealy, for a good color balancing, select a solar type star (a G star).
Example:
SCALECOLOR I J 2 3


Scale the images I1, I2, I3 relative to the image I2. This produce the scaled sequence J1, J2, J3. If J1 is the R component of a tricolor image, if J2 is the G component and J3 is the B component, than you can made:
TRICHRO J1 J2 J3

Usually you assign the red channel 1 and adjust the other colours to the red.
The values will show up in the "output" window.
You can then use these numbers to adjust your tricolour image.
Use

RGBBALANCE [Rcoef] [Gcoef] [Bcoef]

Multiplied components red, green and blue of the 48 bits in memory image
respectively by the coefficient Rcoef, Gcoef and Bcoef. This function is
equivalent to RGB balances... command of Digital Photo menu.

A lot of this stuff is available in the tutorial page at
http://www.astrosurf.com/buil/us/iris/iris.htm#tutorial

Having said all this the commands "black" followed by "white" will give you a rough correction to your image. Just select a black bit of sky and run the black command then select a non saturated star that you would like to be white followed by the white command. Simple and not as accurate but gives OK results.

[CometGuy]

Luka,

1. After you load the image, in menu select "Digital Photo" -> "RGB Balance"

2. Try coefficients 1.93 1.00 1.80 (I've seen these quoted for D70)

3. Select (using the mouse) some sky background relatively devoid of stars/nebula and type the command 'black'. This neutralises the sky background colour.

Hope that helps.

Terry

[luka]

Thanks for your replies.

I have been using black and white so far, but the final result is very dependent on the selection of the star.
I will try doing colour separation from a G2 star as Terry B suggested when the clouds finally clear over Perth.

Regarding comment from the other Terry (commetGuy), unfortunately D70 and D80 have different sensors so the coefficients will be different.

Also a bit more of searching suggested 1.9/1.0/1.4 as good starting values but experimenting with them is a real pain as there is no undo in Iris.