PI can carry out colour calibration on most astronomical images and works on the basis that if you have enough stars or a near by galaxy the sum of these will be white .

This process is recommended to be carried out at the linear stage and have the background neutralization tool run first

We will first have a look at the CC processing consol and then how to use it with a galaxy and then a group of stars

White Reference

A) Reference image-- White reference image. Color Calibration will use pixels read from this image to compute a set of three color calibration factors, one for each channel. If you leave this field blank the target image will be also the white reference image during the calibration process , you will select your galaxy or group of stars here.

B) Lower limit-- Lower bound of the set of white reference pixels. White reference pixels with values less than or equal to this value will be rejected for calculation of color correction levels, black pixels are always rejected. You might need to raise the lower limit on this to isolate your white reference from the background.

C) Upper limit-- Upper bound of the set of white reference pixels. White reference pixels greater than or equal to this value will be rejected for calculation of color correction factors. This parameter allows you to reject saturated pixels, or pixels with very high values pertaining to the nonlinear regions of most CCD response curves. Note that since the maximum allowed value for this parameter is 1, white pixels are always rejected , usually ok at the default limit

D) This is to do with detection of objects , leave selected for star detection and deselect for galaxy detection

E) Manual white balance-- Perform a manual white balance by specifying the three color correction factors, literally. If you select this option, no automatic color calibration routine will be applied, and you'll be allowed to enter the correction factors for red, green and blue, as the next three parameters. I do not use this as if I am unable to carry out the automatic correction I prefer to do it by eye

F) Output white reference mask-- If this option is selected, Color Calibration will create a newer image window with a white reference mask. A white reference mask is white for pixels in the white reference image that have been used to calculate color correction factors, black anywhere else. You can use this mask to check whether the Lower limit and Upper limit parameters are doing a good job selecting the pixels that you intend to use as a white reference .I always out put this to see that my white selection is correct

Background Reference

G ) Reference image-- Background reference image. Colour Calibration will use pixels read from this image to compute an initial mean background level for each color channel. If you leave this field blank the target image will be also the background reference image during the calibration process. Select a image or preview that you have selected for your background.

I ) Lower limit: Lower bound of the set of background reference pixels. Background reference pixels below this value will be rejected for calculation of the mean background levels. usually ok at the default level

J ) Upper limit: Upper bound of the set of background reference pixels. Background reference pixels above this value will be rejected for calculation of mean background levels , you might have to adjust this level to include your background level

K ) Output background reference mask: If this option is selected, Colour Calibration will create a newer image window with a background reference mask. A background reference mask is white for pixels in the background reference image that have been used to calculate mean background levels, black anywhere else. You can use this mask to check whether the Lower limit and Upper limit parameters define a suitable range of values to select the pixels that you intend to use as background reference. I always out put this mask to make sure my settings are correct.

How to use colour calibration

To Start with lets start with a image with a near by galaxy and plenty of background

Below is Pi desk top with a image opened and a STF applied to it and the CC process applied to it

To carry out CC to the above do

1) Create a preview around the galaxy for the white calibration ( Preview 1 ) and select in ( A ) white reference

2) Create a preview for the background reference ( Preview 2 ) and select in ( G ) background calibration

3) Uncheck the structure detection as we are using a galaxy

4) check the options for white and background masks so they are outputted for review

5) Next I need to see what the background level is background level is and to do this move your cursor over the back ground and read the RGB level at the bottom bar and you will see that it just under 0.4200 and bearing this in mind use this setting as follows

6) Set the white reference at 0.4200

7) set the background Reference at slightly higher at 0.43

These settings are individual to your image and the background level gives a very good start level but a little experimentation may be needed

8) Now apply the process to the image and the white reference and background reference mask will be output

You will now need to access the white reference mask first to make sure that only the galaxy ( and some stars ) are only selected as in the example above.

Next you will need to access the background reference mask to make sure only background is selected as again in the example above ( see that stars are excluded )

If this done correctly as above good colour correction will have taken place

Next we will Colour Calibrate a image using stars

Click on image to enlarge

Fig 1

Now looking at the image below I have done the following