Using eXcalibrator With a Color Camera and CCDStack.pdf

Using eXcalibrator With a

Color Camera and

CCDStack

Bob Franke

(http://bf-astro.com)

CCDStack is developed and maintained

Stan Moore

I Introduction

This document shows how to combine several One Shot Color (OSC) camera exposures into separate

R, G and B files, suitable for use with eXcalibrator. Finally, the eXcalibrator filter factors are used to

create a final color corrected RGB image.

II Why Do I Need To Correct My Camera's Color ?

• CCD Quantum Efficiency

All CCD cameras have varying efficiency at different wavelengths. Here is a typical set of quantum

efficiency curves. The increased sensitivity, at the blue end of the sprectum, gives a general

adjustment for the fact that our atmosphere scatters blue and green light more than red. The

author does not know if this is by design.

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Page 1 of 12

• Altitude Extinction.

The below graph shows how green light is darkened more than red and blue more than green.

Once the imaging target gets below about 55 degrees, above the horizon, the color extinction starts

to noticeably affect color balance.

• Variable Seeing Conditions

When data are collected, on multiple nights, the sky transparency will likely vary. Even in a single

night, thin clouds may come and go. These varying conditions will affect color balance, in the same

way as altitude extinction.

III A Short Description Of The Process

•

Load all the color fits image.

•

Calibrate the images.

•

Create all the R, G and B sub exposures.

•

Create the mean combined R, G and B masters.

•

Get the RGB combine ratios from eXcalibrator.

•

Create the final combined RGB image.

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IV The Detailed Work Flow

•

Start CCDStack and load all of the color FITS images.

•

Apply darks and flats to all images.

•

You should get something like this.

If the Filter column is not filled in with "Bayer", you will have to manually tell CCDStack that the data

are from an OSC camera. Use "Edit|Camera Manager" to open the Camera Manager window.

Then find your camera and turn on One Shot Color. Close the window and reload the color FITS

images.

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•

Use the Convert Bayer process to convert the color FITS images into their separate R, G and B

components.

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