Using eXcalibrator With a Color ...

Using eXcalibrator With a Color Camera and CCDStack, ASTRONOMIA, Tutoriale

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Using eXcalibrator With a
Color Camera and
CCDStack
by
Bob Franke
CCDStack is developed and maintained
by
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.
Copyright ¨ 2011, Bob Franke
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.
•
Register the images.
•
Create the mean combined R, G and B masters.
•
Get the RGB combine ratios from eXcalibrator.
•
Create the final combined RGB image.
Copyright ¨ 2011, Bob Franke
Page 2 of 12
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.
Copyright ¨ 2011, Bob Franke
Page 3 of 12
•
Use the
Convert Bayer
process to convert the color FITS images into their separate R, G and B
components.
Copyright ¨ 2011, Bob Franke
Page 4 of 12
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