Image Processing Tutorial
Bernard Miller
CCDStack Work Flow
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Create Master Dark, Bias, and Flat
Calibrate
Hot/Cold Pixel Rejection(Optional)
Debloom (Optional)
Register
Normalize
Data Reject
Combine
CCDStack Work Flow
• Create Master
Dark, Bias, and
Flat
– Bias: min/max
clip (0,1) or (1,1)
– Dark: min/max
clip
• 0-6 frames: (0,1)
• 7-15 frames:
(0,2)
• 15+ frames: (1,2)
– Flat: Median
CCDStack Work Flow
• Calibrate Images
– Calibrate
– Rotate West Images
– Hot/Cold Pixel Removal
• I only do hot pixel removal at this
stage for RGB
• Luminance images I do not do hot
pixel removal
• Hot Pixel Removal General
Recommendation
– If dithered images and more than 7
subframes, don’t do hot/cold pixel
removal
– If not dithered or 7 subframes or less,
do hot/cold pixel removal
– Debloom
• I don’t debloom because I have
deblooming camera
CCDStack Work Flow
• Registering (Stack->Register)
– General
• Get the CCDIS alignment plugin
• After registering, double-click on small to
medium star and arrange subframes by
increasing FWHM
– Luminance
• Use “Nearest Neighbor” algorithm
– Assuming you have 8 or more subframes
– RGB
• Read in the Stacked Luminance image
prior to registering
• Use “Bicubic B-Spline” algorithm
– If you have 8 or more subframes you can
use try not using hot/cold pixel removal and
using “Nearest Neighbor”
CCDStack Work Flow
• Normalizing (Stack->Normalize)
– Use the subframe with the best
FWHM as reference
– Always use Stack->Normalize>Control->Both
– First selection is the background
– Second selection is the signal or
object
– For nebulas, look for dark region
for background selection
CCDStack Work Flow
• Data Rejection (Stack-> Data Reject)
– For 8 or more subframes, use “STD
sigma reject”
– For less than 8 subframes, use “Poisson
sigma reject”
– Use 2.2 for the “sigma multiplier”
– Red pixels go away if you close the
dialog box, but the data is still there
• Combine (Stack->Combine->Mean)
– Save Calibrated and Stacked frames in
separate directory
CCDStack Work Flow
• Deconvolution of Luminance
Image
– Process->Star Selection
• Max ADU:
– About half of your full ADU range
(~30K for 16-bit camera)
• Threshold ADU:
– About 2000 for galaxies and images
with dark background
– Above the ADU for faint part of
nebula
CCDStack Work Flow
• Deconvolution of Luminance Image
– Process->Deconvole
• Select “PSF to fit Moffat”
• Select “Positive Constraint”
• Matrix Radius
– About 2X your FWHM
• Sharpen Count
– Lower numbers do MORE sharpening
– I usually use 0-4
• Bias subdivisions
– Lower values result is less star ringing
– I usually use about 1X my FWHM
– Create two deconvolved images
• Use 20-30 iterations for one image
• Use 150-200 for the other
CCDStack Work Flow
• Applying DDP and Saving Images
– Should have original luminance and both deconvolved images
– Adjust original luminance image
• Window->Adjust Display
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Make sure DDP is selected
Background slider adjusts background brightness
DDP slider adjusts image brightness
Gamma set to 0.90
• Brightest part of image should be about 200 counts
– Not including saturated parts of image like galaxy cores
• Background should be between 20-30
• Use Histogram window to make sure you are not clipping
• When finished with original image, click “apply to all”
– File->Save Scaled Data->This
• Save all three luminance images as 32-bit float and TIF
CCDStack Work Flow
• Missing Value Technique for Combining Images
– Start with long exposure image selected
– Normalize the two images using the long
exposure image as the master
– Select over exposed portion of long exposure
– Stack->Data Reject->Procedures->Reject range
• Reject pixels above about 2/3 of full range (~40K
for 16-bit)
• Remember to select “Apply to this”
– Select Grow and use 3-5 pixels
– Select “Set Rejects to Missing Value”
– Click on “Weight” column of short exposure
image
• Set to very small number (i.e. 1E-7)
– Select Stack->Combine->Mean
Recommended Plugins
• CCDStack
– www.ccdware.com/products/ccdstack
• CCDIS Plugin for CCDSTACK
– http://www.ccdware.com/buy/