Image Processing TutorialBernard Miller

CCDStack Work Flow

• 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– 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/