Image Processing

Overview  Polar Align  Focal Length  Focussing  Find & Take  Processing  Color Imaging

There are many software packages that can be used to process CCD images.  You can use just one package to process an image but is often best to use several packages so you can combine the best features of each one. 

Processing Deep Sky Images

Calibrate, Align, Normalize and Combine the images taken through each filter

  1. Calibrate the images in CCD Stack by subtracting dark frames and  applying flat fields.

  2. Align the images in Registar.  I have tried the registration algorithms in MaximDL, Images Plus and CCDStack  but find Registar to be the most accurate and reliable for registering images.  It is also extremely good at aligning images that only have a small amount of overlap which is great for assembling mosaics. 

  3. Normalize the images in CCD Stack.  This makes each image the same relative brightness.  It is important that the images are normalized if the statistical noise rejection is to work correctly. 

  4. Reject outlier pixels by using Standard Deviation Sigma rejection in CCDstack.  This works best when you have a least 16 images.  It will reject all pixels that fall a set number of standard deviations from the average pixel value.  This technique is very effective at removing satellite an asteroid trails as well as hot/cold pixels if the images were taken using dithering.  I set CCD Stack to reject pixels greater than 2.2 standard deviations from the mean.

  5. Combine the images in CCD Stack using Average and save the master image

Final processing of the luminance image in CCD Stack

  1. Perform deconvolution if required to sharpen the fine detail.  This tends to work best to bring out the fine detail in galaxies.

  2. Perform a mild digital development stretch in CCD Stack.  This brings out the faint detail and avoids saturating the bright areas.

Make the master color image from the master images taken through the RGB filters

Use color combine in CCD Stack or Images Plus to make a single color image out the master images taken through the red, green and blue filters.  You should adjust the ratio of each color based on the G2V calibration for your camera/filter combination.

Create the LRGB image in Photoshop

  1. Align the color image with the luminance image in Registar

  2. Load the color image in Photoshop and use levels to stretch the image and make it brighter.  Then look at the histogram for each color and adjust the level of the black point so that each histogram starts to rise at the same point.  This will ensure a correct color balance in your image.

  3. Use levels and curves on the color image to brighten it and bring out the fine detail.  Gradient Xterminator is a great plugin for Photoshop that makes it very simple to remove gradients from the color and luminance images.

  4. Load the luminance image.  Finish processing it with levels/curves, sharpening, noise reduction, gradient removal etc... 

  5. Copy the luminance image as a layer on top of the color image.  Set the combine mode for the layer to Luminance and now you will have the fine detail of the luminance image with the color of the color image shining through.

  6. Increase color saturation of the color image by applying the Shadows/Highlight filter and boosting the color correction.

  7. Blur the color image to reduce noise.  A 3 pixel Gaussian blur works well.

  8. Flatten the image and switch to Lab mode.  Select the 'a' channel and increase the contrast and then do the same with the 'b' channel.  This increases the saturation of the mage without introducing noise.

  9. Switch the image back to RGB mode and do any last processing to taste.

There are many techniques you can use in Photoshop to enhance your image, far more than I can list here.  I recommend getting a copy of Adam Block's 'Powerful Processing in Photoshop' DVD.  All the techniques you will need to produce great astro images can be found in a series of easy to follow tutorials.  Adam also has a great DVD tutorial on CCD Stack as well.

Starizona has a good online guide to CCD imaging and processing

Processing Planetary Images

For planetary imaging with the webcam it is best to use Registax followed by Photoshop.  Registax is freeware software which can process AVI files captured by webcams.  A planetary imager may capture an AVI file of a minute's duration or more.  It will contain 100s or 1000s of images of the planet.  Registax can sort through these images, automatically picking the ones with the most detail .  It can then sum these together to produce a single image with a much higher signal to noise ratio.  Registax also contains some useful image processing routines that can be used to enhance the detail in the summed image.   For example it contains a very powerful unsharp mask routine.