Once the images of the deep sky objects acquired as well as the images used for the preprocessing, we haves at the preprocessing stage. This step will allow two processes to be carried out:

  1. Deep Sky Image Preprocessing: This process performs the subtraction or division of the preprocessing images from the deep sky object images
  2. Compositing (assembling) the images pretreated in step 1

If you have made your deep sky object images in color (color CCD, Webcam or APN), you must also make your preprocessing images (Bias, Black and PLU) in color. But the ideal is to produce your images in FIT format. Color CCD cameras offer this format. They save images as black and white (or monochrome) RAW files, but they retain colors (some cameras also offer images in RAW format) . They therefore make it possible to carry out the preprocessing of black and white images which is the format ideal for preprocessing images. After preprocessing, the images are converted to color. For monochrome CCD cameras, the format used is also monochrome FIT. These cameras can be used to produce color images using the LRGB technique.

I will explain the preprocessing of images with the software Maxim DL. If you are using other software, then you can take inspiration from this preprocessing.

Before you begin, you should have completed the following images:

  • The 200 Bias images.
  • The 10 (or more according to your preference) Noir (Dark) images.
  • The 10 (or more according to your preference) PLU (flat field) images.
  • The images of the deep sky object.

I will explain the preprocessing of images produced with a color CCD camera.  LThe file format is RAW with the extension .fit. The preprocessing will therefore be done in black and white before converting the images to color. For all the details on the pretreatment with a monochrome CCD camera (LRGB technique), click on this link. 

The formula used for image preprocessing is as follows:

Calibrated Image = (Raw Image - Black1) / (PLU - Bias)

Black1 = Same exposure time as each of the raw images

Here is the preprocessing of the images with Maxim DL :

1- Pretreatment of deep sky images

  • Create a subdirectory Pretreatment.
  • Copy | To stick on all images (raw images, Bias, Dark, Flat) in this directory.
  • File | Open ... Open all the raw deep sky images to calibrate. Images must come from the directory Pretreatmentt.
  • Process | Set Calibration ...
  • Source Folder : Select the directory Pretreatment and click on OKAY.
  • Click on the button Auto-Generate (ClearOld).
  • The 3 preprocessing images will appear.
  • Click on Advanced Calibration.
  • Configure as follows:
  • The alt attribute of this image is empty, its file name is maxim_ava-cal.jpg.
  • This configuration will depend on the wording given: Calibrated image = (Raw image - Black1) / (PLU - Bias)
  • Click on OK to close the window Advanced Calibration
  • Click again on OK to close the window Set Calibration
  • Process | Calibrate All
  • Queue | Save All

Color conversion

  • Put the preprocessed images in a new folder Color Conversion.
  • Queue | open… Open all preprocessed images. Images must come from the directory Color Conversion.
  • Color | Convert Color… ·
  • Set up the color conversion as follows:
  • The alt attribute of this image is empty, its file name is conver_color.jpg.
  • The color conversion depends on the Bayer matrix of your camera. If the above settings do not work for your camera, you need to change them. If you don't know them, try-and-error.
  • Click on OKAY.
  • Close the image and click on OK to save it.
  • Do the conversion individually for all images.

2- Composition

  • Process | Stack ...
  • Deselect Auto CalibrateAuto Color Convertert and Make Pixel square
  • Click on the button Add Files ... and select all the images to be composed.
  • In the tab Quality, deselect all Criteria
  • Click on the button Aling
  • Mode: Auto - Star Matching
  • Click on the button Combine
  • Combine Method : Sigma-Clip
  • SigmaFactor : 3 or 2 (do both)
  • Select ignore Black Pixels
  • Normalization : Linear
  • Click on GO
  • Click on the button Close
  • Queue | Save : file name for example IC434-Ha-Bin 1 × 1-Sigma 3
  • Compare the Sigma image 3 and 2 and choose the best image. Always favor the Sigma 3 image (which rejects fewer pixels). If there are cosmic rays or other parasites present, then choose the Sigma 2 image.

Image processing for import into Photoshop

  • Open the FIT image in Maxim DL
  • Filter | Digital Development ...  
  • FilterType : FFT-Low-Pass
  • FFT Hardness :Mild
  • Click on OK
  • File | Save As ...  
  • Type : TIFF Pictures
  • Size : I6-Bit Int
  • Uncompressed
  • Auto-Stretch must be checked (will keep the maximum and minimum luminances)
  • Click on Save

Richard Beauregard
Revised 2021/01/27

Sky Astro - CCD