The following steps are going to sound very complicated. And if you had to do them manually to each frame, it would be. Luckily, you don't have to do it manually. You can get an astronomical image processing program, like DeepSky Stacker or Images Plus, or Maxim DSLR, or Nebulosity, and they will all perform all of these steps for image calibration automatically.

These specific steps are described in detail here for those who are curious about exactly what is happening during image calibration.

Basic Calibration

1. Convert the Raw Files to 16-bit Linear CFA Files

   • First convert the raw light and dark frames from the camera's proprietary raw file format to linear, black and white, CFA, 16-bit TIFF (or FITS) files. No sharpening or tonal adjustments should be applied. CFA means Color Filter Array where the data from the sensor is not de-Bayerized and turned into a color image.

2. Create a Master Dark Frame

   • Average the dark frames together to create a master dark frame.

3. Calibrate the Individual Light Frames

   • Subtract the master dark frame from each individual light frames.

4. Save the Calibrated Light Frames/

This processing yields basic-calibrated, linear, 16-bit, Color light frames. In the next section, we will align and register these calibrated individual light frames and stack them.

Advanced Calibration

1. <

   Convert the Raw Files to 16-bit Linear CFA Files

   • First convert the raw light, dark, flat-field and bias frames from the camera's proprietary raw file format to linear, black and white, CFA, 16-bit TIFF (or FITS) files. No sharpening or tonal adjustments should be applied. CFA means Color Filter Array where the data from the sensor is not de-Bayerized and turned into a color image.

2. Create a Master Bias Frame

   • Average the bias frames together to create a master bias frame.

3. Create a Master Flat-Field Frame

   • Average the individual flat-field frames to create a combined raw flat-field frame. If you have shot twilight flats with the telescope drive turned off, use a median combine to eliminate any stars that were recorded in the field. Technically, twilight flats shot at a fixed exposure (camera not on auto-exposure) must be normalized first before they are median combined, but if you take twilight flats together quickly enough to minimize the effects of changes in sky brightness as twilight falls, you can get buy without normalizing them first.

   • Average the individual flat-field darks to create a combined flat-field dark frame.

   • Subtract the master flat-field dark frame from the combined flat-field frame.

   • Convert the master flat-field frame to grayscale (if your software doesn't do this automatically).

4. Create a Scalable Master Thermal Frame

   • Average the individual dark frames to create a master dark frame.

   • Subtract the master bias frame from the master dark frame. This yields a master thermal frame.

5. Calibrate the Individual Light Frames

   • Scale the master thermal frame to match the thermal signal in the light frame (usually done automatically in software).

   • Subtract the scaled thermal frame from the individual light frames.

   • Subtract the master bias frame from the individual light frames.

   • Divide the individual light frames by the master flat-field frame.

   • Save the individual calibrated light-frame images.

6. De-Bayerize the Light Frames (Interpolate into Color Images)

   • Since the 16-bit linear light frames were calibrated as CFA black and white images, the Bayer matrix needs to be "de-Bayerized", interpolated, and the color constructed. This cannot be done in Photoshop, it needs to be done in the same astronomical image processing program that you used to calibrate the CFA lights.

7. Save the Calibrated Light Frames

It is important to perform these steps in this order. This processing yields advanced-calibrated, linear, 16-bit, Color light frames. In the next section, we will align and register these calibrated individual light frames and stack them.

If you decide to do basic calibration, all you need to do is create a master dark frame and subtract it from each individual light frame, and then de-Bayerize the light frames and interpolate them into color images.