From raw frames to a finished image

The order matters. Some tools only work on linear data, so they come before the stretch. After the stretch, edits are judged by eye.

1. 1

   Calibrate

   Calibration removes the fingerprints of your camera and optics from every light frame: sensor noise and hot pixels (darks), the pattern of the sensor's own electronic offset (bias), and dust shadows and vignetting (flats).

   • Put your lights, darks, flats and bias frames in separate folders.
   • In Siril the easiest route for a beginner is a preprocessing script from the Scripts menu (a one-shot color camera like yours uses the one-shot-color version). It builds the master frames, calibrates every light and, for a color camera, debayers them.
   • Or do it by hand in the Conversion and Calibration tabs: stack each set of calibration frames into a master, then apply the masters to the lights.

   If you skip it: Dust donuts, a bright center with dark corners, and fixed-pattern noise are baked into the stack and are much harder to fix later.

   Calibration frames guide →

2. 2

   Register and stack

   Registration lines up the stars in every frame. Stacking averages them, rejecting things that appear in only a few frames, like satellite trails and cosmic-ray hits. Averaging cuts the random noise, and that is what makes a faint target usable.

   • Register with global star alignment. Check the frame list and drop frames with bad tracking, clouds or poor star shape.
   • Stack with a rejection method (sigma-clipping variants are the usual choice) so trails and outliers drop out.
   • The result is a single linear image, saved as FITS. This is your master stack.

   If you skip it: Without rejection, satellite and plane trails stay in the final image. Without registration there is no stack.

3. 3

   Prepare the linear image

   Some tools only work properly on linear data, before the stretch. This is the short list of jobs to do while the image still looks nearly black.

   • Crop the ragged stacking edges.
   • Remove gradients with Background Extraction, which models and subtracts the light-pollution glow.
   • Calibrate the color with Photometric (or Spectrophotometric) Color Calibration, which uses the stars in the frame to set the white balance. The image has to be plate solved first.
   • Optional: deconvolution to sharpen slightly, and noise reduction while the data is linear.
   • Save a linear copy as FITS.

   If you skip it: Stretching first makes gradients and color casts much harder to remove, and some tools simply stop working correctly.

4. 4

   Stretch

   This is the step where the faint detail becomes visible. It is the main point of no return: from here on, the image is non-linear.

   • Use Histogram Transformation, Asinh or GHS in Siril.
   • Several small stretches beat one big one, and you stop when the sky is a dark gray, not black or washed out.

   If you skip it: A stack that is not stretched stays black on every screen.

   Stretching guide →

5. 5

   Fine-tune the stretched image

   After the stretch there is usually a flat, slightly dull picture. A little shaping in Siril, or in Lightroom, gives it contrast and color.

   • Curves in Siril: a gentle S-curve for contrast, and a nudge to the black point if the sky is too light.
   • Color saturation in Siril, if you want it, applied modestly.
   • Then save a 16-bit TIFF for the finishing steps in Lightroom.

   If you skip it: It will still be a valid picture, just flatter and less colorful than the data can show.

6. 6

   Finish and share

   Tone and color polish, local contrast on the target, a final crop, and export. Lightroom is good at this part. The next section walks through it.

   • Import the TIFF into Lightroom.
   • Edit, then export a JPEG or PNG for sharing, and keep the TIFF as your master.

   If you skip it: Nothing breaks, but this is where the image goes from correct to good.

Menu names and options change between Siril versions. This follows the current documentation (1.4), so check the labels in your own copy.

Export a 16-bit TIFF from Siril and import it into Lightroom. The big stretch is done, so this is polish. Go in small steps, and compare with the original often.

• Start with small moves in the Basic panel

  The big stretch is already done, so use Exposure only for small corrections. Nudge Contrast, and use Whites and Blacks to set where the sky and the brightest parts sit. Lower Highlights a little if the bright core looks heavy.

• Shape it with the Tone Curve

  A gentle S-curve adds punch to the middle tones. Keep the dark end from touching the bottom so the faint outer detail of the target survives.

• Color: Color Mixer, then Color Grading

  Use Saturation and Vibrance in small amounts, and use the Color Mixer to lift the hydrogen reds or the blue reflection nebulosity without making the whole picture neon. If the sky has a color tint, Color Grading (or the shadows tint) can neutralize it.

• Use masks to treat the target and the sky separately

  A radial gradient over the nebula or galaxy lets you add contrast and clarity there without brightening the sky. The Select Sky and Select Subject masks can help on wide-field shots. Keep each adjustment modest.

• Noise reduction and sharpening: less is more

  Lightroom's Denoise can calm noise in faint areas, but compare before and after at 100 percent, because it can smooth away faint stars and fine texture. Use only light sharpening, and mask it to the target so it does not sharpen the noise in the sky.

• Crop and export

  Crop for composition last. Export a JPEG in sRGB for sharing, and keep the TIFF as your master.

Panel names are from current versions of Lightroom and may differ slightly in yours. Lightroom does not have layers, so it cannot combine a tracked sky with a separate foreground frame. See the Milky Way tips for that.

• Stretching before removing the gradient and calibrating the color, which turns easy fixes into hard ones.
• Skipping flats. Dust donuts and vignetting that were avoidable end up as heavy-handed edits.
• One enormous stretch that burns the stars and the bright core. Take it in steps.
• Pushing saturation and contrast until the picture looks good on a phone and strange on a monitor. Check it on a bigger screen.
• Throwing away the linear stack. Keep the FITS so you can redo the stretch without re-stacking.
• Saving every step as a JPEG. Use FITS or TIFF until the final export.

What order should I process astrophotography images in?

Calibrate, register and stack, then do the linear clean-up (crop, gradient removal, color calibration, optional deconvolution and noise reduction), then stretch, then the finishing touches like contrast, color and a final crop. Steps that need linear data come before the stretch.

Can I stretch in Lightroom instead of Siril?

A stack is too dark for Lightroom's sliders to handle well, and the linear-only steps would be lost. Do the main stretch in Siril, where the tools are made for it, and use Lightroom for the finishing.

Do I have to use a script for stacking?

No. A script is the easiest way to get a correct result, because it follows the standard order of calibration and stacking for you. Doing it by hand teaches you what each step does, and gives you more control once you know it.

Why is my stack still noisy after processing?

Noise falls with the square root of the total exposure time, so the real fix is more data. Processing can hide noise, but a gentle stretch and good calibration help the most. If it is still grainy, add more nights on the same target.

Which file should I keep?

Keep the linear FITS stack, since you can redo everything from it, and the final 16-bit TIFF. The JPEG you export for sharing is only a copy.