Journal · Entry 04 · Processing workflow
From raw light to the final image
Astrophotography post-processing: a practical guide using Siril, SETI Astro Suite / Cosmic Clarity, and Affinity Photo.
Astrophotography is more than capturing light. Processing reveals faint structures, controls noise, and balances the target with its stars. The aim is to reveal information in the data, not invent details. Broadband star colours and assigned narrowband palettes need different approaches.
A working guide, not a fixed recipe. Adjust the order to your data and installed tools. If using linear deconvolution or a linear sharpening workflow, move that work before star separation and stretching; Step 8 also covers later finishing. Keep checkpoints and compare every strong adjustment with the original.
Organize your raw data
File Explorer · Siril
Create separate folders for lights, darks, flats, and bias or dark flats. Lights contain the target; darks measure thermal signal and repeatable sensor patterns; flats correct vignetting and dust shadows.
Match calibration frames to the relevant camera settings and conditions. Dark flats match the flat exposure; bias frames measure the electronic offset. Use the calibration method appropriate to your camera, and do not subtract the offset twice.
Keep your original files unchanged. Good organization makes processing easier to repeat.
↑ Step listCalibration, registration and stacking
Siril
Calibrate the lights using the appropriate master calibration frames. Inspect exposures and reject clouds, tracking errors, and badly distorted stars.
Register the remaining images to align their stars, then stack with a suitable rejection method. Winsorized Sigma Clipping is an option when enough frames are available.
Save a 32-bit FITS master. Crop invalid alignment borders before background modelling. Stacking improves signal-to-noise; the result remains linear. A screen auto-stretch is only a preview, not a permanent stretch.
↑ Step listRemove background gradients
Siril
Open the linear master and use Background Extraction. Place samples away from stars, galaxies and nebulosity, select a suitable model, then preview the correction.
Light pollution, moonlight and changing sky conditions can create gradients. Large nebulae and faint dust may fill the frame, however: do not mistake real structure for unwanted background.
Aim for a balanced background without erasing astronomical signal.
↑ Step listCalibrate the colours
Siril
For broadband data, plate-solve the image and use PCC or SPCC where supported. Configure the relevant camera and filter response information when the tool requires it.
Inspect the resulting star colours. Dual-narrowband data needs a different approach: ordinary broadband calibration does not automatically establish meaningful narrowband colours.
Choose and explain the intended channel mapping. Assigned palettes such as SHO are not the colours an unaided eye would see.
↑ Step listReduce noise carefully
SETI Astro Suite · Cosmic Clarity
Save a working copy and use the denoising mode appropriate to your image state and installed version. Linear data may benefit from early treatment; gentle finishing denoise can also be useful later.
Inspect faint nebulosity and the dark background at full size. Reduce the strength if small structures become smooth or waxy.
If your chosen workflow sharpens linear data before denoising, perform that work first. Keep an untouched reference and avoid stacking several aggressive noise-reduction passes.
↑ Step listSeparate the stars
StarNet · StarXTerminator / compatible integration
Create a starless image for the nebula, galaxy and dust, plus a separate stars image. This allows independent adjustments without making the stars overpower the subject.
Follow your tool’s linear/non-linear input requirements. Siril’s StarNet integration provides a pre-stretch option for linear input; use the documented setting rather than treating every star-removal tool identically.
Inspect the starless image for holes, residual halos and removed compact structures. Keep the original master and both outputs.
↑ Step listStretch the image
Siril
Begin gently with Generalized Hyperbolic Stretch (GHS), Asinh, or another suitable transformation. Increase faint structures gradually rather than applying one extreme change.
Monitor the histogram to avoid clipping the background or bright regions. Protect galaxy cores and bright nebula centres while revealing faint outer structure.
Stretch the star layer separately when needed. A permanent stretch changes the data; a display auto-stretch does not.
↑ Step listSharpening and deconvolution
Cosmic Clarity · suitable deconvolution tool
This is a decision point, not a requirement to deconvolve after stretching. Linear deconvolution belongs earlier in a suitable workflow, normally before star removal and stretching. Later sharpening is a separate finishing option.
Use the image state required by your tool, apply a moderate strength and compare at 100% magnification. Look for ringing, dark halos, artificial edges and invented-looking texture.
AI sharpening is not automatically equivalent to measured deconvolution or proven detail recovery. If a feature appears only after strong processing, do not treat it as confirmed astronomical structure.
↑ Step listRefine the colours
Affinity Photo
Use adjustment layers for Curves, HSL, Vibrance and Selective Colour as available. Masks let you target specific regions instead of changing the whole image.
For emission nebulae, balance the chosen H-alpha and OIII mapping. For broadband galaxies, preserve subtle differences between warm stellar populations and blue star-forming regions.
Avoid saturation and channel clipping. Maintain smooth transitions and keep assigned narrowband colours clearly distinguished from broadband colour calibration.
↑ Step listProcess and recombine the stars
Affinity Photo · compatible star recombination tool
Adjust the stars’ brightness and saturation independently. Apply gentle reduction only if they overwhelm the target, preserving differences in star colour.
Recombine using the reconstruction method intended for the way the stars were separated. Screen and additive blending are not interchangeable in every workflow.
Inspect for halos, dark rings, doubled stars and unnatural edges. Stars belong to the scene: balance them with the target rather than eliminating them unnecessarily.
↑ Step listMake the final adjustments
Affinity Photo
Review background balance, contrast, colour consistency, star shapes and preservation of faint dust. Look for clipped shadows or highlights, over-sharpening and processing artefacts.
Inspect both at full resolution and fitted to the screen. Compare with an earlier checkpoint to see whether the image genuinely improved.
Small, selective changes often work better than dramatic global adjustments.
↑ Step listExport and preserve the result
Affinity Photo
Save the editable project with its layers. Keep a high-quality 16-bit TIFF and the original stacked FITS for future reprocessing.
Export PNG or a high-quality JPEG for publication. Convert the publication copy to sRGB and embed its colour profile; keep high-bit-depth masters in their appropriate working colour space.
Include the target and catalogue name, telescope and focal length, camera and filters, mount and guiding, exposure length and total integration, location and sky conditions, and processing software. Keep acquisition facts separate from artistic colour choices.
↑ Step listFinal thoughts
Processing balances astronomical information with artistic interpretation. Every image begins with photons that may have travelled enormous distances before reaching the telescope. Calibration, stacking, stretching, and careful enhancement help turn faint signals into visible nebulae, galaxies, and cosmic dust. The goal is not to invent detail, but to reveal what the captured light can support.
Siril · SETI Astro Suite / Cosmic Clarity · Affinity Photo