Why Color Calibration Matters
If you have ever stacked your subframes and wondered why your galaxy looks green instead of blue, or your nebula has an orange tint that you know is not real, you are not alone. Color calibration is one of the most important yet overlooked steps in astrophotography image processing. Without it, your final image will not faithfully represent the actual colors of the celestial object you worked so hard to capture.
The problem comes from several sources: your camera sensor has different sensitivities in red, green, and blue channels; light pollution adds unwanted color casts; atmospheric extinction selectively absorbs certain wavelengths; and your filters alter the spectral balance. All of these factors combine to shift the colors of your stacked image away from reality.
Siril’s Three Calibration Methods
Siril offers three ways to calibrate colors, each with different levels of accuracy and ease of use:
1. Manual Color Calibration
The simplest approach. You manually adjust the RGB channel levels by eye, trying to get the sky background to a neutral gray. This method is fast and requires no internet connection or plate solving, but it is inherently subjective. What looks right to you may not match the true colors of the object.
When to use it: Quick previews, or when your image cannot be plate solved for some reason.
2. Photometric Color Calibration (PCC)
PCC takes a more scientific approach. It plate solves your image to know exactly where it is pointing in the sky, then queries the Gaia DR3 catalog for stars within your field of view. By comparing the known photometric magnitudes of those stars (in B, V, and R bands) to how bright they appear in your image, Siril computes a correction matrix that rebalances your RGB channels.
When to use it: When you need reliable color accuracy but do not have detailed sensor and filter response curves for your setup.
3. Spectrophotometric Color Calibration (SPCC)
SPCC is Siril’s most advanced and most accurate color calibration method. It also uses Gaia DR3 data, but instead of just comparing broad-band magnitudes, it compares the full spectral response of each star. SPCC takes into account the spectral sensitivity curve of your specific camera sensor and the transmission curve of your filters, producing a calibration that is tailored to your exact imaging setup.
The result? Colors that are as close to reality as you can get from the ground. SPCC renders PCC essentially obsolete for most users.
When to use it: Always, if possible. This is the recommended method for any serious astrophotography workflow.
Prerequisites: What You Need Before You Start
Before running any photometric calibration in Siril, you need to check a few boxes:
- A stacked, linear image. This is the most critical requirement. Your image must not have been stretched yet. Color calibration works on linear data. If you have already applied a histogram stretch or curves adjustment, the photometric measurements will be wrong and the resulting colors will be unreliable. Run calibration immediately after stacking, before any other processing.
- A plate-solved image. Both PCC and SPCC require Siril to know exactly where your image is in the sky. If your image has not been plate solved yet, you will need to do that first. Siril can plate solve online using the Gaia catalog or offline using a local astrometric catalog.
- An internet connection (for online SPCC) or the local SPCC catalog downloaded. Siril 1.4.0 and later support an offline Gaia SPCC catalog, which is especially useful if the Gaia archive is down for maintenance.
- Your camera and filter information. For SPCC, you will get the best results if you have selected your camera sensor and filter profiles in the SPCC dialog. Siril includes a database of common sensors and filters, and you can add custom ones if needed.
Step-by-Step: Calibrating with SPCC
Here is the recommended workflow using Spectrophotometric Color Calibration:
Step 1: Stack your image. Complete your calibration frames (darks, flats, bias) and stack your light frames in Siril as usual. When stacking is done, you will have a 32-bit FITS image with linear data.
Step 2: Plate solve. If Siril did not plate solve during stacking, go to the Astrometry panel and run plate solving now. You can use the online Gaia solver or a local catalog. Once solved, the image header will contain WCS information that tells Siril exactly where each pixel is on the sky.
Step 3: Open SPCC. Go to the Processing menu and select Spectrophotometric Color Calibration, or press Ctrl+Shift+C. The SPCC dialog will open and automatically check the Gaia archive status. If you are using the offline catalog, it will use that instead.
Step 4: Configure your equipment. In the dialog, select your camera sensor from the dropdown list. Siril includes spectral response curves for popular sensors like the Canon EOS series, ZWO ASI cameras, and others. Also select your filter if you are using one, such as an IR-cut filter, a light pollution filter, or narrowband filters.
Step 5: Choose a white reference. SPCC allows you to balance your image using a white reference. If you want to use sunlight as your reference (the most common choice), select Star, type G2(v) from the white reference list. This tells SPCC to treat a G2V star (Sun-like) as white, which gives you colors as they would appear to the human eye.
Step 6: Run the calibration. Click Apply. Siril will query the Gaia DR3 catalog for stars in your field, compare their known spectral data to how they appear in your image, compute the correction factors, and apply them to your RGB channels. The whole process typically takes a few seconds for a moderately populated field.
Step 7: Check the results. After SPCC finishes, your image may look different than expected. The sky background may have shifted to a slight blue or brown tint, and the object itself may look less vivid than before. This is normal. The calibration has corrected the color balance, but the image is still linear and has not been stretched yet.
Step 8: Background extraction. If you have not already done so, now is a good time to run Siril’s background extraction tool to remove any remaining light pollution gradients. Some imagers prefer to do background extraction before color calibration; the order is a topic of ongoing debate, but both approaches are valid. What matters is that both steps are done on linear data.
Step 9: Stretch and enjoy. Apply your histogram transformation or arcsinh stretch. You should now see colors that are much closer to the true colors of your target. Galaxies will show their natural blue and yellow hues, nebulae will display realistic reds and teals, and star colors will range from warm orange to cool blue depending on their spectral class.
Common Pitfalls and Troubleshooting
My image turned green after calibration
This is the most common complaint. A green cast usually means one of three things: the image was already stretched before calibration, the sensor profile was incorrect, or there is a strong gradient that needs to be removed with background extraction first. Make sure you calibrate on linear, unstretched data, and try running background extraction before SPCC if gradients are severe.
SPCC says no stars were found
If Siril cannot find catalog stars in your field, the most likely cause is that plate solving failed or the WCS data is missing. Check that the astrometric solution is present in the image header. Another possibility is that your field is very sparse or in a region with few catalog stars. In this case, manual calibration or PCC may be your fallback option.
The colors look wrong after stretching
If colors looked correct in linear but shifted after stretching, make sure you are using a stretch method that preserves color ratios. The standard histogram transformation in Siril applies the same stretch to all channels, which is fine. If you are using separate curves on each channel, you may be throwing off the calibration. Always apply the same transformation to all three channels.
SPCC is grayed out
If the SPCC menu item is disabled, your image has not been plate solved. Run plate solving first from the Astrometry panel. You need an active internet connection for online plate solving, or a local catalog for offline solving.
Manual Calibration: When All Else Fails
Sometimes SPCC and PCC are not available: you are offline, the Gaia archive is down, or your image cannot be plate solved. In these cases, manual calibration is still a viable option.
To use manual calibration, open the Color Calibration tool from the Processing menu. You will see sliders for red, green, and blue. The goal is to make the sky background neutral gray. Click on a blank area of sky in your image to sample it, then adjust the sliders until the sampled region shows roughly equal RGB values.
While manual calibration is less accurate than the photometric methods, it is still far better than skipping calibration entirely. Even a rough manual balance will dramatically improve your image compared to the raw stacked output.
The Bottom Line
Color calibration is not optional if you want your astrophotos to look their best. Siril gives you three tools to do it, from quick and manual to scientifically rigorous. If you are using Siril 1.4.0 or later, SPCC with the offline catalog is the way to go for most images. It is fast, accurate, and takes the guesswork out of color balancing.
Remember the golden rule: calibrate on linear data, before you stretch. Get this step right, and the rest of your processing pipeline will produce noticeably better results.
Related Reading
If you found this tutorial helpful, you may also want to check out these posts:
- Siril Workflow: From Stacking to Final Image – A complete walkthrough of the Siril processing pipeline
- Siril Background Extraction: Removing Light Pollution Gradients – How to clean up your image before calibration
- Calibration Frames Explained: Darks, Flats, and Bias Frames – Getting your calibration frames right for clean stacking
Questions about color calibration? Drop a comment below or reach out through the gallery. Clear skies!
