My photograph of the Pillars of Creation, in the Eagle Nebula (M16), is on the Night sky page. I shot it in Guyana in 2023 at 250 mm and f/4.9, with 45 minutes of exposure in total, stacked and processed in Siril. The steps below are how a stack like that becomes a picture.
What the camera records
Each exposure is a grid of numbers: how much light reached each pixel. The nebula is in there, mixed with noise from reading the sensor, noise from heat, noise in the light itself, and the glow that Georgetown throws into the sky. Processing pulls the signal away from the rest.
Calibration
Three kinds of extra frames do most of the cleaning.
- Bias frames are the shortest exposures the camera can take. They record the noise of reading the sensor.
- Dark frames match the exposure time and temperature of the real shots, with the lens covered. They record the noise that heat adds.
- Flat frames are shots of an evenly lit surface. They record vignetting and dust on the sensor.
Subtract the darks and bias from each light frame, then divide by the flat. The maths is simple. The hard part is taking flats at the end of a long night, when you are tired. Skip them and you get rings and gradients you cannot fully remove later.
Stacking
Stacking lines the stars up across every frame, throws out outliers such as satellite trails, and averages what is left. Noise falls with the square root of the number of frames, so four times the frames gives about half the noise, as long as tracking held. Bad guiding or wind caps that gain quickly.
I use Siril for this. It does calibration, alignment and stacking in one place, and it is free.
After the stack
The order matters more than the software.
- Remove the gradient that light pollution leaves across the frame.
- Balance the colour against the stars or a reference area.
- Apply deconvolution, gently, to recover some sharpness.
- Stretch the histogram so the faint gas comes up without burning out the bright stars.
- Reduce noise last, so fine detail is not smeared early.
Keep the files from each step, and note what you changed.
The Moon works the same way at a different scale. The Colors of the Moon is a stacked composite, with the colour pushed until differences in the rock across the surface show.
What Guyana adds
- Cloud. Tropical cloud ends a session without warning. On a clear night with nothing to shoot, build a library of calibration frames.
- Heat. Warm nights warm the sensor, so darks have to match the temperature of the session.
- Sky glow. Around Georgetown, narrowband filters help if you have them. Without them, keep exposures short and remove the gradient hard.
- Power. A blackout mid-sequence corrupts frames. Use stable power or a battery.
Where it goes
A processed image is where teaching starts: what the object is, how far away it is, and why it has those colours. On AstroPhotosGY that is how processing turns into outreach.