Your First DSLR Stack: From a Folder of Raw Files to a Picture

A camera on a tracker gives you a memory card full of raw files, and getting from there to a photograph is where most people stall. The steps are not hard, but nobody says plainly what goes where and what happens next. This post does, using two real sessions with the same Canon, one on the Pleiades and one on Orion, and quoting what the software reported at each stage.

The folder

Calibration is decided by where each file sits. This is the layout every tutorial teaches and every stacking program reads:

Pleiades/
  lights/    111 x IMG_7156.CR2 ...   the photographs of the sky
  darks/      37 x IMG_7260.CR2 ...   same exposure, same ISO, lens cap on
  flats/      17 x IMG_7290.CR2 ...   a plain, evenly lit surface
  biases/     17 x IMG_7724.CR2 ...   the shortest exposure the camera allows, cap on

Drop the top folder onto Akastroid. It reads the four subfolders by name (dark/darks, flat/flats, bias/biases/offset all work) and reports what it found before doing anything:

Calibrating with 37 darks, 17 flats, 17 bias.

A raw file has no header field saying what kind of frame it is, so the folder serves as the label. Put a dark in lights/ and it will be graded as a very bad photograph of nothing and rejected. That is the right outcome, but a confusing one.

If you only have lights, that is fine. The session runs without calibration and says so:

No calibration frames detected. Continuing with light-frame optimization.

Hot pixels are then repaired statistically from the frames themselves, before demosaicing. The timing matters: once a hot pixel has been interpolated into its neighbors it is no longer an isolated speck and cannot be found. (What each calibration frame fixes.)

What happens to the darks

Thirty-seven 24-megapixel darks held in memory at once as floating point take eleven gigabytes. Akastroid builds the master dark as a median when the set fits in a couple of gigabytes and as a running mean when it does not. On a set this size you will not see the difference in the result.

It also checks that the darks match the lights. A dark shot at a different exposure or ISO measures a different sensor state, and subtracting it leaves a residual pattern. If they don’t match, the session says so, applies them anyway, and tells you to reshoot:

The darks do not match the lights: the darks were exposed for 30s and the lights for 120s …

On both of these sessions they matched and no warning appeared.

Grading, and the frames you lose

Every light is measured: how many stars it has, how sharp and how round they are, how bright the sky is, whether a satellite crossed it. The frames are then scored against each other, because “good focus” only means something relative to the rest of the night.

The Pleiades session had 111 frames and 67 were selected. The rest scored below the session’s own threshold (soft focus, a passing cloud), and the quality verdict was “Challenging”. Orion had 134 frames, with 119 selected. Both sessions carried the same warning:

Stars are slightly oval across the whole session (eccentricity 0.76). That is the optics rather than the tracking (coma, field curvature or camera tilt), so no frames were rejected for it.

The last clause is what matters. Oval stars in every frame come from the lens, not from tracking, and rejecting frames for a fault they all share would leave nothing to stack. The app mentions it once, at session level, and carries on.

Registration

Frames are aligned on star patterns, not on where the mount thinks it pointed. A camera lens distorts the field slightly, stretching the corners a little differently from the center. So after the usual rotation-and-shift fit, a second pass lets the fit stretch too, and keeps that only if it lands the corner stars closer. A few frames in each session could not be matched at all (three of the Pleiades, seven of Orion) and were excluded, with their names in the log.

Integration

This is where the two sessions diverged, and the log says why:

Pleiades: Integration: Sigma clipping (64 frames: deep enough for sigma clipping to remove trails cleanly)

Orion: Integration: Linear fit clipping (112 frames whose sky level drifted through the session: fitting each pixel’s stack rejects outliers without mistaking the drift for one)

The Orion sky brightened through the night. A plain sigma clip treats every frame as a sample of the same sky, so a smooth drift makes the earliest and latest frames look like outliers, and those are often the best data. Linear-fit clipping fits the drift first and rejects what is left. Orion rejected 8.7% of pixel samples (a DSLR’s hot-pixel population plus the drift) and the Pleiades 3.4%.

Neither session was drizzled, and the log explains that too. Stars spanned 3.6 and 3.9 pixels, which is sampled finely enough that a finer grid would enlarge the file without recovering detail.

The border

Field rotation and dithering leave a ragged edge where only a few frames overlap. The trim follows where that edge is rather than taking a fixed percentage:

Trimmed the frame edge where field rotation left partial coverage: 5 px left, 40 right, 0 top, 43 bottom.

Color and gradient

Both sessions had the background gradient removed and the color balanced from the stars in the field: 1473 on the Pleiades, 1500 on Orion. The Pleiades needed a quadratic gradient model rather than a plane. The lens vignettes red and blue slightly differently from green, and the flats correct brightness more exactly than color. The app detects that from the residual and steps up the model by itself. (Why the reference star matters for the balance.)

Finishing

The stretch, noise reduction, star handling and contrast are set from measurements of the finished linear stack: how much of the frame is sky, how bright the brightest structure is against the noise, and how clean the stack is. The log lists each decision:

Tuned background: 95% of the frame is empty sky, so the background is set darker to keep it from washing out Tuned highlights: the frame has a very bright core, so highlight protection is increased to keep its structure

Each of those has a slider in the Adjust tab, set to AUTO until you move it.

What you get

Pleiades: 64 frames, noise reduced 4.5× against a single frame, the reflection nebulosity around Merope and Maia visible, and round stars out to the corners. Orion: 112 frames, noise reduced 6.5×, the Trapezium region held short of white, the Running Man blue, and clean dust lanes.

Neither is a competition image. Both are photographs, made from a folder of raw files in about four minutes, with a log of what was done and why. On your second stack you will probably start reading that log and disagreeing with parts of it.

Summary

• Put lights/ darks/ flats/ biases/ under one folder and drop the folder.
• The app reports what it found and whether the darks match. Read that line.
• Frames are scored against the session. A fault every frame shares is not a reason to reject any of them.
• The integration method is chosen from the data, and the log tells you which and why.
• Every automatic decision has a slider. Start on AUTO.