Deep sky stacking

The Deep sky workspace turns a night's worth of light frames into one clean, deep image. The workflow is simple (add frames, register, stack), with modern rejection methods, drizzle, comet stacking and mosaics.

Contents

• Overview of the process
• Taking calibration frames
• The Deep sky workspace
• Adding frames
• The frame list
• Registering
• Choosing frames
• Stacking settings
  • Calibration
  • Registration
  • Combination and rejection
  • Normalisation and weighting
  • Output area
  • Drizzle
  • Comets
• Stacking
• The result
• File lists
• Recommended settings

Overview of the process

1. Calibration removes the camera's own signal from each light frame: the bias (read-out offset), the dark current and hot pixels (darks), and vignetting and dust shadows (flats).
2. Registration finds the stars in every frame and works out how each frame is shifted, rotated and scaled relative to a reference frame.
3. Stacking warps every frame onto the reference and combines them pixel by pixel, rejecting satellite trails, aircraft, cosmic rays and hot pixels.

Stacking N frames improves the signal-to-noise ratio by about √N, so 16 frames are four times less noisy than one.

Taking calibration frames

Frame	How to take it	How many
Lights	The target	As many as possible
Darks	Same exposure, gain, offset and sensor temperature as the lights, with the telescope covered	20–50
Flats	Evenly illuminated (flat panel, sky flat or T-shirt), same focus, camera rotation and filter as the lights, exposed to about half the full well	20–50 per filter
Dark flats	Same exposure as the flats, telescope covered	20–50
Bias / offset	Shortest possible exposure, telescope covered	50–100

You need either dark flats or bias frames for the flats. With cooled cameras you can reuse darks and bias frames for months at the same temperature and gain. You can capture all of these with the Observatory or the sequencer.

The Deep sky workspace

Area	Contents
Left	Action buttons in workflow order
Centre, top	Preview of the selected frame, or the stacked result
Centre, bottom	The frame list
Right	Stacking settings (a property grid) and Stacked image adjustments
Summary line	Numbers of lights (total, checked, and total exposure in minutes), darks, flats, dark flats and bias frames

Adding frames

Button	Adds
Light frames…	Images of the target
Dark frames…	Darks
Flat frames…	Flats
Dark flat frames…	Darks for the flats
Bias / offset frames…	Bias frames
Master frames (FITS)…	Ready-made master bias, dark or flat files. The type is taken from the file name (it must contain "bias", "dark" or "flat")

Supported formats are FITS, XISF, TIFF, DNG, CRW, CR2 and CR3 raw files, and PNG, JPEG and BMP. Colour (Bayer) sensors are detected from the FITS BAYERPAT keyword or the raw file, and debayered after calibration. You can also drag files onto the preview.

The frame list

Column	Meaning
☑	Whether the frame is used
File	File name
Type	Light, Dark, Flat, DarkFlat or Bias
Exposure	Seconds
ISO/Gain	From the file
Temp	Sensor temperature
Filter	From the file header
Score	Quality score after registration (★ marks the reference frame)
Stars	Stars detected
FWHM	Mean star full width at half maximum in pixels
Ecc.	Mean star eccentricity (0 is perfectly round; above about 0.5 shows trailing)
Background	Sky background level
Status	Registration result or error

Click a column heading to sort, for example by Score or FWHM. Click a row to preview the frame. Check all, Uncheck all, Remove selected and Clear list manage the list.

Registering

Register checked frames analyses every checked light frame:

1. Calibrates it (if masters are available), so hot pixels are not mistaken for stars.
2. Detects stars with an adaptive moment method and measures their brightness, FWHM, half-flux radius and eccentricity.
3. Computes a score: the sum of the square roots of the stars' signal-to-noise ratios, divided by FWHM × (1 + eccentricity). Many sharp, round stars give a high score.
4. Picks the best frame as the reference (unless you chose one with Use selected as reference).
5. Matches each frame's stars to the reference with triangle-similarity pattern matching and fits the chosen transform with RANSAC, which ignores false matches. The number of matched stars and the residual error are recorded.

Frames that cannot be matched (clouds, wrong target, trailing) are marked in Status and excluded from stacking.

Choosing frames

After registering, sort by Score, FWHM or Ecc. and uncheck poor frames: those with trailing, clouds, or much larger stars than the rest. The settings Keep percent and Min score ratio can do this automatically at stacking time.

Stacking settings

The settings grid on the right has the following options.

The stacking settings can be changed while frames are registered or stacked: the run keeps the settings it started with, and the changes apply to the next run.

Calibration

Setting	Meaning
Dark combine, Flat combine, Bias combine	How calibration frames are combined into masters. Defaults: median kappa-sigma for darks, median for flats and bias
Optimize dark	Scale the master dark to minimise noise in each light. Use it when darks were taken at a slightly different temperature or exposure
Hot pixel removal / Cold pixel removal	Replace defective pixels, found from the master dark and by comparison with their neighbours, by the median of same-colour neighbours
Pedestal	A small constant added after dark subtraction so that noise is not clipped at zero
Save calibrated / Calibrated folder	Also save each calibrated light frame

Calibration order follows standard practice: subtract bias (or use the bias-subtracted dark), subtract the dark (scaled by exposure time when it differs), and divide by the flat normalised to a mean of 1 for each colour of the Bayer matrix. Flats are calibrated with dark flats or bias first.

Registration settings

Setting	Meaning
Detection sigma	Star detection threshold in units of background noise (5 by default). Lower it for sparse fields, raise it for noisy frames
Max stars	Maximum stars used per frame (1500)
Alignment	Transform model: Translation, Similarity (shift, rotation, scale), Affine (default), Homography (perspective), or Polynomial2 (second-order, for wide fields with distortion)
Debayer	Bilinear, SuperPixel (2×2 binning, no interpolation), MalvarHeCutler, or EdgeDirected (default, sharpest)
Bayer override	Force a Bayer pattern (RGGB, BGGR, GRBG, GBRG) if the file does not state it correctly
Interp	Interpolation when warping frames: Nearest, Bilinear, Bicubic or Lanczos3 (default, sharpest)

Combination and rejection

Light combine chooses how the aligned light frames are combined:

Method	Description	Best for
Average	Plain mean; no rejection	Very few frames
Median	Median of each pixel	Quick results, 5–10 frames
KappaSigma	Mean after iteratively rejecting values more than κ standard deviations from the mean (default: κ 2.5, 5 iterations)	General use, 10+ frames
MedianKappaSigma	Like kappa-sigma but rejected values are replaced by the median	Darks and bias
WinsorizedSigma	Sigma clipping with Winsorized (robust) statistics	10–20+ frames
LinearFitClipping	Fits a line to the sorted values and rejects points far from it	15+ frames with varying sky
GeneralizedEsd	Generalised extreme Studentized deviate test (Rosner 1983)	Large stacks (50+)
PercentileClipping	Rejects values outside percentile limits	Very small stacks
AutoAdaptiveWeightedAverage	Iterative re-weighting that down-weights outliers smoothly (Stetson)	Many frames, no hard cut-off
EntropyWeightedAverage	Weights each frame locally by the information content of its histogram, keeping bright cores from frames where they are not saturated	High-dynamic-range targets such as M 42
Maximum / Minimum	Brightest or faintest value	Star trails, special effects

The κ low and high values, iterations, percentile limits and the ESD significance and maximum outlier fraction are in the expandable Light combine group.

Normalisation and weighting

Setting	Options
Normalization	None, Additive (match background offsets), AdditiveScaling (match offsets and noise, default), Multiplicative (match levels by scaling)
Weighting	None, Noise (inverse noise variance, default), Score (registration score), Snr
Per channel background calibration	Equalise the background of the red, green and blue channels so the sky is neutral grey
Keep percent	Use only the best N% of frames by score
Min score ratio	Reject frames scoring below this fraction of the reference frame's score

Normalisation compensates for the sky getting brighter or fainter during the night (moonrise, twilight, light pollution changing with altitude), so that rejection compares like with like.

Output area

Mode	Result
Standard	The reference frame's area
Mosaic	The union of all frames: everything any frame covered, for mosaics and drifting fields
Intersection	Only the area covered by every frame, with no ragged edges
Custom	A rectangle you specify in Custom rect (x, y, width, height in reference pixels)

Drizzle

Drizzle scale (1, 2 or 3) reconstructs a higher-resolution image from dithered, undersampled frames (Fruchter & Hook 2002). Each input pixel is shrunk to a "drop" of size Drizzle pix frac (0.8 by default) and dropped onto the finer output grid. Drizzle helps when your stars are only 1–2 pixels wide and you dithered between frames; it needs many frames (30 or more) and costs memory and time.

Comets

Comets move against the stars during the night. Comet chooses how to handle this:

Mode	Result
None	Normal star-aligned stack; the comet is smeared
CometAligned	Aligned on the comet's nucleus; the comet is sharp and the stars are trailed
StarsAndComet	Both stacks are made and combined, keeping sharp stars and a sharp comet

Mark the comet's nucleus with Mark comet in selected frame: select a frame in the list, click the button, then click the nucleus in the preview. Do this in at least two frames, preferably the first and the last; Sidralune interpolates the comet's position in every other frame from the observation times.

Stacking

Click Stack checked frames. Stop, under it, ends registering or stacking part way; nothing is kept from a stopped run. Sidralune:

1. builds master bias, dark, dark flat and flat frames from the calibration frames (or uses the masters you loaded),
2. calibrates and debayers each light frame and warps it onto the reference,
3. writes the warped frames to temporary files, so memory use does not grow with the number of frames,
4. combines them in strips, in parallel on all processor cores, with the chosen rejection,
5. neutralises the background if requested.

Progress and the current stage appear in the status bar. When it finishes, the result is shown and a summary reports the number of frames used, the total exposure, the time taken and the method.

The result

The stacked image is linear (unstretched) 32-bit floating point data. The preview shows it auto-stretched. Stacked image adjustments changes the preview only:

Slider	Effect
Black point	Where black starts
Midtones	Brightness of the midtones
Saturation	Colour saturation

Button	Action
Save stacked image…	Save as 32-bit FITS (Autosave.fits by default), XISF or TIFF to keep the full data, or as PNG or JPEG with the preview stretch applied
Open in Process	Send the result to the Process workspace for background extraction, stretching and so on

Keep the 32-bit linear file: all processing should start from it.

File lists

Save file list… stores the frame list, check marks and types in a .sdstack file; Open file list… loads it again. Use this to come back to a project or to add frames from a second night.

Recommended settings

Situation	Settings
Fewer than 10 frames	Median, Alignment Affine
10–30 frames	KappaSigma or WinsorizedSigma, AdditiveScaling, Noise weighting
30+ frames	WinsorizedSigma or LinearFitClipping; consider GeneralizedEsd for 50+
Satellite trails	WinsorizedSigma or GeneralizedEsd
Bright cores (M 42, M 31)	EntropyWeightedAverage
Undersampled, dithered frames	Drizzle scale 2, pix frac 0.7–0.8
Wide field with a camera lens	Alignment Polynomial2 or Homography
Two nights with different framing	Mode Intersection (or Mosaic to keep everything)
Colour camera	Debayer EdgeDirected, Per channel background calibration on