Turn a folder of frames into a stack animation.

Deep Stack takes a folder of frames of one target and averages them one at a time. Each step of that running average becomes one frame of an animation, so the viewer sees noise fade and faint detail appear as integration time grows. You can put live measurements, plots, and your own labels on top of the animation.

• Folder of frames in, animation out. Frames are read from one folder in filename order. Step 3
• Display control. Stretch, curves, and invert set the look of both the preview and the export. Step 4
• Crop. Limit alignment, measurement, and export to one rectangle of the reference frame. Step 5
• Measurement regions. Choose where signal and background noise are measured. Step 6
• Live values and plots. Show Signal and Noise labels and plot Frame count, Total integration time, FWHM, SNR, Signal, Noise, Moon Illumination, or Moonlight Impact. Step 7
• Annotations and layers. Add circles, rectangles, and text, and reorder or remove layers. Step 8
• Presets. Save a layout with its crop and regions and reuse it on other image sets. Step 9
• WCS alignment. Reproject plate-solved frames onto the first frame, or skip alignment for data that is already registered. Step 10
• GIF and MP4 export. Set frame rate, output size, looping, and Fast mode. Step 11

Images

A folder with two or more frames of the same field. Deep Stack reads FITS (.fit, .fits), XISF, TIFF, PNG, JPG, and common camera RAW files, in grayscale or simple RGB/RGBA. Only files directly in the folder are used; subfolders are ignored.

File order

Frames are stacked in filename order. Name the files so that this is also time order, and so that the frame you want as the reference sorts first.

Plate solving

Needed if CAst should align the frames, and for the Moonlight Impact metric. Every frame then needs a celestial WCS in its header. Deep Stack does not plate-solve frames itself. Frames that are already registered by another program can be stacked without a WCS.

Header metadata

EXPTIME gives the integration time. DATE-OBS gives the observation time for the Moon metrics.

Observing site

Optional. Needed only for Moonlight Impact. Set it in Settings > Open Settings > Setup > Observing Site.

MP4

MP4 export needs FFmpeg from the imageio-ffmpeg package, which the installer packages are built with. GIF needs nothing extra.

Disk space

The export writes a temporary folder named astrostack_export_… inside your image folder, with one aligned FITS copy per frame when CAst aligns. The folder must be writable. The temporary folder is deleted when the export ends.

1. On the mode picker, under Visualizations & Tools, click Deep Stack. If CAst is already open in another mode, choose Mode > Deep Stack instead.
2. Click Open in the toolbar. You can also choose File > Open Folder or press Ctrl + Shift + O.
3. In Select Deep Stack image folder, choose the folder with your frames and confirm.

CAst lists the supported files in the folder, sorts them by name, and loads the first one. A small Deep Stack dialog shows Loading Deep Stack folder... while it does this. The counter at the right end of the toolbar then reads Frames: N, the number of frames that will be stacked.

The first frame is the reference. It is the image you crop and annotate, and it defines the pixel grid that every other frame is aligned to.

If the folder has no supported files, the image area reads No supported images were found in the selected folder.

The panel has three areas: the toolbar across the top, a control column on the left (Plot Properties, Functions, Layers), and the image. A plot card titled Stack SNR is already placed on the image. It shows the SNR curve, an Integration line, and a frame counter such as 1/42.

Move around the image

• Mouse wheel: zoom in or out around the cursor
• Hand tool (the default) + drag: pan
• Display > Reset View: fit the whole image again

Set the display

1. Click Display in the toolbar.
2. Pick a stretch: Auto Stretch, Linear, Asinh, Sqrt, or Log.
3. For finer control, click Curves and adjust the black point, midtone, white point, or curve in the Curves dialog. Click Apply, or Reset to start the curve over.
4. Tick Invert if you want dark stars on a light background.
5. Click Reset Display to clear curves and return to the default stretch.

The exported animation uses exactly the display you set here, applied to each cumulative stack.

Crop when the target fills only part of the frame, or when the frames do not cover exactly the same area.

1. Click Crop. The strip under the image reads Drag on the image to select a crop region. Click Crop again to cancel.
2. Drag a rectangle around the area you want to keep.
3. Release the mouse. The preview now shows only the cropped area, and the button changes to Reset.

To remove the crop, click Reset. To leave crop mode without cropping, click Crop again before you drag.

The crop is used everywhere after this point: as the alignment grid, for every measurement, and as the frame size of the exported animation.

Signal, Noise, and SNR are measured on each cumulative stack. Without regions, CAst uses statistics of the whole (cropped) image. With regions, you choose what counts as signal and what counts as sky.

1. In the tool row at the top right, click the Signal tool (the S icon).
2. Drag a rectangle over the part of the target you care about, such as a galaxy arm or a bright part of a nebula. It is drawn in green.
3. Click the Background tool (the B icon).
4. Drag a rectangle over empty sky with no stars. It is drawn in orange.

Both regions appear at the bottom of Layers as Signal region and Background region. Drawing a region again replaces it. To delete one, select it in Layers and click Remove.

How these regions turn into numbers is described in R7.

Live Signal and Noise labels

1. In the Functions group on the left, click Signal (the S icon) or Noise (the N icon).
2. Click on the image where the label should go. A label such as Signal: 1.438e-04 appears.
3. Optionally type a Prefix, and set the text color, font, size, and style in the same group. With a prefix, the label reads <prefix> <value>.

On export, each label shows the value for the current cumulative stack.

The Stack SNR card

The card that appears when you open a folder plots Frame count against SNR. Below the chart it shows Integration, the total exposure time stacked so far in HHhMMm, and a counter such as 12/42. Click Hide Info at the bottom of the left column to hide this card from the preview and the export; the button then reads Show Info.

Draw your own plot

1. Click the Plot tool in the tool row.
2. Drag a rectangle where the plot card should go.
3. In Plot Properties > Data, type a Title and choose an X metric and a Y metric.
4. Optionally type an X label and Y label. Empty labels use the default label for each metric.
5. Style the card in the other Plot Properties sections. Every field is listed in R3.

Both axes can use any of these metrics:

Plots drawn with the Plot tool show the chart only. The Integration line and frame counter belong to the Stack SNR card.

Use these to mark objects or add a title, an observer name, or equipment details.

Circles and rectangles

1. Click the Circle or Rectangle tool. The Properties strip next to the tools shows Stroke, Fill, Weight, and Opacity.
2. Set the colors and line weight first if you like.
3. For a circle, press at the center and drag out the radius. For a rectangle, drag from one corner to the opposite corner.

A click without a drag creates nothing.

Text

1. Click the Text tool. The Properties strip shows the text color, font, size, and style.
2. Click on the image where the text should start, without dragging.
3. In Add Text Layer, type the text and click OK.

The words of a text layer cannot be changed after it is placed. To change them, remove the layer and add it again. Its color, font, size, and style can be changed.

Select, move, resize, and remove

1. Click the Mouse tool.
2. Click a layer on the image to select it, or click its name in Layers.
3. Drag the layer to move it. Drag a corner handle to resize a rectangle or plot card, or the edge handle to resize a circle.
4. Change any property in Properties, Functions, or Plot Properties; the selected layer updates immediately.
5. To delete it, right-click it and choose Remove Layer, or click Remove under Layers.

In Layers, the top item is drawn in front. Up moves the selected layer toward the front and Down toward the back.

A preset stores all overlay layers and their styles, the crop, and the signal and background regions. Positions are stored as fractions of the image, so a preset made on one image set fits another of a different size.

Save

1. Under Layers, click Save Preset.
2. Choose a name and folder. The default is astrostack-layers.astrostack.json in the image folder.

Load

1. Open the image folder first (Step 3).
2. Under Layers, click Load Preset and choose the file.
3. If the session already has layers or a crop, confirm Replace the current overlay layers and crop with the preset? with Yes.

Presets do not store the display settings or the export options.

1. Click Stack in the toolbar.
2. The Deep Stack Alignment dialog asks Is this an already aligned dataset? Choose one of the answers below.

Cancel stops without exporting. After you answer, the Deep Stack Export dialog opens (Step 11).

Alignment and stacking happen after you confirm the export file, with progress shown in a Deep Stack dialog. The method is described in R6.

1. In Deep Stack Export, leave Fast mode on for a first try. It writes up to 60 evenly spaced frames instead of one per stack.
2. Choose the Format: GIF or MP4.
3. Set the speed with Frame Rate or Frame Time. They are linked: 50 ms per frame is 20 fps.
4. Set Output size, from 10% to 100% of the stacked frame.
5. For GIF, keep Loop forever ticked under Playback to repeat the animation, or untick it to play once.
6. Check the summary line, which gives the number of animation frames and the length of one pass in seconds. Click Export.
7. In Export Deep Stack Animation, choose where to save. The default is <folder name>_astrostack.gif or .mp4 in the image folder.

A Deep Stack progress dialog shows each phase: alignment ([Align 3/42] ...), then stacking (Building cumulative Deep Stack 12/42..., or in Fast mode Accumulating Deep Stack 12/42 (exporting 5/60 GIF frames)...). Click Cancel at any time to stop. When the export finishes, the status bar names the file.

Play back the result

Deep Stack does not play the animation inside CAst. Open the exported file in a web browser, an image viewer, or a video player. A GIF with Loop forever repeats on its own. An MP4 plays once unless the player is set to loop.

Make the final version

If the Fast mode result looks right, click Stack again and export with Fast mode off. Every cumulative stack then becomes one animation frame and every plot gets one point per frame. For long sequences, MP4 gives a smaller file than GIF.

Open

Opens a folder of frames. Same as File > Open Folder (Ctrl + Shift + O).

Stack

Starts the alignment question, the export dialog, and the export. Enabled once a folder with frames is open.

Crop / Reset

Crop enters crop mode; click it again to cancel. After a crop is drawn the button reads Reset and clears the crop.

Display

Menu with the stretch list, Curves, Invert, Reset View, and Reset Display. Enabled once an image is loaded.

The initial stretch and invert come from Settings > Open Settings > General > Image Display.

Stretch

Auto Stretch, Linear, Asinh, Sqrt, Log.

Curves

Opens the Curves dialog for black point, midtone, white point, and curve points. Changes the display only, not the data that is measured.

Invert

Inverts the display.

Reset View

Fits the image in the view.

Reset Display

Clears levels and curves and restores the default stretch and invert.

Frames: N

Number of supported frames found in the folder.

Hide Info / Show Info

At the bottom of the left column. Hides or shows the Stack SNR card in the preview and the export.

Properties

The strip next to the tools. With Circle or Rectangle active: Stroke, Fill, Weight, Opacity. With Text active: text color, font, size, style. With other tools it reads Select a drawing tool to edit overlay properties.

Weight 0.5–12 px. Opacity 5–100%, set with the slider under the percentage button or by dragging left and right on the Opacity label. Text size 6–120 pt. Styles: Regular, Bold, Italic, Bold Italic.

Functions

Signal (Place a live Signal value label on the export.) and Noise (Place a live Noise value label on the export.). When a label tool is active or a label is selected: Prefix, text color, font, size (6–120 pt), style.

Layers

All overlay layers, front to back, followed by Signal region and Background region. Selecting an entry loads its settings into the property controls.

Up / Down

Move the selected layer toward the front or the back. Not available for measurement regions.

Remove

Removes the selected layer or measurement region.

Save Preset / Load Preset

File filter: Deep Stack Preset (*.astrostack.json);;JSON Files (*.json). Save Preset is enabled when at least one layer or region exists. Both need an open folder.

These fields edit the selected plot card and set the starting values for the next plot you draw. Fields that show Auto at 0 let CAst size the element from the card.

Data

Title, X metric, Y metric, X label, Y label. The metric list is in Step 7.

Card

Stroke and Fill colors, Weight (0.5–12 px), Radius (corner radius, 0–48 px), Opacity (background opacity of the card).

Text · Title

Horizontal (Left, Center, Right), Vertical (Top, Middle, Bottom), Offset X and Offset Y (−80 to 80 px), Color, Font, Style, Size, Opacity (0–1).

Text · Labels

Color, Font, Style, Size, Opacity, Accent (color of the Integration label on the Stack SNR card), and X and Y offsets for the X label and the Y label.

Plot area

Left, Right, Top, Bottom margins, 0–50% of the card.

Chart

Curve color, Opacity (0–1), Width (0–12 px). Highlight color for the current point, Opacity, Radius (0–24 px).

Fast mode

Writes at most 60 evenly spaced animation frames, always including the first and last stack. All frames are still aligned and stacked; metrics are measured only on the written frames. Off: one animation frame and one measurement per cumulative stack.

GIF

One 256-color palette built from the first, middle, and last frames, with dithering. All frames are held in memory until the file is written.

MP4

H.264, yuv420p, quality CRF 23, no audio. The frame is padded to a multiple of 16 pixels by repeating the edge. Needs FFmpeg; without it CAst shows MP4 export unavailable with install steps.

Output size

Scales the image and all overlays together, so the layout stays the same.

Output file

Default <folder name>_astrostack.gif or .mp4 in the image folder. The extension is corrected to match the format.

All are in Settings > Open Settings.

Observing Site

Setup tab: Observing Latitude, Observing Longitude, Observing Elevation. Latitude and longitude are needed for Moonlight Impact and make Moon Illumination topocentric.

Image Timestamp Timezone

Differential Photometry tab, Science Export group. Used to read DATE-OBS values that have no time zone.

Shared Analysis Workers

Advanced tab. Number of parallel workers for alignment.

Auto by default. Alignment uses at most 2 workers for full reprojection and 4 for the fast affine path unless you set a number.

Image Display

General tab. Default stretch and invert for the preview.

Keep mode memory when switching

General tab. Off by default. When off, leaving Deep Stack and returning shows the empty preview, and you reopen the folder.

Alignment (answer No)

1. CAst checks that the first frame and every other frame have a usable celestial WCS. A missing one stops the export with the file name and the reason.
2. The target grid is the first frame, or the crop of the first frame when a crop exists.
3. Each frame is reprojected onto that grid. When the mapping between the two WCS solutions is affine to within 0.08 px, CAst uses a fast affine transform. Otherwise it uses bilinear reprojection with the reproject package.
4. CAst then detects up to 96 stars in the reference and in the reprojected frame, finds the most common offset between them, and shifts the frame by that offset when at least three stars agree and the shift is at least 0.05 px.
5. Aligned copies are written as FITS files to a temporary folder inside the image folder.

Pixels that a frame does not cover are set to 0.

Already aligned (answer Yes)

The frames are read as they are. If a crop exists, the same pixel rectangle is cut from each.

Running mean

The stack is an unweighted running mean. Frame 1 starts it, and each later frame n updates it as mean += (frame − mean) / n. Every frame has equal weight; there is no outlier rejection.

Rendering

Each written step is displayed with your stretch, curves, and invert settings, then the measurement regions and all visible layers are drawn on top, and the result is scaled to the output size. The temporary folder is deleted at the end, whether the export finished, failed, or was canceled.

Every metric is measured on the current cumulative stack, inside the crop. For color images the channels are averaged into one brightness plane first. MAD is the median absolute deviation; multiplying it by 1.4826 gives a noise estimate that matches the standard deviation for Gaussian noise and ignores stars.

Noise

With both regions: 1.4826 × MAD of the background-region pixels. Without both: 1.4826 × MAD of all pixels. If the MAD is zero, the standard deviation is used.

Signal

With both regions: mean of the signal-region pixels minus the median of the background-region pixels. Without both: the 99.5th percentile of all pixels minus their median. Never below 0.

SNR

Signal divided by Noise. For random noise, averaging n frames lowers the noise by about √n, so SNR tends to rise roughly with √n until gradients or pattern noise limit it.

FWHM

CAst takes the brightest pixel in the central part of the image (a margin of one-eighth of the width and height is ignored on each side), cuts a 15 × 15 pixel box around it, subtracts the local background, and finds where the radial profile drops to half its peak. Falls back to 3.0 px if that fails.

Frame count

Number of frames in the stack at that step.

Total integration time

Sum of the exposure times of the stacked frames, from EXPTIME or from the filename. Frames without an exposure add 0. The card shows it as HHhMMm, or -- when it is 0; plots use seconds.

Moon Illumination

The Moon's lit fraction, in percent, at each frame's mid-exposure time (DATE-OBS plus half the exposure), from the Sun–Moon–observer angle computed with Astropy.

Moonlight Impact

A 0–100 score from the Krisciunas–Schaefer (1991) scattered-moonlight model. It uses the Moon's altitude and distance, the target's altitude, their separation, and the Moon's phase at the frame time and your site. The target is the center of the first frame's plate solution. CAst estimates the moonlight brightness with V-band extinction of 0.172 mag per airmass, converts it to a brightening of the sky against a dark zenith of 79 nanoLamberts, and scales 4.5 magnitudes of brightening to 100. The score is 0 when the Moon is below the horizon.

When a plot uses Moonlight Impact, the axis label includes the current category, for example Moonlight impact (Moderate). Live Signal and Noise labels use scientific notation with three decimals, such as 1.438e-04.