BROWSER-BASED FITS PROCESSING FOR STACKED ASTROPHOTOGRAPHY
Process, stretch, and explore stacked FITS files directly in your browser. Background extraction, channel neutralization, and GHS stretching — no installation, no cloud, no account.
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DROP YOUR FITS FILE HERE
or click to browse · stacked-16_*.fits
OR
ABOUT THIS TOOL
I built FITS Studio to learn how to stretch astrophotography images. I wanted something that delivers results comparable to Siril without the learning curve of professional software. I'm not a developer — just a Dwarf 3 owner who enjoys deep-sky imaging from a Bortle 6 backyard in New England. The tool is Dwarf 3 specific for now and is designed around the linear stacked FITS files the scope produces after a session. It does not stack frames — it takes the stack the Dwarf 3 already made and gives you a proper processing pipeline to bring out the detail.
WHAT IT DOES
✦Background extraction with POLY or TPS-RBF gradient models
✦Automatic channel neutralization to remove LP colour cast
✦GHS, Asinh, and MTF stretching with multi-pass history
POLY-2 handles most Bortle 6 gradients. TPS-RBF for complex multi-source sky glow.
Sample Grid
Rejection Tolerance1.0
Lower = stricter star rejection. 1.0 is a good default.
Green circles = accepted. Red = rejected. Click to toggle. Scroll to zoom · drag to pan.
Calculate Model
Generate sample grid first
Strength & Apply
Strength100%
Real-time preview. Reduce if edges over-darken.
Apply bakes the correction and runs channel neutralization automatically.
2
Channel Neutralization
Automatic
▶
Runs automatically as part of Apply — no action needed from you.
After gradient subtraction the three channels often have different residual sky levels — light pollution tints red and green unevenly, and the polynomial fit can leave small per-channel biases. Left uncorrected this shows as a brownish or greenish background cast. Neutralization samples the accepted background regions and subtracts the per-channel offset so R, G, B share the same sky floor before any stretch. A colour cast baked into linear data is very difficult to remove after GHS or Asinh amplifies it.
R offset
—
G offset
—
B offset
—
3
Statistics Refresh
Automatic
▶
Runs automatically after neutralization — no action needed from you.
The histogram peak, noise cluster edge, and per-channel sky medians all shift after background correction and neutralization. If the STRETCH tab used pre-correction statistics, BP and SP would be placed against the wrong sky level and the first stretch pass would be miscalibrated. Statistics Refresh discards all pre-correction numbers and recomputes from the corrected linear arrays. The values below are exactly what the STRETCH tab receives.
Sky peak
—
Cluster edge
—
Sky median R
—
Sky median G
—
Sky median B
—
SP position
—
4
Gentle Linear Denoise
Optional
▶
Gentle luminance-only denoise on the corrected linear image. Reduces background noise before stretching while preserving colour balance. Hue and saturation are unchanged. Skip this and go straight to STRETCH if your stack is deep.
Strength0%
Start at 20–40% for linear data. 0% = no effect.
Apply background correction first
5
Ready for Stretch
Waiting
▶
Complete Step 1 to proceed.
Stretch
⚡ AUTO PREVIEW ACTIVE
Background Clip0.000
Drag the orange marker on the histogram. Raise until background turns clean black.
Midtone (m)0.490
Left = no stretch (m=0.490). Right = very aggressive (m=0.010).
White Point1.000
Drag the red histogram marker. Clips highlights. Right = no clipping.
Stretch D0.00
Logarithmic. Left = no stretch. Right = very aggressive.
Stretch Point SP0.005
Place just left of where the histogram data begins to rise. Green marker shows recommended position. Drag the blue marker on the histogram.
Stretch D0.00
Overall stretch strength. Increase until you can see your target. Stop before stars bloat.
Local Intensity b0.0
Negative = log/Asinh behaviour. 0 = pure GHS. 1–6 = contrast focused at SP. 8–15 = very concentrated at SP for initial linear stretches.
Lower Protection LP0.000
Pixels below LP are returned toward identity — preserves deep shadow detail. Drag green LP marker on histogram. Keep LP ≤ SP.
Highlight Protection HP1.000
Pixels above HP are returned toward identity — protects galaxy cores and star colours. Drag yellow HP marker on histogram. Lower = more protection.
Highlights0.00
Power-law compression above midpoint. Reduces blown star cores only.
Star Roll-off0.00
Smooth asymptotic cap. Final star shape control after Highlights.
White Balance
Red1.00
Green1.00
Blue1.00
Adjust R and B until white stars look neutral.
Colour Palette
Multi-Pass Stretch
PASS 0 OF 0
Bakes current stretch into pixel data and resets sliders to identity. Click any pass in the log to undo back to that point.
Export & View
01 — GETTING THE FILE FROM YOUR DWARF 3
Retrieve the stacked FITS file via USB-C
Connect your Dwarf 3 to a laptop or desktop using the USB-C cable supplied. Power on, wait for the device to mount as a storage drive. Navigate to /Astronomy/ and open the session folder DWARF_RAW_TELE_{target}_EXP_{n}_GAIN_{n}_{timestamp}. Copy stacked-16_*.fits to your computer.
The stacked-16 file is the internally calibrated stack. Individual sub-frames are not needed.
02 — THE RAW SIDE (LEFT OF SPLIT)
What you are seeing on the left is real colour, gamma lifted
The split-screen left side applies pow(channel, 0.28) to each RGB channel independently. This is a strong gamma lift — it takes the nearly-black linear data and makes it visible. The colours you see are real: the actual R, G, B values from the three FITS data planes. Nothing is processed or stretched. It is dim and low-contrast by nature because linear data contains almost no contrast.
Drag the split to the right to compare before and after. The left side does not change when you move sliders — it always shows the raw data.
03 — THE HISTOGRAM
Drag, resize, and read the floating histogram
The histogram floats over the image and can be dragged by its title bar. X axis: 0 = sky background, 1 = brightest stars. Y axis: logarithmic bin count. The tall peak near 0 is normal — most pixels are sky. The tail to the right contains nebulosity and stars. Switch channels (LUM, RED, GRN, BLU) to check colour balance. Three draggable vertical markers: orange = Background Clip, red = White Point (MTF), blue dashed = Stretch Point (GHS). Click anywhere on the histogram to set BP directly.
Colour imbalance shows as one channel's histogram peak sitting at a different position from the others. Use white balance to align them.
04 — AUTO STRETCH
One click to a usable starting point
Press AUTO STRETCH at the top of the PROCESS tab. Computes median and MAD from the luminance channel. Sets Background Clip to median − 2.5×MAD and solves for the stretch parameter so the background maps to approximately 20% output brightness — the same target PixInsight AutoSTF uses.
Switch stretch mode before pressing Auto Stretch — it solves for the correct parameters for whichever mode is active.
05 — MTF MODE
Midtone Transfer Function — three-point curve
Three independent controls: Background Clip (clips below sky), White Point (clips above and renormalises), and Midtone m (applies T(x)=(m−1)x/((2m−1)x−m)). At m=0.490 the curve is identity. Lower m = more aggressive contrast. This is the same algorithm as PixInsight's HistogramTransformation. Use the red histogram marker to set White Point and the orange marker for Background Clip.
MTF is the recommended starting mode for most Dwarf 3 images.
06 — ASINH MODE
Inverse hyperbolic sine — smooth highlight compression
Applies asinh(D×x)/asinh(D). Logarithmic D slider. At low D nearly linear. At high D very aggressive in shadows, smooth in highlights. Better star colour preservation than MTF at equivalent stretch levels because highlights are compressed smoothly rather than clipped.
No White Point in Asinh mode. Use the Highlights and Star Roll-off sliders to manage bright stars.
07 — GHS MODE
Generalised Hyperbolic Stretch — Siril's method
Stretches around a pivot point SP. Shadow side: Möbius pre-warp (B) boosts faint detail before asinh stretch, with LP fade near zero. Highlight side: log compression with HP fade. Set SP to background median — use Auto Stretch or drag the blue histogram marker. Start with B=0 and D at a moderate value.
D and SP are the primary controls. B, LP, HP are refinements once the basic stretch looks right.
08 — WHITE BALANCE
Correct colour after stretching
Inspect white stars in the processed side. If they appear orange, lower Red or raise Blue. If blue, raise Red. Green is the reference channel and is usually left at 1.00. Check the histogram channel selector — a balanced image shows all three channel curves overlapping closely.
Apply white balance before selecting a palette. An uncalibrated image will look unpredictable in HUBBLE or COOL mode.
09 — EXPORT
Export at native 3856 × 2180 resolution
Press EXPORT FULL RES PNG. Renders at native Dwarf 3 resolution without the split overlay. Filename is built from the OBJECT and DATE-OBS header fields. Zoom and pan do not affect the export — always exports the full frame.
The PNG contains all current stretch, white balance, and palette settings baked in. It is ready to share or open in Affinity Photo or Photoshop for further work.