Astro Dither & Drift Analyzer
Browser-only analysis. Images stay on your computer.
Pixel size override is optional. Leave it blank to use XPIXSZ/YPIXSZ from the FITS header; enter a value (e.g. 2.9 µm for Seestar S50) when the FITS does not contain pixel size. Dither coordinates remain measured in image pixels.
Star detection is adaptive: the threshold is automatically lowered on faint/sparse fields (useful for Seestar short exposures) while stopping once enough reliable stars are found.
* FITS/FIT is parsed directly in the browser (8/16/32-bit integer and 32/64-bit floating-point primary images). TIFF support depends on the browser.
FITS / sensor data analysis
Digital range of the loaded FITS data. This estimates stored/output bit depth; it is not the sensor's physical dynamic range in EV.
Inter-frame shift / dithering
XY dither path. Auto scale fills both axes independently.
The table gives numerical shifts. The visualization is an XY displacement plot in pixels. Each red point is the measured frame position relative to frame #1; grey lines connect consecutive frames, so even small dither steps remain clearly visible.
| Frame | ΔX vs #1 | ΔY vs #1 | Offset | Step X | Step Y | Step | Type | Rotation vs #1 | Step rotation | Score |
|---|
PSF / FWHM / noise / SNR
This cannot recover chronological motion from an ordinary single exposure. It measures star elongation/trailing from detected stellar PSFs inside the selected frame and reports a median trail vector in pixels. A single integrated image contains no information about whether the star moved from A→B or B→A, so the direction has a 180° ambiguity.
The primary quality metric for a single linear FITS is stellar SNR. Background median and raw pixel σ are shown only as diagnostics: they use the file's current ADU scale and must not be compared directly between differently scaled stacks. Stellar apertures follow measured FWHM and aperture noise is measured empirically from blank sky, so correlated noise from registration, debayering and drizzle is included. With exactly two compatible FITS files, Compare 2 FITS matches the same unsaturated stars, derives a stellar-flux scale, and reports flux-normalized aperture noise plus paired-star SNR. Conversion gain is optional and means e⁻/ADU—not the camera's GAIN setting.
This analysis measures stellar PSF shape only. For FITS files, FWHM is measured directly from native linear pixels using local background subtraction and interpolated half-maximum stellar profiles. Elongation/eccentricity remain independent PSF-shape measurements. It does not infer guiding RMS or motion in pixels from a single integrated exposure.
Sensor tilt / field analysis
Measures native-pixel stellar FWHM across the field. A linear FWHM gradient is a useful indicator of sensor/focal-plane tilt; symmetric degradation toward the edges is more consistent with field curvature or other optical aberrations.