Astrophotography storage calculator: FITS file size per session

What it estimates

The calculator answers a practical question before a long session: how much disk space will this night take, and how much more will processing need? It counts three groups of files:

1. Light frames: the number that fit in the hours you enter, given the sub-exposure length and the gap between subs (download time, dithering, settling).
2. Calibration frames: darks, flats and bias or dark-flat frames at the same resolution and bit depth.
3. Processing copies: the intermediate files a stacking program writes, typically one calibrated and one registered copy of every light frame, stored as 32-bit floating point.

How a FITS file’s size is set

Astronomy cameras normally save FITS files. The FITS Standard fixes the layout precisely:

• every part of a file is made of whole 2880-byte blocks; the image data is padded up to the next block;
• each header block holds 36 keyword records of 80 characters;
• BITPIX = 16 means 16-bit integers, 2 bytes per pixel. Cameras that produce unsigned 16-bit values store them in the same 2 bytes, with the standard’s BZERO = 32768 convention, so the file is the same size;
• BITPIX = −32 means 32-bit IEEE floating point, 4 bytes per pixel. The calculator assumes this format for processing copies; if your stacking program saves 16-bit intermediates, halve that part of the estimate.

frame size = header blocks × 2880 + ⌈ width × height × bytes per pixel ÷ 2880 ⌉ × 2880

The calculator assumes two header blocks (up to 72 keyword records). Capture programs that write more keywords add 2880 bytes for each additional 36, a difference of a few kilobytes per frame.

Worked example

A ZWO ASI2600MC Pro (6248 × 4176 pixels, 16-bit) taking 180-second subs with a 5-second gap, for 4 hours:

Item	Count	Size
One light frame	1	52.2 MB
Lights (19 per hour × 4 h)	76	3.97 GB
Darks, flats, bias	30 + 30 + 30	4.70 GB
From the camera	166 files	8.66 GB
Calibrated + registered copies, debayered to RGB, 32-bit	2 × 76	47.6 GB
Total		56.3 GB

Two things stand out. Calibration frames can take as much space as the lights on a short night, because their count does not depend on how long you imaged. And processing dominates: each debayered 32-bit copy of a colour frame is 6248 × 4176 × 3 channels × 4 bytes = 313 MB, six times the original. A mono camera’s 32-bit copies are twice the original, so the same session in mono would need about 15.9 GB of processing copies instead of 47.6 GB.

Other sensors

Sensor	Resolution	16-bit FITS frame
IMX455 full frame (e.g. ASI6200MM Pro)	9576 × 6388	122.4 MB
IMX571 APS-C (e.g. ASI2600MC Pro)	6248 × 4176	52.2 MB
IMX585 (e.g. ASI585MC)	3840 × 2160	16.6 MB

Planetary imaging is a different pattern: short videos of thousands of frames, often in 8-bit. At 8 bits an IMX585 frame is 8.3 MB, so a capture of 6000 frames is about 50 GB before any frames are discarded. Use the calculator with your frame count and 8-bit setting to plan these runs. Capturing a smaller region of interest, where the capture program supports it, reduces the size in proportion to the pixels kept.

Assumptions

• Sizes use decimal units (1 GB = 1,000,000,000 bytes), the way drive capacities are labelled.
• Compressed formats (some programs offer compressed FITS or XISF) are not modelled.
• DSLR raw files use their own compressed formats; the calculator’s FITS sizes do not apply to them.
• The number of processing copies depends on your software and settings. Enter 0 to count capture files only.

For what the calibration frames are for and how many are typically taken, see the calibration frames guide; for how integration time relates to signal-to-noise, see the integration time guide.