Common File Formats
The image file formats presented here are standard formats, readable by all image manipulation software. These formats were designed to meet the needs some time ago and may be obsolete. Moreover, none of these formats have been designed to handle astronomical data. They must therefore generally be used at the end of the processing chain.
Files with the .bmp extension are bitmap image files used to store digital bitmap images. These images are independent of the graphics card and are also called Device Independent Bitmap (DIB) file format. This independence allows the file to be opened on multiple platforms such as Microsoft Windows and Mac. The BMP file format allows data to be stored as two-dimensional digital images, both in monochrome and in color, with different color depths.
Nowadays, this format is not really used anymore and other file types are preferred.
Probably the most used file format for sharing images on forums, by e-mail or usb sticks. This format allows a more or less strong (destructive) compression which gives ideal file sizes for exchanges. The extension of this type of file is .jpg or .jpeg.
The JPEG format is however only coded in 8-bit. With the compression that produces artifacts, this format is not very suitable for astronomy images and we generally prefer the PNG format.
Portable Network Graphics is a raster-graphics file format that supports lossless data compression. The extension of the format is .png. PNG grayscale images support the widest range of pixel depths of any image type. Depths of 1, 2, 4, 8, and 16-bit are supported, covering everything from simple black-and-white scans to full-depth medical and raw astronomical images.
Calibrated astronomical image data is usually stored as 32-bit or 64-bit floating-point values, and some raw data is represented as 32-bit integers. Neither format is directly supported by PNG.
However, this format is an excellent choice for saving the final image, after processing.
TIFF or TIF, Tagged Image File Format, represents raster images for use on various devices that conform to this file format standard. It is capable of describing bi-level, grayscale, palette color and full color image data in multiple color spaces. It supports both lossless and lossy compression schemes to allow applications that use this format to choose between space and time. The extension is either .tiff or .tif.
The advantages of the TIFF format are multiple. It supports encoding up to 32-bit per pixel and offers a wide variety of possible fields in the metadata making it a good candidate for storing astronomical data.
Using the TIFF format, and in collaboration with other developers, we have set up a pseudo standard, Astro-TIFF.
Several graphics formats are used and defined by the Netpbm project. The portable pixmap format (PPM), the portable graymap format (PGM) and the portable bitmap format (PBM) are image file formats designed to be easily exchanged between platforms. Possible file extension are .pbm, .pgm (for gray scale files) and .ppm.
These formats, supporting up to 16-bit per channel, are marginally used and should only be used for final image storage.
It's a film container, able to contain data with various audio and video codecs. Some lossless video codecs exist and they have been used for astronomy imaging purposes in the past, but it's a format that does not contain metadata usable for astronomy, that is limited to 8-bit images and that does not give any warranty that the data it contains is raw.
This input file format is now deprecated. We recommend to use SER format instead.