When using a colour Altair camera in SharpCap or Altair Capture, you will have some colour space options to choose from. This blog outlines the various choices of colour space and their pros and cons.
You select the colour space mode like this:
1) in AltairCapture under the Selected in AltairCapture under the Capture & Resolution > “Format” menu. Select bit-depth separately under the “Bit depth” menu. E.g. Format “RAW” & Bit a depth of 8 bit gives you RAW8 colour space.
2) In SharpCap you select from the list under the Capture Format Area menu > “Colour space” > RAW8.
In both capture programs, the options will not appear for all file formats because some don’t support some spaces. (See other blog Choosing the right file format for my Altair Camera).
1) The RGB colour space:
RGB Pros: Simple to use and simple post-processing. Images should look correct when viewed in any application. Camera based adjustments like white balance, gamma, brightness and contrast are available in the capture software, although these are usually performed in software on the PC. This is best for EAA or Video Astronomy.
RGB Cons: Files are large as they are typically 3 bytes per pixel. Bit depth limited to 8 bits. Debayering (turning the raw image to full colour) is performed by the camera driver typically using a simple but fast algorithm. Adjustments like gamma, brightness and contrast lead to data loss when they are performed as they happen in digital space.
2) The RAW8 colour space:
RAW8 Pros: Exact data that comes off of the camera sensor with no post-processing. Post-processing (including debayering) can be done later at a higher quality. File size is small (only 1 byte per pixel).
RAW8 Cons: Smaller range of post-processing applications that can work with the output files. Post-processing is more complex. Output files may appear to have ‘chessboard’ effect if opened in applications that don’t understand raw formats. Bit depth limited to 8 bits.
3) RAW12 colour space:
RAW12 Pros: Exact data that comes off of the camera sensor with no post-processing. Post-processing (including debayering) can be done later at a higher quality. Higher bit depth may give more information and more dynamic range if images are low noise with short exposures, like in solar/lunar imaging. (See other blogs on bit depth for more info).
RAW12 Cons: Smaller range of applications that can work with the output files. Post-processing is more complex. Output files may appear to have ‘chessboard’ effect if opened in applications that don’t understand raw formats. Files are larger (2 bytes per pixel) hence slower frame rates due to slower write speeds to PC.
4) MONO colour space:
MONO Pros: smaller file size (1 byte per pixel), ideal for monochrome targets (using RGB filters on planets, narrowband filters on deepsky objects, Hydrogen Alpha solar imaging, lunar imaging, etc) when using a colour camera.
Mono Cons: Processing to produce mono involves a debayer to produce a colour image and then that is made monochrome, so the following cons for RGB apply : Debayering (turning the raw image to full colour) is performed by the camera driver typically using a simple but fast algorithm. Adjustments like gamma, brightness and contrast lead to data loss when they are performed as they happen in the “digital” space. Therefore, it may be better to capture as RAW8/12 and then make the final processed image monochrome.
(Special thanks to Robin Glover)
