Telescope Reviews: Astrophoto for newbies????

Standard images such as JPEG images use 8bits of data per colour channel. Confusingly you often see the channels added up so that 8bits for Red, 8bits for Green and 8bits for Blue is a "24 bit colour" image.

8bits of data lets the computer store 256 levels of intensity for each colour. Combined together that is plenty of data to represent what our screens can display so 8bits-per-channel is good for most photography.

16bits per channel lets the computer store not double but 65535 levels of intensity in each colour! FAR more.

With astrophotography we are often capturing very bright stars and VERY dim nebulas or galaxies in the very same image - in that case 256 levels of intensity is just not enough to capture the detail we want! Thankfully most cameras can produce "RAW" files that are either 12bit or 14bit deep so we *can* capture more levels of intensity with the equipment. Likewise all the astrophoto software upscales that 12 or 14 bit RAW data to a full 16bits-per-channel so that you can work with(modify) the data without loosing the detail. Some AP apps even work at 32bit per channel!

One of the first things that is done with most astrophotography is the bring those dim details up to a brightness level where we can see them, this is "stretching" the data. Eventually the final output will be a 8bit per channel image, but especially for those early stretch operations being able to work in 16bit is a large advantage.

GIMP, since it does not support 16bit per channel images yet, converts any file you load into 8bit, so if you load an un-stretched stacked image into GIMP it will unintentionally toss out a whole bunch of data that you most likely wanted. For example a dim nebula may take up 500 or so intensity levels in the 16bit image, but if you convert over to 8bit right away that nicely detailed nebula will now take up only 2 intensity levels! This may make you think that your image was no good when in reality it was just the conversion to 8bit happening too soon.

This is why I use other applications for the early processing stages and only move to GIMP for a few finishing touches. DeepSkyStacker has a rough ability to do an initial stretch and IRIS, while not that easy to use, is free and can do all sorts of processing. Likewise inexpensive apps like Nebulosity have post-processing tools as well. There are many other options such as Photoshop or one of the higher-end astrophoto processing applications as well.

I should note that you can pretty much ignore all of this when you are doing things like Planetary Imaging, it mostly applies to Deep Sky Imaging where you have long exposures and lots of them stacked to bring out your images.

I hope that has cleared things up a bit and not added to your confusion!