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Walking on the Moon

What is RAW image

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Hi Mark, RAWs are the building blocks for making quality long exposure images, in general you would usually take many long exposure raw images (called lights, light frames or subs) of the same subject until you had say an hour or 2 of exposure time between them and then using a program like deep sky stacker or maxim dl which will allow you to add all of the exposures together.

This process is called 'Stacking' and its done in such a way that resulting image is of a much higher quality than any single exposure in the 'Stack' :hello2:

It's highly desirable to use raw frames when stacking because they haven't been touched by the internal noise reduction routines on the camera which is great at sorting out daytime pictures but might remove stars and other detail from our astronomy images thinking it was something that shouldn't be there.

When you take the raws for the stack you will usually try and keep all the exposure lengths the same (unless you are trying something exotic like HDR but thats another story), you will also need to look into darks or dark frames and flats or flat frames. When these are put into either of the above programs they are used to remove noise, vignetting if present and dust spots on the sensor. You don't have to take darks and flats but its highly recommended to do so. Its best to take dark frames the same length exposure time and ISO as your RAW images of the object and I am yet to master flats properly so I'll leave that for someone else to explain.

Personally as I know your just starting out with a DSLR I would say just start taking some long exposure raw images and take 10-20 dark frames of the same length/iso and have a fiddle with them in Deep Sky Stacker (it's free) worry about flats later when you've got your head around the rest of it.

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A RAW file, aside from being significantly bigger, is basically the exact image data that the sensor picked up. It is unprocessed. This means, for normal daylight shooting at least, that you must process it, apply some sharpening etc. What RAW will allow is the ability to adjust the data far more than a compressed format like jpg will. As an example, the Adobe RAW processor will allow you to adjust the exposure of a shot by up to 4 stops in either direction. You probably wouldn't want to, but it'll allow you to recover a shot that JPG would totally ruin. Here's an example.

This is what the JPG would have looked like, roughly...


And here's my adjusted RAW version..


There would have been only a little that could be done to the jpg to recover it, and almost certainly to get the same result would have meant problems and loss of the image quality itself. The same ability and image data capture holds true for Astro but they are processed differently.

RAW data is normally 12 or 14 bits per channel but for practical purposes it can probably be considered to be 16 bit, whereas jpg is 8.


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Thinking photographically, the RAW file is the digital camera equivalent of the film negative. It holds the original data captured by the CCD. The jpeg produced by the camera is the camera's onboard image-processing software's version of the image. Having a RAW file is very useful as you can return to it again and again as your processing skills progress, and apply what you have learnt.


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