ISO

[Herzy]

What is the difference between ISO and exposure time? They both make brighter images so what settings should I use? (For DSO's)

[wxsatuser]

http://www.astropix.com/HTML/I_ASTROP/HOW.HTM

For DSOs use ISO 800 or 1600

[jambouk]

ISO is difficult to explain, mostly because I don't fully understand it  But it is not a measure of the sensors sensitivity to light. Start by thinking about global exposure settings. There are three main ways to determine how light your final image is. One is to alter the aperture of your lens; the "f-stop". The lower the number (eg f/3 cf f/12) the wider open the iris is inside the lens and more light gets through. The second way is the length of time the shutter is open, the exposure time or shutter speed. If the shutter is open for a long time lots of light gets through to the sensor so the image will appear lighter than if the shutter is only open for a short period of time. So a shutter speed of one 4000th of a second (1/4000) is a very short exposure time and 1/25th is a long exposure time (in day light anyway). In astrophotography you might use a shutter speed of 20 minutes! The third way is to alter the ISO; the sensor is fixed so its impossible to alter its sensitivity so how I imagine it is that it multiplies the value in each pixel. ISO 100 say just multiples the results from each pixel by 1, ISO 200 causes the camera to multiple every pixels value by 2, up to high ISO where it is multiplying the value from each pixel by a high number. The issue is though that each pixel is not only stimulated by incoming photons but also stimulated by heat, and other forms of noise, so when one increases the ISO you are multiplying the good signal as well as the unwanted background noise, which is in part why high ISO images often appear more grainy than low ISO images.

When a DSLR is at prime focus on a telescope it isn't generally easy to alter the aperture, so that is a fixed value and the only two ways to alter the exposure is to alter the ISO or to alter the exposure time / shutter speed.

I highly recommend this book:

http://www.amazon.co.uk/Digital-Astrophotography-Practical-Amateur-Astronomy/dp/0521700817

James

[Herzy]

Thanks for taking the time to write that! I think I understand now but one more question...

Suppose the brightness of each pixel is represented as a number value; the higher the number the brighter. Increasing ISO increases the brightness of each pixel. So why is ISO permanent? Why can't ISO be adjustable if it is only represented by numbers? If ISO is a direct representation of the brightness of the pixels, shouldn't I be able to go to my already taken photos and mess with the ISO?

[Davey-T]

You can mess with your photos to your hearts content in Photoshop or similar, some pixels may be zero and some saturated so you need a way to mess with some but not others usually by selective stretching.

Dave

[jambouk]

ISO is permanent. The processors inside the camera effectively count how many photons are in each bucket; imagine each pixel is a bucket which catches photons. Unfortunately noise is counted as photons too (not really but this is how I imagine it and if anyone else gets their knickers in a twist by their explanation then they are welcome to sit down and type out the real reason in language which us thickos can understand); this is where the problems of increasing ISO come in. So the processors count the number of photons in each bucket, and if you have set the ISO to say 200, the camera will multiply the number of photons by 2, then as it writes the information to the memory card it writes the value as 2 x number of photons; so if there were 13 photons in bucket A1, it will write on the memory card A1 contained 26 photons. That information is then fixed for that image forever.

 

However, as Davey has said, when you upload that image to your PC, you can work in Photoshop (or other software packages) to alter the levels and change the "curve" to adjust the relative brightness of the pixels; so you can do fancy things like make all the pixels which are quite dark (not many photons) appear brighter without necessarily increasing the brightness of the brighter pixels... But there is only so much stretching of that data you can do before you again start introducing artifacts and just making the noise in the image more apparent.

 

The key is to capture the best possible data in the first place - so the best signal to noise ratio (the optimum ISO (there are threads and websites which indicate this for various cameras and for most run of the mill DLSRs it is about ISO 400)), and as many photons as possible [1]. Remember the aperture is fixed (you can't increase the aperture of your telescope unless you buy a bigger telescope) and now the ISO is fixed at say 400, so the only other way to capture more photons is to increase your exposure time. But to achieve long exposure times (anything over 15-30 seconds in my book) you need a pretty good mount and you really need to guide.

James

[1] The other thing you need to think about further down the line is your sampling, how big an area of the sky does each pixel cover. Someone else can explain this, I'm well out of my depth now