The parameter jungle of astrophotography
(Telescopes and astro cameras only)
Refer to the update information on the left.
How to come to a good exposure of your photos.
This is not like taking your shots with a normal camera. Though the basic are the same the way you apply the many parameters is quite different from setting to 'automatic' and a computer in you camera making the best of it. It even differs quite a lot from the 'M' or fully manual setting that you are used to on your system camera or DSLR. The nice thing: all those basics about digital image building that you can forget or even just neglect in normal photography come to life here...
Start with this explanation of Cuiv. The information on this page and Cuivs video is crucial for determining the best way to expose.
His explanation is very technical but just because of that it is very thorough.
I only try to clear things around the parameters, you really have to watch Cuivs video for the complete story.
ADC = Analogue to Digital Converter. This has a bit range. The bit range is the amount of bits that it can comprise for the value (which in our case is a shade of light) of one pixel. So a 12 bit ADC can hold 0-4096 as a value for a pixel and a 16 bit can hold 0-65535
ADU = Analogue to Digital Unit. The carrier that delivers the value that the pixel produced to the ADC. So for a 12 bit ADC we have 4096 ADU available. Each ADU has to be filled with electrons coming from the pixel. The trick is in how many electrons fit in one ADU before it is full and the next ADU is needed to take the rest of the electrons still coming from the pixel. Much like a bucket size... or just a cup. The ADU however is a fixed thing so in order to regulate the moment that we want the ADU to stop filling we need to put a tap on it. That tap is your gain.
Dithering. There is no better explanation about dithering than in this video of Astro Escape.
Gain. The more electrons you need to fill the ADU the 'less sensitive' the pixel seems to be. There is just one sensitivity for a pixel/sensor, it is just the amount of electrons that you allow to pass to fill an ADU before moving on to the next ADU. So when you allow for lesser electrons to pass to the next ADU your pixels seem faster.
Unity gain. The gain at which one electron 'fills' an ADU so it can close and call for the next to be 'filled'.
Full well. (video from 09:33). In short: the maximum value the camera can measure. The maximum number of electrons that can be transferred to the maximum ADUs available. When exceeded you will loose the rest of the electrons causing clipping your image.
Read noise. (video from 11:50). In short: the minimum value the camera can measure.
Swamping the read noise. (video from 14:35). Adjusting exposure time or gain staying well above the read noise with your exposure time. Otherwise it will be hard to tell if a electron was generated as noise or a low energy photon.
Increasing the gain reduces the read noise so that leads to the observation that while reducing the read noise you also reduce the full well by increasing the gain. This leads to the dynamic range.
Dynamic range. (video from 15:39). The wider the dynamic range the more details. It is the difference between the points of details in the brightest and the darkest parts of your image.
Gain offset. (video from 24:44). Extra number added to the values over the entire range in order te prevent for total black pixels that contain no data at all.
Noise
(Optimum) Exposure time. (video from 28:43). This is the time needed to swamp the read noise. But all about this is quite much so part 2 of this subject: what is the optimum exposure time
Offset (separate video on setting the offset)
