ASIAir Exposure Calculation

There are quite a few guides out there that attempt to explain how to choose the optimal exposure for astrophotography.  Most are rather complicated to a beginner.  Since the ASIAir is supposed to make astrophotography easier lets explore this within the context of asiair. 

First though lets go over some basics.  By now you are likely familiar with stacking and with that the calibration frames that go with it, bias, darks, and flats.  For the sake of exposure calculation we will focus on bias.  What is Bias?  In a practical sense this is the absolute noise floor that an imaging sensor produces just to read the data from the sensor.  It has nothing to do with exposure and everything to do with the all of the analog to digital conversions and getting the data read quickly from the sensor.   I wont go into the technical details here.  We use a bias exposure to determine the baseline from which to measure our exposures.  To take a bias frame you simply take an exposure with the shortest exposure time possible and the lens or telescope covered. 

So what?  Well ASIAir makes it easy to measure the noise floor exposure.  We call this the Average ADU value.  Its an average value of all of the pixels in the image.  Since bias is relatively flat across the sensor we can use this as starting point for our exposure. The average ADU value can be read right on the preview information screen as AVG.  This value will change at different gain settings so you may want to make a chart.

We can use this same AVG value to measure our target images or light frames.  In all cases we are all skyglow limited so when we expose our light frame the AVG value is almost always the skyglow with the minimum AVG value being our bias noise.  For some large objects this value can be skewed slightly higher but in general this is the limit of detail that you can record at your location.

The rule of thumb is to expose your target light frames to about 400-600 AVG ADU above the bias level for monochrome cameras abs about 1200 above for color cameras.  Why so much higher for color cams?  Its because of the introduction of the bayer matrix over the mono sensor to create red green and blue pixels.  I wont go into the details here but most CMOS color cams use an RGGB quad of pixels to give your color representation.  There are two green pixels for every red and blue.  We therefore really need to expose for those single color pixels and to compensate for the color filter we have to expose about 2 to 3 times higher in ADU.

Why not just expose much higher and adjust later in processing?  In astro imaging we want to preserve as much dynamic range as possible but expose the image high enough so we can get our skyglow far enough above the bias noise that we can absolutely send that noise to black in our final image and be able to process the fine detail higher above it.  We also want to preserve as much star color as possible so its a balance between preserving star color and getting the bias noise down to black away from our subject.

If you live in highly light polluted areas you will find that using this method gives you really short exposures.  This is about all you can record given the skyglow you have.  In those instances you can add filters to block light pollution.   The same method holds true with filters.  Once you reach your target AVG ADU then recording longer will only clip your stars.

These are just starting values and depending on the filters you may never get to the AVG ADU value for skyglow within a reasonable amount of time.  Trial and error will give you an idea of where you need to be.  Make notes on the AVG ADU values that work best with each filter and use that as your basis.

I hope this helps get you started and gives you an idea of how to determine the optimal exposure for location and setup.  ASIAir makes it easy to calculate exposure and the AVG value should be your goto metric to determine that.