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Understanding the ZWO ASI 294MM Pro Camera
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Understanding the ZWO ASI 294MM Pro Camera
By Steven Bellavia
Like other new CMOS cameras being introduced into the astrophotography market, the ZWO ASI 294MM Pro seems to be a strange beast (but not in a bad way).
The first thing to notice is that it has a “jump” in performance at Gain 120.
I have highlighted the graph from the ZWO web page:
There is a drop in read noise from over 6 electrons to below 2 electrons, which causes a 2-stop increase in Dynamic Range, going from 11-stops to 13-stops.
But this is not where the strangeness ends.
That performance is in the “BIN2 mode”, which is supposed to be the default mode for the camera. (which it is not).
In BIN 1 mode, it becomes a 47 Mega-Pixel camera, with 2.31 micron pixels, performing as a 12-bit, high-resolution camera:
As you can see, no “jumps”, and 11 stop Dynamic Range at a similar Gain, with less than 2 electrons Read Noise. Not bad for a high-resolution camera, and similar to the ZWO ASI 183, but with a larger format sensor.
So my problem:
How do I know if I have “activated” the 14-bit BIN2 mode, and I am not just 2x2 binning the 12-bit BIN 1 mode?
So I started by running a script in PixInsight called “Basic CCD Parameters”.
You take two identical Flat Frames, two identical Bias Frames, one 30-second Dark Frame and one 300-second Dark Frame, and enter them into the Script. For all these images I selected 2x2 Bin from my image Capture software. Below are the results:
Not good. That is only 11 stops DR, and not the Full Well Count it should be, with a very strange Gain of 0.221 It should have been close to unity Gain (1.0 electrons/ ADU). But the read noise was low at 1.673 electrons. (An ADU is an Analog Digital Unit)
So was I not activating the BIN 2 mode, with the higher Dynamic Range? How could I tell?
So I reverted to a measurement method that was developed either by Craig Stark or Richard Berry (I don’t know who came up with this first, but it is ingenious). Nothing new. From around 2003, and likely for CCD cameras, not CMOS, but it should work.
And why is it ingenious? Here is the first part:
Part I - Gain and Full Well Count
Light, i.e., photons, hitting pixels and emitting electrons (what Einstein got his only Nobel prize in) are discrete events, so they follow a Poisson Distribution. Without getting too much into the mathematics, one very nice thing about the Poisson distribution is that the Standard Deviation = square root (Mean). That is quite amazing in itself, and very fortunate for me.
So by doing several pairs of flat frames, and then plotting the Mean against the square of the Standard Deviation of the difference between each pair, you are essentially plotting the Mean against the Mean. And anything plotted against itself should be a straight line with a slope of 1.0
But the ADC (Analog Digital Converter) in the camera is not reporting single electrons for each ADU it puts out. That is a function of the Gain. At low Gain, the camera is collecting several electrons for each ADU, and at high Gain, only a fraction of an electron for each ADU. At Unity Gain, it becomes 1.0 electron/ADU. So the slope of the line of Mean versus the square of the standard deviation (the Variance) IS the Gain. And then multiplying the Gain by the maximum ADU output, you also get the Full Well Count. Genius!
So after converting all my measurement (done in AstroImageJ) to 14-bit (essentially just dividing all the numbers by 4), this does now look like I am getting the “correct” Gain, and Full Well Count.
Why did the PixInsight script show a different result? I don’t know.
And because of that nagging question I had to go further. So back to Craig Stark and Richard Berry.
Part II - Read Noise
By taking a number of Bias Frames (I did 60), and then subtracting out a single Bias Frame from the stack, you can determine the Standard Deviation. This IS the Read Noise. This is how much each Bias frame differs from the Mean. (Noise, by definition, is the uncertainty. Since this frame has no photons hitting the pixels, and is too short for dark current, the only remaining uncertainty is the Read Noise in the electronics chain).
So how am I going to use this to determine if I am getting the BIN 2 mode performance?
This was my idea (but standing on the shoulders of Stark and Berry):
Do Bias Frames in the BIN 1 mode, on either side of the “magical Gain” of 120, where the “jump” occurs. Then repeat while binning 2x2, to see if there is the jump in the Standard Deviation, i.e., Read Noise.
And this is that result:
And look at that! When capturing images with a 1x1 Bin, there is no jump in Read Noise (StdDev), going from Gain 119 to Gain 121. But when capturing images with a 2x2 Bin, the jump is very noticeable.
Also note that this is in 16-bit ADU’s. It is less in electrons by at least a factor of 8 for the 12-bit BIN 1 mode, and at least factor of 4 less for the 14-bit BIN 2 mode. I say “at least” as you also need to multiply by the Gain, which might be a little less than 1.0 for BIN2 mode, based on the Gain test shown earlier.
Simply by “asking” the camera to bin 2x2 turns the “switch” to allow the improved performance.
I later found out that this is in the latest firmware of the camera, so it is independent of the software asking it to bin 2x2. It is not the “Default” mode, but very easy to get to with an image capture program. And I also just found out there is a BIN 3x3 mode, and a Bin 4x4 mode. In 3x3 you are in the 12-bit mode and cut your resolution from the 47 mega-pixel 8288x5644 to 4144x2822. Then if you go to bin 4x4 you get the performance boost, going from the 4144x2822 pixels, to 2072x1411.
Appendix A: The ZWO ASI 294MM
- Bob Campbell, Spot On and B@echoes like this