Cooled CMOS Camera – P3: Image Quality

In the previous post I successfully obtained the test pattern with custom VDMA core. The next step will be to implement an operating system and software on host machine. In order to obtain real time live view and control, both software should be developed in parallel. Thus in this post, let’s take a look at the image quality with a simple baremetal application.

The sensor is capable for 10FPS 14Bit, 30FPS 12Bit, or 70FPS at 10bit ADC resolution. For astrophotography, 14bit provides the best setting for dynamic range and achieves unity gain at default setting. The sensor IR filter holder and the camera mounting plate are still in design. I will only provide a glimpse into some bias and dark images at this moment.

To facilitate dark current estimation, the cover glass protective tape was glued to a piece of cardboard. The whole sensor was then shielded from light with metal can lid. Lastly, the camera assembly was placed inside a box and exposed to -15°C winter temperature. During the process, my camera would continuously acquire 2min dark frames for 2 hours, followed by 50 bias frames.

Bias Hist

Pixel Intensity distribution for a 2×4 repeating block (Magenta, Green, Blue for odd rows)

The above distribution reflects a RAW bias frame. It appears each readout bank has different bias voltage in its construction. The readout banks assignment is a 2 rows by 4 columns repeating pattern, each color for each channel. A spike in the histogram at certain interval implies a scaling factor is applied to odd rows post-digitalization to correct for uneven gain between top and bottom ADCs.

Read Noise Distribution

Read Noise – Mode 3.12 Median 4.13 Mean 4.81

The read noise distribution is obtained by taking standard deviation among 50 bias frames for each pixel. Then I plot the above distribution to look at the mode, median and mean. The result is much better compared to a typical CCD.

Dark_current_minus_15

Finally the dark current in a series of 2-minute exposures is measured by subtracting master bias frame. Two interesting observations: 1. The density plot gets sharper (taller, narrower) as temperature decreases corresponding to even lower dark generation rate at colder temperature. 2. The bias is drifting with respect to temperature. This could be in my voltage regulator or in the sensor, or a combination of two.

The bias drift is usually compensated internally by the clamping circuit prior to ADC. But I had to turn this calibration off due to a specific issue with this particular sensor design. I will elaborate more in a later post. Thus to measure dark generation rate, I have to use FWHM of the noise distribution and compare against that in a bias frame. At temperature stabilization, FWHM was registered at 8.774, while a corrected bias is 8.415 e-. For a Gaussian distribution, FWHM is 2.3548 of sigma. Thus the variance for the accumulated dark current is 1.113 given the independent noise source. As such, the dark generation rate at this temperature is less than 0.01 eps. Excellent!

Preliminary Summary

The sensor performs well in terms of noise. For long exposure, the dark generation rate in this CMOS is more sensitive to temperature change than CCDs. The dark current is massively reduced when cooled below freezing point. The doubling temperature is below 5°C.

LEXP_001

An uncorrected dark frame after 120s exposure showing visible column bias and hot pixels