January 28, 2017 Leave a comment
When you are deep into astrophotography, you’d probably start doing monochromatic deep sky imaging. A typical choice would be cooled CCD imager. These CCD cameras come in a variety of size format and architecture. The most affordable are interline CCD offered by Sony and Kodak (now ONSemi). Then the expensive full frame CCD requiring mechanical shutter from Kodak. Now however, as most of my previous posts and other similar studies have pointed out, CMOS holds a much better edge comparing to CCD. The only problem is, not a lot of CMOS based monochromatic devices are out there for your choice.
One option is the sCMOS from EEV and Fairchild. But I would imagine those to be expensive. Then CMOSIS who offer global shutter ones with monochrome in various format. But their dark current (~125 eps) and read noise (>10e-) figures are not a clear competitor to CCD in any way. Sony makes small format B/W CMOS but nothing bigger than 1 inch format. As such, we saw many specialized conversion service that scrape away the Bayer filter layer these years. Unfortunately, by doing so, you essentially remove the microlens array which boost the quantum efficiency. So in this post, I’m going to investigate the QE loss and gain with such modification.
The modification steps involve camera disassembly, filter stack removal, followed by prying open the cover glass, protecting the bonding wire and finally scratching the pixel array. For the last step, the scratching actually happens in layers. We’ll use IMX071 cross section EM image from Chipworks again for illustration.
The surface texture of an image sensor, as described by ChipMod, varies in resistance to scratching. The first layer to come off, are the microlens array indicated in green arrow. This layer is usually made of polymer. Further applying force would strip away the RGB Bayer filter as well, indicated by the red arrow. The yellow region represents the pixel pitch with the blue defining the photodiode boundary. Comparing the length of blue to yellow, we could estimate the fill factor is 50%. Because of the channel stop, overflow drain on the other axis, the fill factor is typically 40%. The gapless microlens above, focus the light rays onto the photodiode to bring the fill factor close to 90%.
The sensor was scraped into 3 vertical regions. From top to bottom, A: the microlens array is removed; B: both layer removed and C: the original one. Comparing A/B tells you how much light the color dye absorbs at that wavelength. A/C tells you how effective are microlens. Finally, B/C gives you the gain/loss after mod.
An identical test condition was set up with a 50F6.5 ED telescope in front of a white screen. 2 wavelength, Ha and Oiii are tested with 7nm FWHM filter in the back. Field is sufficiently flat so center regions are used to calculate mean intensity.
The microlens array performs as expected, it typically boost QE to 2x in native channels. Even in non-native color channel, the uLens still boost signal by 50% or more. Losing the uLens array is a major downside. But considering the absorption of color dye even in its peak transmission, stripping CFA actually minimize the QE loss. For example, in the red channel of H-alpha, signal was at 64% even though losing the uLens should impact the QE by more than half. The same is more apparent in Oiii wavelength. Because green channel peaks at 550nm, at 500nm the absorption is nearly half for this particular sensor. Thus the net result is no different from the original sensor.
In conclusion, mono-mod sacrifices some QE for resolution and all spectrum sensitivity. My estimation puts final peak QE at around photodiode fill factor, or around 45%. The state of art CMOS process maximized the photodiode aperture, making such mod less prone to loss of QE after microlens removal. This is in vast contrast with Kodak interline CCD structure where a 5 fold QE penalty should microlens are stripped away. The mod should perform well for narrowband imaging, especially for emission nebulas. However, a fully microlensed monochromatic sensor is still preferred for broadband imaging.