Quote:
Originally Posted by gregbradley
Ray I don't think the difference between mono and osc is 3X the time. That's because of interpolation. Where the firmware calculates likely values for a pixel based on its surrounding neighbours. In a sense this is similar to binning.
I would estimate the difference between a mono and a osc CCD to be more like 50% or even less. I have had an STL11 colour and it was quite sensitive. Where it had trouble was in the dim areas or dusty areas. They often were noisy.
Same with DSLRs. Some DSLRs have very high QE compared to CCDs. Nikon D800E Soony Exmor 36.3mp QE is supposed to be 59%. That is the same as my FLI Microline 8300. Having used both that seems about right.
Greg.
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Hi greg
OSC makers normally quote peak QE at the maximum of the green Bayer filter - ie if the chip is illuminated with narrow band green light of just the right wavelength, 59% of the photons will be detected by those pixels that are under green filters. What they do not say is that 0% of that green light will be detected by either the red or blue pixels - it won't even get to them. So the QE claims for OSCs are not remotely apples to apples when comparing with figures from mono chips - QE data will only be comparable at one chosen wavelength and then for only half of the pixels in the OSC.
The Bayer filters do not redirect photons, they absorb them, so if you illuminate the chip with white light, the Bayer filters will stop the red and blue light from getting to the green pixels, the red and green light from the blue pixels and the blue and green light from the red pixels - ie, about 2/3 of the light is absorbed before it gets to any one pixel. If you remove the filters, each pixel will now see the full red+green+blue of the white light, so it will have ~3x as many photons pouring into it - this is a mono chip. The mono chip will be much more sensitive to broadband light (~3x), simply because it sees so many more photons.
The Bayer matrix is limited in fundamental resolution to that of the 2x2 pixel group that make up a Bayer cell - ie Bayer chips have lower resolution than mono chips. The interpolation process attempts to estimate what the missing data might have been to try to recover some of the lost resolution and it is generally very successful in doing so. However, it does not increase sensitivity - that was irrevocably lost when the Bayer filters removed 2/3 of the photons.
OSC chips seem to have higher internal gain than mono chips so that you see the same sorts of signal levels when you switch between mono and OSC. However, the OSC is giving you the same signal from a lot fewer photons by running at higher gain and that will show up as noise in darker regions of the image - which is what you have observed.
rest assured, your 8300 will eat any OSC for breakfast when it comes to broadband SNR. That doesn't mean that OSCs are no good - far from it, they have many advantages, but they are definitely less sensitive than mono chips. regards ray
EDIT: just realised that I have both colour and mono versions of the original QHY5. Its hardly a top line chip, but I can see vastly more guide stars with the mono version - 3x more sensitivity would be about right.