When I first bought my STL-11000 I was concerned about the fact that QE charts indicated the sensitivity of the STL-11000 was much lower than the camera I was currently using (ST10XME), particularly in the red where the H-Alpha band lies.
Back in 2006 I had received my STL11000 just days before I left for the annual Okie-Tex star party. I hadn’t even had a chance to take an image with the new camera through a scope, so first light for the camera would be at the star party. As the first sub-integration came up at the star party, I was bracing for a much noisier image than I was used to. When it appeared on the screen however I was delightfully surprised at how smooth the sub-integration looked.
What happened is that the sub-integration that was displayed had automatically been resized to fit on the laptop screen and the resizing had made what would have been a fairly noisy image, very smooth and contrasty. Notice the following image. It appears fairly noisy. Move your mouse over the image however and notice that if 4x more pixels are captured and resampled to the same image size how much smoother the image becomes (note: this is the same exact image simply resampled downward 4x by area):
I had noticed this effect of resampling a long ago when I had first started producing thumbnails for images. Thumbnails always looked so smooth even if the regular sized image was noisy. If the STL11000 image is seen at full resolution then yes, the low QE is very apparent, but as you start resampling downwards in size for display on a website it becomes smoother and smoother as if it had been taken with a much more sensitive camera.
So the upshot of all this is that QE is not the only measure one should consider when deciding which camera to buy. One camera may be 2x more sensitive than another camera which has 4x more pixels, but when the images of the camera with 4x more pixels are resampled downward in size to match the first camera, the 4x pixel camera may produce images that are as just as smooth or even smoother.
The trade off is image scale. The image of the second camera will capture 4x the sky area of the image taken by the first camera (if 4x larger physical area), but the first camera will show more details if the images are converted to the same size.
One other advantage of the larger size chip camera is that image processing can be done on a higher resolution image before it’s resampled downwards. This can help bring out or otherwise preserve details before the resampling.
So the bottom line is that a ccd camera with a larger pixel density may produce images that are as smooth or smoother than a camera with a smaller pixel density but a much better QE when the images are converted to the same size.