Dear All
I have recently borrowed a ST7 with an KA0401E chip with antiblooming. My camera is a Genesis CCD with the non antiblooming version of the same chip that I build many years ago. I have been amazed at the difference in the sensitivity. Some of this may be related to using different programs to take the pics. Is it due to the antiblooming or just different design of camera?
The antiblooming does make a large difference in the QE of the chip. This is simply because of the physical difference - the fact that antiblooming chips have tiny walls between all the pixel cells to prevent light spreading to neighbouring cells - these walls take up space and you loose the light which hits them, so hence your QE drops.
While antiblooming has a big impact on QE there seems to be more to QE than antiblooming/NABG. I'm not sure what the other factors are, I suspect pixel size is one.
Thank You Roger.
Both of the chips are the same size with 9 um pixels so this shouldn't be a factor. I will try to image an item with both cameras (when this cloud goes away ) and measure the difference.
Being a real noob to CCD myself my understand is exactly what Roger has mentioned...the antiblooming chips are approx half the sensativity as non antiblooming.
Pixel size as specified in data sheet for various CCD's is not really a pixel size... it is rather pixel center distance.
So it may well be that pixels on your other chip iare actually bigger.
Also, some manufactures are placing a greed of tiny lenses above the chip surface, thus effectively maximizing the individual pixel size.. up to a pixel spacing.
To check this is true, you just have to divide the sensor area lenght by the number of pixels in the line and you will get the pixel distance... which will be the pixel size from data sheet.
Bojan
Could the difference be the presence/absence of microlenses over the cells? On most CCD chips you can buy either with or without microlenses, and they make a big difference in sensitivity.
Indeed pixel size can increase sensitivity as the larger the pixel, the more photons are collected. The CCD camera then turns these into electrons and are read-out. Considering both cameras are using the KAF-0401E chip, the pixel size of 9 x 9 μm is identical. The difference will solely be related to Non Antiblooming (NABG) vs Antiblooming (ABG).
A little about Blooming...
"Blooming is a phenomenon that occurs when electrons fill the well of a given pixel and spill over into adjacent pixels, causing a bright, vertical streak that destroys the data contained within those adjacent pixels. CCD chips that bloom are called "non anti-blooming gate" chips (NABG) and are typical of the Kodak KAF series. Anti-blooming chips do not have this problem. They contain an "anti-blooming gate" (ABG) that bleeds off electrons before they can spill over into adjacent pixels. ABG chips are typical of the Kodak KAI series and the Sony Exview series. Sounds like we should all be using ABG chips to avoid blooming, right? In order to appreciate why the choice isn't always so simple, take a look at the Quantum Efficiency (QE) curves of a NABG versus an ABG camera, and you will see the problem (QE curves are usually available on CCD vendor websites). QE is a measure of how efficiently a chip converts photons to electrons. Because the ABG technology takes up space in the pixel, less surface area is available for detecting photons. Thus, the QE of ABG chips is comparatively quite low when compared to a NABG camera." http://www.starrywonders.com/ccdcame...derations.html
Recent improvements to the Kodak CCD sensor range also includes Microlensing which Bird points out. However the chips are flagged as ME or M. Your KAF-0401E does not have this. The E series are sensitive not only at the red ~650nm, but also the blue ~450-500nm wavelengths.
Thank you all.You are correct Jase. My CCD does not have any micro lensing. When I purchased it from kodak I deliberately chose the NABG version. They even had an option of having the cover slip glued or just taped on. I chose the glued version. I assume the more modern chips are different and the relative price has dropped. It is still a good camera but I never put a shutter in it and it would be very difficult to do so now. The SBIG camera has a shutter and I was hoping that I could use it instead. I have been pretty underwhelmed by its sensitivity so will probably just keep to my old camera.I have never been terribly interested in spending many hours aiming at one object to make an pic that is just put on the web but do enjoy finding dim objects. The sensitivity is important for this end. Some of my pics are OK (see below) but not of the standard of those spending much nore time than I do.
) and measure the difference.
