I came across an ad for a CCD camera that was non-antiblooming and was advertised as specifically designed for photometry. The camera was easily 5-figures in price. And that got me thinking that no one would spend that sort of money if they could just disable the antiblooming gate; that the camera has to built without one. The question that came up was, according to the photometry courses I took so far, the CCD pixels will go non-linear before being fully saturated and need ABG. So my question would be
- Is it impossible to disable the ABG in a CCD camera
- Would it be redundant to either disable the AGB or buy a camera without one since the readings would be non-linear before the electron start leaking into the surrounding pixels?
Anti-blooming CCDs are non-linear over most of their ADU range, strongly so near the saturation ADU count, and thus appeal to astrophotographers.
Non-anti-blooming CCDs are (should be) linear to nearly the saturation ADU count, and thus appeal to photometrists.
My understanding is that the behavior difference is embedded in the CCD chips themselves, not just in the camera holding the chips.
I have and am been using ABG CCDs for photometry and they are more than useful. My FLI CCDs are linear and can be used to very hgh ADUs (more than 55000). I also used in the past NABG CCDs mainly from SBIG (ST8,ST10) and they have been linear, too.
So there is no objection to use ABG CCDs for photometry. You should give it a try.
...while warmly encouraging everyone to check any camera for linearity, across its ADU range, before starting photometry with it (just as Josch has done).
What Eric says is true - ABG cameras become nonlinear about 1/2 full well for the sensor. However, almost all vendors adjust the electronic gain so that 65535 counts = 1/2 full well, and so they are linear over the entire digitized range. There are some ABG limitations. The pixel drain takes up some of the pixel space, and so full well tends to be less than for a NABG device, for example. Some vendors do give you an option as to how strong the antiblooming feature is applied. However, I've used both for photometry without any problem. As Eric and Josch say, I also strongly recommend that you check your sensor for linearity before using it for serious photometry,