Thu, 04/04/2024 - 07:17
Hi, I am a total newly. Reading about checking the linear response of my camera---I am planning to use my ASI533 mm-cpro mono-- I have worked out how to get the curve in Sharpcap and I understand this is important so the image doesn't saturate. But the graph scale is ADU's--how do I use this to calculate optimum/maximum exposure times?
Thanks
Mike
I have the…
Hi, Mike,
I have the same camera and I did the test already.
This camera is linear almost up to 65000 ADU. I advise keeping the level no higher than 50000-55000 ADU and not shooting with exposures shorter than 5 seconds.
Nikola
Mike,
You can't actually calculate what exposures to use for photometry from the linearity data for your camera. The data just tells you at what ADU value your camera departs from linearity and at what ADU value the sensor saturates.
My practise is to find a comp star in the field of view which is just a little brighter than the variable when the latter is at peak brightness. Do trial exposures, find the peak ADU value in the image of that comp star, and make sure the peak value is within the linear range of your sensor.
How far below saturation should your peak ADU be? You will probably read various recommendations if you go looking. Some observers say have your peak ADUs at about 50% of saturation. I try to get a bit higher than that because I have a 12 bit camera. The problem is that if you try to push your luck at high peak values you may find that some stars will saturate, particularly if you are doing time series photometry, during which the signal at your sensor will change.
I do time series over several hours, sometimes taking a few hundred exposures. I find it useful to plot peak ADUs for the target, comp and check stars against JD. It is revealing to see what the peak ADUs are and how much they change over time.
Roy
Not sure how relevant, but your camera may be linear with respect to ADU (which is most important) but not necessarily time, i.e. I'm guessing if you perform linearity measurements ranging from ms to minute range with a dim source (instead of just ms and a brighter source that say SharpCap uses by default) you might see some very nonlinear results at short exposures. My IMX533 camera (Player One) becomes very non-linear below about 2s exposures.
Keith