From: boyd@apt0.sao.arizona.edu   
      
   sgpopticsguru@gmail.com wrote:   
   > On Saturday, March 22, 2014 11:40:21 PM UTC+8, Mikko OH2HVJ wrote:   
   >>   
   >>   
   >>> I repeat the test for various exposure durations under the same   
   >>> light   
   >>   
   >>> and do a linear regression on pixel values. The linearity of the   
   >>   
   >>> pixels are very good but they do not cross from zero, the   
   >>> regression   
   >>   
   >>> line has a DC offset.   
   >>   
   >>   
   >>   
   >> There's a bias voltage in the sensor output signal by both the   
   >>   
   >> chip amplifiers and purposefully added so that AD converter   
   >>   
   >> does not have to start from zero, where it may be non-linear.   
   >>   
   >>   
   >>   
   >> This is removed in astronomy by taking 'bias images' in dark with   
   >>   
   >> zero exposure time and substracting this from the image.   
   >>   
   >>   
   >>   
   >> --   
   >>   
   >> Mikko OH2HVJ   
   >   
   > Mikko, I do this. If you look at my description, I first measure FPN   
   > and take it out. Despite this I see a DC offset later which makes me   
   > wonder.   
   >   
      
   What is the magnitude of the effect your seeing?    (measured A/D counts   
   out of full range A/D counts) at the zero crossing?    What kind of   
   sensor and camera?  Manufacturer & model of both?   
      
   How are you  determining that the exposure is uniform over the nominal   
   exposure time?  What is the light source?   What method are you using to   
   start and stop the exposure.  Switching the light source on and off? A   
   mechanical shutter in the camera?  Interline transfer on the sensor?   
      
   This is just a guess based on limited information, but you may be   
   measuring the ramp of the opening  and closing of the camera's shutter   
   if it's a Scientific grade camera with a mechanical shutter.   That can   
     be removed in the reduction process.a   It can be a simple constant   
   subtracted from the exposure  if the shutter is always allowed to open   
   fully  before closing.   It's not an error.  It's just a fact that   
   doubling the time between opening the shutter and closing it allows a   
   little more than twice the light to pass though as opening it and   
   closing it twice.   
      
   You may also care if the exposure  at the center of the sensor is equal   
   to the exposure off axis.  Most scientific cameras put mechanical   
   shutters right in front of the sensor and have two leaves which open   
   from the center which  gives a longer exposure in the center of the   
   sensor.  That can be more  important to precision photometry than   
   linearity vs exposure time.   
      
   What is your specific application?   
      
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