I spent considerable time testing the properties of my Canon T3i/600D and would like to share my results with the community. I was motivated in doing so by 3 things:
- Correcting what appears to be a. errors in this frequently cited post, and b. a lack of clear data for this camera esp. since sensorgen is no longer available. (e.g. imagers are often looking for the gain to use in PixInsight's Subframe selector process and are presumably using incorrect nos: ex1, ex2)
- Understanding the behavior of my own specific camera
- Developing a better understanding of the various terminology and testing procedures.
I'll describe my testing process in brief right after the results:
I largely followed the testing procedure shared by Craig Stark here and this description of CCD testing from the Handbook of Astronomical Image Processing. Testing makes use of the following statistical properties:
- The variance of a diff of images (bias, flats) is an estimate of twice the variance in one of the individual images (and addresses variations in individual pixel means, assuming the noise is the same across pixels)
- When taking flats, fluctuations in photons striking sensor and subsequent electron charge in the pixel follow Poisson statistics (Mean = Variance)
- The Gain and Read noise were each measured 4 times with diff data image pairs at each ISO
- All image analysis was performed in PixInsight
- When taking image diffs for variance measurement, a constant was added to ensure no clipping of values
- Since I'm dealing with an OSC, when measuring diff of flats, I only used nos. from one channel (Red)
- To ensure vignetting in flats did not affect results, I used a crop of the central area for flats for mean and variance eval
- I did not bother with dark current measurements for two reasons: 1. it is heavily temperature dependent and there is no way to truly measure or control the sensor temperature; 2. estimation will have to be indirect (through noise) since Canon does some auto dark current compensation for every image
Some notable observations:
- The Mean Bias signal at all ISOs is ~2048 DN
- Max ADU at saturation at all ISOs except ISO 100 is ~15831. At ISO 100 it is 14043. This means that of the 16383 DN (14 bit), only 15831 - 2048 = 13783 are actually used. Less at ISO 100
- Full Well capacity based on ISO 100 data is 24828 electrons
- Unity Gain is roughly at ISO 200. All gains are significantly different from Alexander Vasinin's work here which is the result that shows up most often during a search for gain data for this camera.
- The results reinforce my decision to image at ISO 800. It roughly marks the start of the ISO less state of the camera (read noise roughly varies linearly with ISO above this). Any lower and the read noise is much higher. Any higher and I'm losing dynamic range and amplifying other noise without any benefit.
Questions, feedback welcome. Hope this is useful for other T3i users.
Edited by arvindsh, 19 September 2019 - 05:08 PM.