Differences between various DSI imagers.
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The following information has been either obtained from the spec sheet of the DSI imagers or measured using methods desribed below.
Only the imagers that I have access to are listed here since I cannot make measurments on other devices types.
This page is under construction and will be updated once the measurments have been performed.

DSI C DSI Pro DSI Pro II
Resolution (pixels) 510x492 510x492 752x582
Pixel size (micron) 9.6x7.5 9.6x7.5 8.3x8.6
Relative sensitiviy 1= DSI-C (to be measured) N/A N/A N/A
Read Noise 15e-RMS 8e-RMS 9e-RMS

Gain

 0.53 0.22 0.24

 

Test Conditions

For the DSI-C the High gain box Checked and Color sharp model used.

For the DSI Pro the High gain box is Checked. 

 

Measurement method for Gain measurement

 

 

 

1) Set the exposure time to 1 second and adjust the offset in Envisage keeping the cap on the nosepiece of imager, adjusting the black level to maximum usable range, to get a reading as close to 0 as possible without cliping the data as shown by the histogram. Using an acquisition time of 1 seconds or more ensures that Envisage is using the slow read mode.

 

2) Take a series of pairs of flat frames frames at various light levels (2 at each level). I used a couple of polarizer filters to allow changing the relative light levels applied to the CDD sensor easily.

 

3) Using a small section near the center of the frames captured in step 3, about 100 to 300 pixels in size, calculate the mean value of each section in both frames. This mean value is the signal level for the frame and we use it as the x coordinate values of the point to add to the graph. Now  normalize the section of the second frame by multiplying it by the ratio of the Mean value of the first frame to the Mean value of the second frame This step compensate for the pixel to pixel gain variation, that would otherwise be intrepreted as being part of the read noise. We now substract the first section of the first frame from the second frame and calulate the standard deviation of the resulting frame. Squaring the value (computing the variance) and dividing it by 2 (since it comes from the difference of 2 frames the variance is doubled in the process). This value is used as the y cordinate of the point to graph

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4) Repeat step 3 for as many light levels as possible and add each point to the graph.

 

5) We can now curvefit the data in the graph to obtain equation of the variance to input level for the CCD and determine the slope of the resulting curve. The inverse of this slope is the gain.

 

 

Measurement method for Readout Noise

 

We begin by taking 2 bias frames. A bias frame is an image acquired at the fastest exposure time that the camera/sotware supports in the case of the DSI imagers this is 1/10000 sec. Use a section of center area of the frames (100-300 pixesl in size) and substract the first bias frame  from the second bias frame and calculate the standard deviation of the result. Calculate the readout noise by multiplying the standard deviation by the gain value measured above and dividing it by 1.414 (Square root of 2).

 

 Readout Noise = gain x (BiasFrame Std Dev)
                            1.414

 

 

Additional Info

Exposing the DSI Pro and DSI Pro II to approximately the same light level resuslts in approximately the same Reading on both cameras. However the noise level of the DSI Pro II appears to be approximately half the level of the DSI Pro. This is shown in the histrogram display, the maximum - mininum values was about 600 for the DSI Pro versus 300 for the DSI Pro II. The standard deviation of  the center area being about 60% higher on the DSI Pro, this is very much apparent as seen in the following single frames of 1 seconds in the following picture.

 

DSI Pro Single 1second frame DSI Pro II Single 1 second frame

 

The pictures below also shows the improvement the DSIPro II provides compared to the original DSI Pro. The following picture on the left was taken using the DSI Pro camera and a total of 42 minutes of exposure, while the one on the right totals only 20 minutes on the DSI Pro II. Both images were taken using a lumicon LF3085 Hydrogen Alpha long pass filter. The image on the left had to be blurred to reduce the apparent noise in the dimmer part of the nebulosity while the one on the right was sharpened using a unsharp mask.