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• Drakkith
In summary, the conversation discusses the results of testing an astro camera and finding a temperature dependent readout noise value. The results show a standard deviation of pixel values in ADU's and e- for different temperatures. The camera being used is an SBIG ST-2000xm with a KAI-2020M sensor. There is a discussion about the source of the noise and the possibility of using temperature dependent bias calibration frames to model it. It is also mentioned that newer cameras have less read noise and higher sensitivity.
Drakkith
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Hey all. I tested my astro camera a while back and found what looks like a temperature dependent readout noise value.
Which is odd, as everything I've ever read has told me that readout noise is independent of sensor temperature.

I took 50 bias frames at 5°c intervals, from -15°c to +20°c and recorded the value of a single pixel in each frame (the same pixel in all frames).
I found the standard deviation of these pixel values in ADU's and then converted that to e- for each temperature.
Results are as follows:

 Temp in c -15 -10 -5 0 5 10 15 20 Stdev in e- 8.368 9.056 9.492 9.525 10.502 10.278 12.106 14.979

Camera: SBIG ST-2000xm, with KAI-2020M sensor.
Specs can be found here: http://www.company7.com/library/sbig/sbwhtmls/st2000xm_new.htm

I'm wondering if I'm doing something wrong in my analysis or if my understanding of how CCD's function is incorrect. Any ideas?

That sounds a little odd indeed. The noise in a CCD image comes from the signals it records. Since the signals are all Poisson distributed statistical processes the noise from each signal source is the square root of the signal.

The readout noise, often called bias noise since it comes from the bias signal that is added to the the rest of the signal (thus biasing it) to create a minimum level (a pedestal) that makes sure that the values after the A/D converter are positive, should as you say be independent of temperature.

How long are the exposures of your bias frames? What are the average pixel vales i.e. do they also increase in the expected way (square of noise) with the increasing noise?

ETA: pintle ->pedestal

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glappkaeft said:
How long are the exposures of your bias frames?

Essentially zero.

glappkaeft said:
What are the average pixel vales i.e. do they also increase in the expected way (square of noise) with the increasing noise?

Here are the average pixel values:

 Avg Pixel Value (ADU's): 1023 1021.42 1017 1028.02 1028.5 1043.1 1073.88 1125.44 Temp in c: -15 -10 -5 0 5 10 15 20

As you can see, there is negligible or no increase until around +10 C is reached, at which point it rapidly increases. Note that the amplifier circuitry is uncooled and remained at ambient temperature the entire time.

While you kept the off-chip circuitry at a constant temperature, the on-chip amplifier is at the same temperature as the CCD. This amplifier has some non-zero noise associated with it, and to me it is not surprising that the noise introduced by this amplifier decreases as the temperature decreases. In principle, you could use your data to extract this component of the noise and separate it from the Poisson noise, as discussed by @glappkaeft above.

trurle
Looking at your additional data I agree with @physguy that something in the CCD camera system is adding noise in a non-Poisson fashion. I'm not a CCD sensor expert but the amplifier sounds like the most likely suspect.

Considering the relatively low levels of the noise and the way low noise sources sort of disappear when added to larger sources (typically the photon and dark current signals) I doubt it really has a big effect on your images but it could in principle be modeled. The simplest way would probably be to use the temperature dependent bias calibration frames you already have made.

It would be interesting to see if it has an non-linear effect on dark frame calibration.

Drakkith said:

Camera: SBIG ST-2000xm, with KAI-2020M sensor.
Specs can be found here: http://www.company7.com/library/sbig/sbwhtmls/st2000xm_new.htm

I'm wondering if I'm doing something wrong in my analysis or if my understanding of how CCD's function is incorrect. Any ideas?
KAI-2020 sensor has a built-in amplifier. It is natural for such amplifiers to become noisier at high temperature. And noise you reported is under sensor specs. The rapid noise increase from 10C is likely self-heating effect when high-current output stage of video amplifier is heating the input stage. It is commonly happens when chip maker overdo "cost-performance optimization" by cramming too much circuits in too small area.

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trurle said:
And noise you reported is under sensor specs.

What do you mean? Sensor specs are about 7.6 to 7.9e- rms. All of my measurements are above that. Almost double at higher temps.

trurle said:
The rapid noise increase from 10C is likely self-heating effect when high-current output stage of video amplifier is heating the input stage.

Hmmm. Why would this be a larger problem at slightly higher temps? I should also note that all of my frames were taken with a small pause in between in order to allow for the amplifier to cool back to ambient temp after readout.

trurle said:
KAI-2020 sensor has abuilt-in amplifier. It is natural for such amplifiers to become noisier at high temperature.

phyzguy said:
While you kept the off-chip circuitry at a constant temperature, the on-chip amplifier is at the same temperature as the CCD. This amplifier has some non-zero noise associated with it, and to me it is not surprising that the noise introduced by this amplifier decreases as the temperature decreases.

Ah, okay. I wasn't aware that there were on-chip amplifiers in addition to the off-chip amplifier.

Drakkith said:
Ah, okay. I wasn't aware that there were on-chip amplifiers in addition to the off-chip amplifier.
The signal being read out of the CCD is extremely small, often just a few electrons. There is no way to pipe this tiny signal off chip without losing it or introducing a lot of noise. So there is an amplfier on chip (often just a singe transistor) that converts this charge signal into a voltage signal. This voltage signal is then routed off chip where it is amplified further by the off-chip circuitry.

Drakkith
Well, I guess this is just more reason to keep my chip cooled and hope I can afford a new camera in the near future. My ST-2000XM was probably about 5 years old when I bought it used in 2010. Been itching for a long time to get a new camera. Newer ones commonly have less than half the read noise as mine and their sensitivity is double or more in some areas of the spectrum. It would be nice to have more than ~33% QE in the Hydrogen-Alpha band.

Drakkith said:
What do you mean? Sensor specs are about 7.6 to 7.9e- rms. All of my measurements are above that. Almost double at higher temps.
Which specs? I refer to
onsemi.com/pub/Collateral/KAI-2020-D.PDF
which lists 16-20 e- "readout noise".
Drakkith said:
Hmmm. Why would this be a larger problem at slightly higher temps? I should also note that all of my frames were taken with a small pause in between in order to allow for the amplifier to cool back to ambient temp after readout.
Because video amplifier cores are commonly driven with constant current, the thermal dissipation grows typically as T^2.5, where T-absolute temperature. Also, the thermal conductivity of chip fall at higher temperature, providing a positive temperature feedback. In worst case of self-heating effect, step-like performance dependence on temperature or even temperature-performance hysteresis may happen.

trurle said:
Which specs? I refer to
onsemi.com/pub/Collateral/KAI-2020-D.PDF
which lists 16-20 e- "readout noise".

I don't think that's accurate for my chip. If I understand it correctly, the readout noise depends on how fast the pixels are read out, with decreasing noise the slower the readout. The 16-20 is listed for 20 and 40 MHz readout rates I believe, but I think mine operates at around ~400 KHz, as it takes about 4.5 seconds to read out the entire 1.9 megapixels.

trurle said:
Because video amplifier cores are commonly driven with constant current, the thermal dissipation grows typically as T^2.5, where T-absolute temperature. Also, the thermal conductivity of chip fall at higher temperature, providing a positive temperature feedback. In worst case of self-heating effect, step-like performance dependence on temperature or even temperature-performance hysteresis may happen.

Got it. Thanks!

trurle
Yes, circuit noise is definitely temperature dependent. In imaging sensors it is called 'dark noise.'

Serious astronomical imaging sensors are always cooled. For a few decades there was a company, Santa Barbara Instrument Group (SBIG) (since purchased by diffractionlimited.com), that was THE supplier to the advanced amateur and professional astronomer market of thermo-electrically cooled cameras.

If there is an Astronomy group near you, ask them about the SBIG cameras, and what replaces them.

Cheers,
Tom

1. What is CCD readout noise?

CCD readout noise refers to the electronic noise present in the output signal of a CCD (charge-coupled device) sensor. It is caused by random fluctuations in the electronic components and can affect the accuracy of the sensor's measurements.

2. How does temperature affect CCD readout noise?

Temperature has a significant impact on CCD readout noise. As the temperature of the sensor increases, the readout noise also increases. This is because higher temperatures cause more thermal energy, which can introduce more electronic noise into the sensor's output signal.

3. Why is it important to consider CCD readout noise vs temperature?

It is important to consider CCD readout noise vs temperature because it can affect the overall performance and accuracy of the CCD sensor. If the readout noise is too high, it can obscure the signal from the object being measured, leading to inaccurate data. By understanding how temperature affects readout noise, scientists can optimize the sensor's performance for their specific needs.

4. How can CCD readout noise vs temperature be measured?

CCD readout noise vs temperature can be measured by taking multiple images of a uniform light source at different temperatures and analyzing the noise levels in the resulting images. The readout noise can then be plotted against the corresponding temperatures to determine the relationship between the two.

5. How can CCD readout noise vs temperature be minimized?

CCD readout noise can be minimized by cooling the sensor to lower temperatures. This reduces the amount of thermal energy and therefore decreases the amount of electronic noise in the output signal. Additionally, using a sensor with lower readout noise capabilities can also help to minimize the effects of temperature on the noise levels.

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