Interested ASI294MC Camera Folks,
OK, here I go again. This exceedingly long diatribe covers what I have learned about the ASI294MC over the past few months. Be forewarned. This write-up filled up 8 pages of text in OpenOffice Writer and will have to be split into multiple postings here.
TLDR / Executive Summary
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With quite a bit of testing of this camera, I now believe that calibration issues some users have reported with this camera are due to not one but several factors. I think these may be overcome but some users will balk at or simply disagree with the methodologies suggested here to improve calibration. While this information is based on considerable indoor bench testing of the camera, limited time under the stars has been available for full corroboration.
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I was not sure whether to put this post in one of the several ongoing ASI294MC threads or start a new topic specifically on ASI294MC Image calibration. I guess I will start here as a separate thread and post a couple of pointers to here as appropriate.
There has been a lot written on the "problems" with calibrating images from the ASI294MC camera as well as “problems” with the camera itself. Much of what I read was short on data and long on opinions, often from folks who have never used the camera. I had long been interested in this camera as a complement to my SXVR-H694 mono camera. I wanted an OSC camera with similar characteristics in terms of pixel size and overall chip size and field of view. Back in the middle of July, I purchased the ASI294MC and have been besieged with cloudy, rainy weather ever since. I have only had three sessions under the stars so far. I hope my luck changes so that I can get back outside with the camera.
I had researched the camera and read all I could from actual users of the camera. I have tried to contribute to the discussion about why this camera may behave differently from CCDs and other CMOS camera offerings. CMOS users have had to adapt new methods for calibration compared to the existing well established methodologies long used for CCD cameras. I suspect this camera could need some adaptation of our methods once again. I do think this camera still has a shot at being a very nice addition to the astrophotographer's tool bag. Here are some background links to a few of my previous posts.
Comments on Sensor Packaging Notes:
Comments on Sensor Cooling Notes:
Musings on Outlandish Camera Calibration Methods
All of the boredom brought on by our miserable weather lead me to do a lot of characterization of the camera to try to understand why some folks have problems such as those discussed in various threads here on CloudyNights and elsewhere. I have come to believe that the issues are due to no one single problem but rather several idiosyncrasies that can combine to cause issues with the calibration of images. I have reached my conclusions after shooting more than 3,500 Bias, Dark, and Flat frames spanning exposures from 0 to 960 seconds. I have done extensive analysis of the data and have sliced and diced it in many different ways as I investigated what I was seeing.
I will preface all this by saying that I still don't completely understand and cannot yet explain all of what I am seeing. The following overly long article will just relate what I am seeing as I run tests on my camera. My conclusions and (soft) assertions are based on my testing to date. Due to the volume of material, I will have to split this document into multiple posting on this thread.
My observations lead me to believe that the issues folks encounter from this camera can stem from the following areas:
- A Bias isn't always a Bias (in the sense of that obtained from a CCD camera)
- Short exposures (of less than 2 to 3 seconds) should be avoided (including Flats and Flat-Darks)
- Stable thermal control of the BSI (Back Side Illuminated) sensor is critical
- Long exposures should be followed with short Bias-like exposures for best results
- Fixed Pattern Noise in Bias Frames is color channel dependent
On the plus side, I have also concluded the following from my testing:
- The "amp glow" seen from the sensor is very linear with respect to exposure length
- Dark Frames may be successfully scaled with proper calibration
What Is A Bias Frame?
In my opinion, a contributor to calibration difficulties with this camera is an unexpected behavior at short exposure times.
In the CCD world, a Bias Frame is a dark exposure of zero seconds. It is used in calibration to eliminate the Fixed Pattern noise of the readout row(s) of the sensor as well as remove the Offset added to the A/D measurement result. Many to most CCD sensors can be made to produce this zero exposure time image frame and readout. On the other hand, many to most CMOS sensors are designed for the video market and have finite (but small) minimum exposure times. I suspect compromises may have been made in the design of the sensor and its operation with this single (video) market in mind.
We often think of a Bias Frame as we would a Dark Frame. We (or least I) have always assumed that a Bias Frame is exactly equivalent to a Dark Frame of 0 second exposure length. My testing of the ASI294MC has shown that this is not true for this camera. Any use of a Bias Frame straight out of the camera in calibration will result in an invalid calibration of the target frame. This is also true for other CMOS cameras and is one of the reasons calibration is done with uncalibrated Dark Frames and uncalibrated Flat-Dark Frames.
While many may still be using Bias-like Frames for calibration of the ASI294MC, following graph should show why this is problematic. The plot is from a test run with my camera using exposures of 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 3.0, 4.0, 5.0, and 6.0 seconds. A total of 140 frames were taken at 0 seconds and 10 frames each at all other exposure values. The Mean ADU value for each set of frames is plotted. The sensor was cooled to 0° C for these tests. Gain and Offset were set to Unity at g=120, o=30 in the ASCOM Driver. SGP (Sequence Generator Pro) was used for image frame capture.
In the plot, note in particular that the value of the Mean ADU at an exposure time of 0 seconds is greater than the Mean value at all exposures greater than 1 second but less than 3 seconds. Recall that a Bias (at exposure of 0 seconds) should equal the Mean ADU value of a Dark frame at 0 seconds. If we extrapolate the slope of the Dark Frames greater than 3 or 4 seconds back to 0, we find that a CCD-like Bias should have a value of 1914.04 ADU rather than the value of 1914.36 that is measured. For this camera, using a Bias out of the camera from calibration will give us some problems down the line. While the problem here seems small, the error can become noticeable after calibration after we stack our image calibration and target frames and try to aggressively stretch an image in post processing.
Fig. 1 - Mean Frame ADU Value vs Exposure Time (@ 0° C, Gain = 120, Offset = 30)
Short Exposure Timing For A CMOS SOC Sensor
In the plot above, also note the sharp increase in ADU values for frames between 0.2 and 0.8 seconds. I believe this is caused by timing these short exposures with circuitry residing on the sensor itself. Unlike most CCD camera sensors, CMOS camera sensors implement an SOC (System On a Chip) whereby many other imaging system functions are implemented right on the same silicon sensor chip. In the CCD world, these additional functions in the camera circuitry are external to the sensor itself.
Most Astro-Camera designs use in-camera timing for short exposures and the PC Device Driver timing for longer exposures. From the plot above, I believe the SOC is being used for all exposures up to 1 second. The sharp slope of the plot for values less than 1 second is most likely due to extra heat generated by on-sensor timer circuitry being active during the exposure. Above one second, the PC running the camera takes over the exposure timing functions. The slight inconsistency of the ADU values between 1 and 2 (or 3) seconds is due, I believe, to inconsistent timing from the device driver. PC systems are generally not good at real time operations. Longer times are more accurate as a percentage of the measured value. That is why things smooth out to a linear slope by the time we get to 4 second and greater exposures.
One side effect of this SOC timing of sub-second exposures is that amp glow becomes quite visible in the short exposures. If sub-second exposures are used with this camera (for Flats and Flat-Darks for instance), calibration issues may arise if everything is not matched exactly. As an example, looking at the plot above, think of what would happen if you calibrate a 1.5 second Flat with a 0.5 second "Bias" or Flat-Dark. (I think this practice may be common since most of us have come to think that Dark current in a 0.5 second exposure can be safely ignored.) If we do a calibration like this example, then the calibrated Flat will not be a true Flat and is likely to cause problems in the calibrated image later. Residual "negative amp glow" will be present and introduce gradients that are likely to be hard to account for later in post-processing.
This phenomenon of increased amp glow and heat generation / Dark Current for short exposures is present at all Gain and Offset values I have tested. The plot below shows similar data for Gains of 200, 300, and 390 in addition to the Unity Gain (g=120) plot shown before. Note that the slopes of both short and longer exposures become steeper as would be expected from increasing gain values.
Fig 2 - Mean ADU Value vs Exposure Time (@ 0° C, Gains = 200, 300, 390; Offset = 30)
Thermal Control Stability Of A BSI Sensor
The next area of difficulty with calibration of images from this camera is, I believe, associated with the nature of the BSI sensor construction. I think we will see similar effects as more large area BSI sensor cameras become available in the future.
A short introduction to BSI sensor construction is in order. “Normal” sensor construction is shown below. Here, the light sensitive pixels reside on the top of the sensor chip along with the chip control and SOC circuitry. The contact points to connect to the chip are also on the top side of the chip. Electrical connections are made between the top of the chip and the top of the ceramic carrier using short wires bonded to each. The back of the chip is bonded directly to the ceramic chip carrier with a special adhesive. This gives a lot of area of thermal contact. The cold finger of the camera is in contact with the bottom of the ceramic carrier. Any heat generated by the chip has lots of thermal contact area down through the ceramic carrier to the cold finger. Changes in temperature of the sensor chip due to self-heating and changes in set-point control of the cold finger are quickly equalized because of the extensive thermal contact area between all parts.
Fig 3 - Diagram of a typical Wire Bonded Sensor Chip Construction
Now, a BSI sensor is constructed differently as shown in the next figure below. In this case, the light sensitive pixels are at the top of the stack which is actually the back side of the silicon chip. The chip control and SOC circuitry reside on the bottom of the chip is this view. The electrical contacts for the chip also reside on the back / bottom side. Electrical connections are made between the silicon sensor chip and the ceramic carrier with the use of microscopic solder balls. Thermal contact between the chip and the carrier is solely through these solder balls. There are at least 248 solder balls making the connections. (There may be a few more to supply power in parallel to the chip. These extra connections are not brought out through the ceramic carrier. The carrier itself has a total of only 248 connections to the circuit board of the camera.) Solder balls in this type of application tend to be between 30 uM to 50 uM in size. Thus the total thermal contact area is much, much smaller than the thermal contact area for a non-BSI sensor chip. (The ratio of thermal contact area between a BSI and non-BSI sensor like the IMX294CJK is about 0.5 to 248 if my calculations are right.)
Fig 4 - Diagram of typical BSI Sensor Chip Construction
In the case of BSI sensors, thermal conduction between the silicon chip and the cold finger under the ceramic carrier is limited by the limited number of solder balls in the path. For BSI sensors, this thermal “bottle-necking or throttling” has some implications. Under steady state conditions, temperature control is not much different from non-BSI sensors. Both types of packaging can reach similar temperatures – they just equalize at different rates. However, when extra heat is generated by the sensor, it will take longer to stabilize to the temperature of the ceramic carrier and cold finger. Further, changes to the set-point temperature of the cold finger will take longer to normalize at the sensor chip compared to non-BSI structures.
In my mind, the above observations have implications to how we capture images with the camera. I think it is prudent to allow some additional “thermal soak time” after reaching the desired set-point temperature. Since the temperatures on the chip take longer to reach steady state after quick changes, the sensor probably won’t reached the same stable temperature that is measured at the ceramic carrier.
In the testing of my camera, I noticed that the stability of the sensor / carrier temperature can vary under some conditions. I have found that for very deep cooling where the TEC is operating at high power loads, the variation of temperature in the images is less stable than when the TEC is operating at lower power levels. Because the silicon sensor chip lags behind changes at the ceramic carrier, that likely means that sensor temperatures will also not be completely stable. After a number of ad-hoc experiments, I have set a personal goal of not running the TEC at more than 75% to 80% of full power. This can be easily verified with any camera by running the TEC at various power levels (and set-points) and then examining the variation in actual temperature recorded in the FITs Header of the captured images. A plot of recorded temperature over time for an imaging run (even with Dark Frames) can be very telling. In addition, a slow change in median ADU values for Dark or Bias Frames can be seen when the data is plotted against time. Furthermore, I have noted inconsistencies in the Dark Frame ADU levels for several minutes after the camera reaches the desired set-point.
At set-points that use the TEC to 50% power or less, the variation is held to +/-0.2° C for the most part in my camera. Mean values of the Dark Frames at a given temperature appear quite stable after 5 to 10 minutes into a run. At TEC power levels higher than 80%, variations of up to 1° C were noted – usually warmer. As TEC power approached 95%+, even more variations were seen and the overall recorded temperatures rose as time went by. I attribute this rise to self-heating of the camera body as a whole. The temperature of the body of my camera rose considerably at TEC Power levels above 95% when compared to 80% or less power. I think the camera’s fan reaches its limit for extracting heat and the excess TEC heat is then dumped into the camera body rather than the surrounding ambient air which will indirectly affect the sensor temperature after a period of time.
It is my opinion after testing my camera that temperature stability is more important than absolute sensor cooling in terms of getting steady repeatable Dark Frame Mean ADU values. Cooling to -30° C or less below ambient gave better calibration results than cooling more deeply. Since Dark Current is rather strongly influenced by sensor temperature, this has an effect on calibration of image frames. As I live in an area with quite warm Summers, I tend not to cool more than about 20° C to 25° C below ambient. (I actually have to stretch that in mid-Summer when my ambient daytime temperatures approach and exceed 40° C. At such extremes, it is hard for me to even reach my modest goal of cooling to 0° C before midnight.) A cooling of 25° C below ambient generally puts me in the range of 40% to 60% TEC Power with good thermal stability through a session.
Part Two To Follow (Multiple Long Exposure Result Peculiarities)
Edited by jdupton, 10 October 2018 - 10:38 PM.