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Old 27-09-2013, 11:47 PM
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naskies (Dave)
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Overscan calibration to improve dark frame scaling (SBIG STF-8300M + PI)

Before I begin... thanks must go to RickS and the authors of various astro texts/webpages for informing me about overscan calibration and for helping me to get it to work in PI

Background: most of us are familiar with bias, dark, light, and flat frames. With bias frames, we can use scaled darks - i.e. take a single set of dark frames say 10 min in duration, and mathematically scale them for calibration against other sub durations (5, 10, 15, 20, 30 min etc).

The problem is that when you do the pixel math calculations by hand, you may find that it doesn't actually work. For example, a 20 min master dark frame might have only 150% of the signal in a 10 min master dark frame - rather than the 200% you'd expect according to theory.

It turns out there are two separate components to the bias signal: a fixed column-by-column (or row-by-row) pattern, PLUS an offset that affects every pixel equally but which changes with every frame. Master bias frames calibrate for the former, but for the latter we need overscan calibration.

Overscan calibration: The overscan region is a non-imaging part of the CCD sensor used purely for calibration. That is, these pixels don't receive light (they may be physically opaque up on the sensor), but they are still connected to the same circuitry for reading pixel data. Think of them as pixels that are permanently wired to take bias and dark frames (even when the rest of the CCD is actually taking a light frame).

It seems that until recently, most amateur astro imagers and camera manufacturers just ignored overscan pixels. Depending upon the camera manufacturer, they can be downloaded as extra rows and columns in an image with a bit of driver tweaking*.

The median value of the overscan pixels is then subtracted from all frames - bias, dark, flat, and light frames (so you need data reduction/processing software such as PixInSight that supports overscan to do this).

I've done some number crunching with dark frames on my SBIG STF-8300M operating at -10 deg C (see the table in the attached images). For simplicity's sake, just look at the "600 sec --> 3600 sec" row, which shows a mean ADU/pixel error of 199.9 and 32.8 for standard and overscan calibration, respectively. The 199.9 figure means that if I just use standard bias / dark / flat / light frame processing and scale a 10 min master dark up to 60 mins, the scaled dark will overshoot a 60 min master dark by 199.9 ADU/pixel on average. Using overscan calibration, the error is only 32.8 ADU/pixel - a 83.6% reduction!

Note: these numbers are based on the same image pixels - the only difference is whether the median overscan offset value is subtracted from each frame. Furthermore, overscan calibration would also improve the accuracy of flat frames.

Step-by-step: Here are the precise steps that I follow for an SBIG STF-8300M:

1. Upgrade to the latest camera firmware (mid-2013) - earlier versions of the firmware had bugs that resulted in grossly incorrect bias frames on my camera. Note, this will invalidate your bias/dark library.

2. Enable the overscan region in the SBIG 64 Bit Driver Checker Utility by clicking on the "Config Drvr" button and enter 60 into the Overscan Rows/Columns field. Turn off Auto Bias Level Correction*.

3. Launch MaximDL - it now detects the camera as 1706x1296 (instead of 1676x1266 using 2x2 binning). See the attached image - it adds dark bars to the right/bottom of the image. Capture bias + dark + light frames normally.

4. When reducing data in PixInSight, enable the Overscan function (see attached image) for all frames - bias, dark, flat and light. This crops the image back down to the original 1676x1266 pixels, and subtracts the median ADU from a 1676x24 pixel overscan region below the image. You may also need to set the Output pedestal to prevent negative values (I set it to 200 in this case).

5. You can integrate the resulting subs as per normal. I generated the numbers in the first table using the Statistics process in PI using the same subs, with and without overscan calibration.

Conclusion: The difference with overscan is not huge - you probably won't notice it in the final stacked output unless you're going really deep with the processing. That said, the KAF-8300M has a read noise a bit over 8e- (22 ADU) so the improvement here is significant. Since this reduces noise for "free" (takes no extra imaging time, equipment, or $$$), I think it's worth doing

I'm more than happy to clarify anything that I've explained poorly


* I believe some manufacturers (QHY maybe?) have added overscan calibration functionality directly into the driver so that you don't have to do it yourself. I don't know any specific details.

** The Auto Bias Level Correction function - which I assume is meant to be automatically applying overscan calibration - on my SBIG STF-8300M has never worked correctly and gives erratic results, despite numerous firmware updates.
Attached Thumbnails
Click for full-size image (overscan_cal_results_-10degC.png)
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Click for full-size image (enabling_overscan_sbig.png)
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Click for full-size image (raw_sub_with_overscan.jpg)
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Click for full-size image (pixinsight_overscan_calibration.png)
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