Q博谈天文摄影之
暗场比偏置场还暗问题 与 过扫区矫正
经常会碰到一些同好询问一个问题，拍摄的暗场比偏置还更暗，也就是说暗场的背景平均值比偏置场的平均值还低。在通常理解里面，偏置场曝光时间为零或者非常短，而暗场的曝光时间是长曝光，大家会觉得长曝光下，热噪声在增加，因此暗场的平均值应该比偏置场更高才对。为什么反而更低了呢。这时候大家首先可能会怀疑相机出问题了，或者驱动有问题。
其实这个现象是很正常的，而且这个现象还与一个名词：过扫区校正 非常相关。
对于CCD而言，这个问题产生的原因来源于AD转换器的参考电压的温度漂移。CCD输出的是模拟信号，要通过AD转换器转换为数字信号。AD转换器需要一个电压作为基准进行转换，这叫做参考电压，就好比我们测量一个长度，必须有一把标准的尺子。如果这个尺子的刻度变化了，那么测量出来的值也会跟着变化。
因此，参考电压如果变化了。那么AD转换器转换出来的值也会变化，就会引起图像的整体亮度变化。
参考电压是通过一个基准电源源提供的。通常这些电压源都会有一定的温漂。以常见的高精度基准电源源TL431为例，它的温漂是50ppm每度. 就是每一摄氏度变化百万分之50.乍一看这个值很小。但是对应到16位的ADC的范围而言，每一度就会变化好几个ADU. 十度就会是几十个ADU。这个就很容易看出来了。
由于通常相机是CCD芯片制冷并且进行温控，而不对ADC进行温控。因此就会导致环境温度变化了，图像整体会略微变亮或者变暗。
这个就容易解释为什么暗场会比偏置场更暗的原因了。通常大家会连续拍摄多张暗场和偏置场进行叠加。由于偏置场的曝光时间极其短，因此主要时间都在读出，ADC处于连续的工作状态，自身温度就升高。引起参考电压发生漂移。而连续拍摄暗场的时候，由于暗场的曝光时间长，在曝光的时候，ADC不工作，只有读出的过程才工作，因此温升没有那么大。
所以就出现了暗场的平均值比偏置场还要暗的情况。
另外，环境温度的变化也同样会引起这个效应。比如我们拍摄偏置的时候，室温是25度，而拍摄暗场的时候，室温是-10度，那么就会产生可观的漂移。
如何使用OVERSCAN校准解决该问题
解决该问题的方法可以有若干种。这里我们首先介绍手动校准方法。以便让大家深入理解校准的原理。下述方法以MAXIMDL软件为例。
首先我们可以拍摄一系列的偏置场，暗场，明场，平场。拍摄的时候注意需要保存OVERSCAN区域，不能选择“忽略OVERSCAN区域”
分别将偏置场，暗场，明场，平场进行叠加，获得主偏置场，主暗场，主明场，主平场。
打开主偏置场。可以看到在图像右边有一个黑边，这个就是OVERSCAN区域，然后用鼠标拉一个框。这个框值需要取一部分黑边即可。记录下这个框内像素的平均值。例如500（用INFO窗口可以显示）
步骤1
使用PIXELMATH工具。将图像A设置为主偏置场。计算方法选择NONE。然后在OFFSET里面，输入上述平均值的负数，-500
执行PIXELMATH. 完成主偏置场的OVERSCAN 校准。
用同样方法，对主平场，主亮场，主暗场进行OVERSCAN校准。
步骤2
然后用PIXELMATH工具，将图像A设置为主平场，图像B设置为主偏置场。计算方法选择SUBTRACT(减法）。OFFSET设置为0。运行。得到的结果就是经过了精确校准的平场。
用PIXELMATH工具，将图像A设置为主亮场，图像B设置为主暗场。计算方法选择SUBTRACT(减法）。OFFSET设置为0。运行。得到的结果就是经过了精确校准的亮场。
用PIXELMATCH工具，将图像A设置为精确校准的亮场，将图像B设置为精确校准的平场。计算方法选择DIVIDE(除法）。OFFSET设置为0。运行。得到的结果就是经过全套校准之后的图像。
上述计算方法依据的公式为
校准后的图像=（L-D)/(F-B)
从这个公式也可以看出来。偏置场主要是在平场校准的时候其作用。暗场主要是在对亮帧做暗场校准的时候其作用。
平场如果不准确，容易出现的问题是校正过度(四个角比中心还亮），或者欠校正。
亮帧校准不准确，就是热噪点扣不准确。（有时候CCD温度已经是相同了，还扣不准，这个是一个可能性）。
自动校准方法
目前一些软件具备了OVERSCAN自动校准功能。比较典型的例如SGP软件。因此可以对此进行设置。具体方法请咨询相关软件使用高手
注：假如出现了过扫区平均值比图像有效像素去还要高，或者很接近怎么办？
这样使用减法就会导致图像被全部减成零。或者很接近导致有部分像素被减成零。这种情况下，对于步骤A中的-500，应该再加上一个常数，比如1000. 这样结果是+500. 然后执行过扫区校正。注意这个仅限于步骤1，步骤2的OFFSET还是为0
CMOS的情况
CMOS相机也存在此类问题，并且影响CMOS的更为复杂，因为某一些CMOS芯片内有具有光学黑电平校准功能。会自动做一次光学黑电平校准。光学黑电平校准和OVERSCAN区域校准是有区别的。区别在于光学黑电平区域，是包含热噪声的。而OVERSCAN区域是不包含热噪声的。因此当曝光时间较长的时候。CMOS的光学黑电平区域的暗电流增加，导致光学黑电平校准的时候扣的值，比曝光时间短的时候扣得更多。从而也导致了暗场比偏置场还要暗的情况。关于CMOS的校准问题，请参考《从单反相机到QHY163M》一文。该文章的英文版在http://www.alessiobeltrame.com/wp-content/uploads/2017/09/QHY163M_review_EN.pdf
中文版在“QHYCCD天文摄影”QQ群的群文件中可以找到。
参考文献
1. CCD Theory http://astro.ufl.edu/~lee/ast325/handouts/ccd.pdf
2. Using the Overscan Bias Correction http://www.mirametrics.com/tech_note_overscan_bias.php
3. Calibration http://www.astro.caltech.edu/~aam/science/thesis/total/node15.html
English Version
(Machine Translated by GOOGLE Translator)
Dr.Qiu's Talking about astronomical photography
Why Dark Field Is Darker Than Bias Field & What's Overscan Calibration
Often encounter some problems with the same question, dark field is even moe dark than the bias. In the usual understanding, the bias field exposure time is zero or very short, while the dark field exposure time is long, we will feel for long exposure frame has more dark current, so the average value of the dark field should be higher than the bias field. Why is it lower? At this time we may first suspect that the camera problems, or drive a problem.
In fact, this phenomenon is very normal, and this phenomenon is also a noun: over the sweep correction is very relevant.
For CCD, this problem arises from the temperature drift of the reference voltage of the AD converter. CCD output of the analog signal, through the AD converter to convert digital signals. The AD converter requires a voltage as a reference for conversion, which is called the reference voltage, as if we were measuring a length and had a standard ruler. If the scale of the ruler changes, then the measured value will follow the change.
Therefore, if the reference voltage changes. Then the value of the conversion of the AD converter will also change, it will cause the overall image brightness changes.
The reference voltage is supplied via a reference supply. Usually these voltage sources will have a certain temperature drift. Taking the common high-precision reference source TL431 as an example, its temperature drift is 50ppm per degree, that is, every change in degrees Celsius by 50 per 1 milion. At first glance this value is small. But corresponding to the 16-bit ADC range, each degree will change several ADU. Ten degrees will be dozens of ADU. This is easy to see out.
As the camera is usually the CCD chip cooling and temperature control, without the temperature control of the ADC. So it will cause the ambient temperature to change, the overall image will be slightly brightened or darkened.
This is easy to explain why the dark field will be darker than the bias field. Usually we will continue to shoot multiple dark field and bias field for superposition. Since the exposure time of the bias field is extremely short, the main time is read out, the ADC is in continuous operation and its temperature rises. Causing the reference voltage to drift. And continuous shooting dark field, because the dark field exposure time is long, in the exposure time, ADC does not work, only read the process to work, so the temperature is not so big.
So the average of the dark field appears to be darker than the bias field.
In addition, changes in ambient temperature also cause this effect. For example, when we shoot the bias, the room temperature is 25 degrees, and shooting dark field when the room temperature is -10 degrees, then it will produce considerable drift.
How to use OVERSCAN calibration to solve the problem
There are several ways to solve this problem. Here we first introduce the manual calibration method. In order to let everyone in-depth understanding of the principle of calibration. The following methods take the MAXIMDL software as an example.
First we can shoot a series of bias field, dark field, bright field, flat field. Note that you need to save the OVERSCAN area during shooting, you can not select "ignore OVERSCAN area"
Respectively, the bias field, dark field, bright field, flat field superposition, access to the main bias field, the main dark field, the main field, the main flat field.
Open the main bias field. You can see in the image on the right there is a black side, this is the OVERSCAN area, and then use the mouse to pull a box. This box value need to take a part of the black edge can be. Record the average of the pixels in this box. Such as 500 (with INFO window can be displayed)
step 1
Use the PIXELMATH tool. Set the image A to the main bias field. Calculation Method Select NONE. Then in OFFSET, enter the negative value of the above average, -500
Execute PIXELMATH. Complete the OVERSCAN calibration of the main bias field.
In the same way, the main field, the main bright field, the main dark field OVERSCAN calibration.
Step 2
Then use the PIXELMATH tool to set the image A to the main plane and the image B to the main bias field. Calculation Method Select SUBTRACT. OFFSET is set to 0. run. The result is a precisely calibrated flat field.
With the PIXELMATH tool, set the image A as the main bright field and the image B to the main dark field. Calculation Method Select SUBTRACT. OFFSET is set to 0. run. The result is a precisely calibrated bright field.
Use the PIXELMATCH tool to set the image A to a precisely calibrated bright field and set the image B to a precisely calibrated flat field. Calculation method Select DIVIDE (division). OFFSET is set to 0. run. The result is a full set of calibrated images.
The above calculation method is based on the formula
The corrected image = (L-D) / (F-B)
From this formula can also be seen. The bias field is primarily centered on the leveling field. Dark field is mainly in the bright frame to do dark field calibration when its role.
If the field is not accurate, the problem is prone to over-correction (four angles are lighter than the center), or under-correction.
Bright frame calibration is not accurate, that is, hot noise deduction is not accurate. (Sometimes the CCD temperature is already the same, but also not allowed, this is a possibility).
Automatic calibration method
At present, some software has the OVERSCAN automatic calibration function. For example, SGP software is typical. So you can set this. Please contact the relevant software experts
Note: If there is an average over the sweep area than the effective pixels to go even higher, or very close to how to do?
This will result in subtraction of the image by subtraction. Or very close to causing some pixels to be reduced to zero. In this case, a constant, such as 1000, should be added to -500 in step A. The result is +500, and then the sweep correction is performed. Note that this is limited to step 1, step 2 of OFFSET or 0
CMOS case
CMOS cameras also have such problems, and the impact of CMOS is more complex, because some of the CMOS chip with optical black level calibration function. Will automatically do an optical black level calibration. Optical black level calibration and OVERSCAN area calibration are different. The difference is that the optical black level region is containing thermal noise. While the OVERSCAN region does not contain thermal noise. So when the exposure time is longer. CMOS dark black area of the dark current increases, resulting in optical black level calibration when the buckle value, shorter than the exposure time when the buckle more. Which also led to the dark field than the bias field but also dark situation. For CMOS calibration problems, please refer to "from the SLR camera to QHY163M" article. The English version of the article is at http://www.alessiobeltrame.com/wp-content/uploads/2017/09/QHY163M_review_EN.pdf
Chinese version in the "QHYCCD astronomical photography" QQ group of group files can be found.
references
1. CCD Theory http://astro.ufl.edu/~lee/ast325/handouts/ccd.pdf
2. using the Overscan Bias Correction http://www.mirametrics.com/tech_note_overscan_bias.php
3. Calibration http://www.astro.caltech.edu/~aa ... s/total/node15.html
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