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改进单反相机的照片效果 ——正确曝光和仔细校对：优秀天文照片的关键
拍摄那些暗弱的星云和星系的照片全是与讯号和噪点打交道。二者的关系被叫作讯噪比（S/N），它的比值越大则图像的效果越好。因为数码相机能够以短时间的曝光捕捉到暗弱的星体，而长时间的曝光则可以拍摄出高质量的照片。这样你就可以做两件简单的事情来极大地提高你的深空天体照片的讯噪比了——获取更多的讯号并对其进行逐帧的校对以移除那些不需要的讯号。
亮帧、暗帧以及本底帧
我们通过拍摄照片来获取讯号，它们通常被叫作亮帧。但什么是校对帧呢？简单地说，就是可以识别那些不需要的信息的图片，那上面还有每一个亮帧上都有的相关噪点。一旦被识别出信息和噪点，你就可以从亮帧中将其去除，以此提高相片的质量。而捕捉本底和较暗的校对帧则不需要相机传感器的曝光。暗帧是最重要的校对图像，用天文摄影者的话来说叫作“a dark”。而一个本底帧会记录下相机自身固有的电子背景讯号，这在你拍摄到的每一张照片上都会有。
暗帧需要以相同的曝光时间、感光度和温度来拍摄，因为曝光时这些因素会在校对时用到。它们用相机传感器自身产生的热流来记录讯号。 由于单反相机上的传感器并不像大多数专业天文照相机上的一样会冷却，单反相机上的热噪点在常温和长时间的曝光下会非常明显。不过你可以移除亮帧上的暗帧来去除热噪点。确实，这就是相机内部降噪功能所要做的，同时这也是你不能在快门关闭后马上就拍下一张照片的原因。这段相机的繁忙期对于晴夜是有利的，因为它自身可以有效利用这段时间从目标拍摄接收更多的光子。你也可以阅读相机说明手册来取消这项功能。
拍摄暗帧
由于单反相机不会制冷，使相机里的电子会在使用的过程中积聚起来，并且它的内部温度会一直随着图像的增加而增加，一直到几个小时后冷却下来。由于适当的校对要求暗帧与亮帧相匹配，那么我们要怎么解决这一问题呢？
我发现最好的解决方法就是拍摄一系列暗帧然后制作一张可扩展的主暗帧，接下来就可以用专业软件比如Images Plus(mlunsold.com)、Max-DSLR(cyanogen.com)或者Nebulosity(stark-labs.com)来做图像校对。
用一张暗帧来校对在一个晚上的不同时段拍到的照片并不会提高图像的质量，这与对一系列暗帧的平均图像进行校对的效果一样。对多张暗帧进行平均处理会降低主暗帧上的噪点。如果气温没有太大变化，你也可以在最后记录暗帧。我觉得在多云的夜晚的可控环境下甚至是在白天时记录暗帧更方便些。
为了做到这点，晚上的时候，我在阳台架起了我的佳能20Da 相机并用可编程的TC80-N3无线快门进行整晚曝光，我将镜身盖罩在相机上，同时把取景器遮住，就这样拍摄了许多暗帧。我定时监控阳台的温度以使它保持恒写。我还记录下了曝光的次数和曝光时的温度。 由于我的相机整晚都处于曝光的状态，我整合了许多单帧来生成不同的感光度及温度下的主暗帧。而我所处的位置温度变化又很大，这使我得以在40、50、60、70和76度（华氏度）记录下不同的暗帧。这样，在这些观测点记录下的暗帧与我的各个主暗帧相差都不超过5华氏度。这一结果已经十分相近了，所以我可以运用自动暗帧匹配校对程序，它可以使我的记录结果同那些在每个亮帧之间拍摄的暗帧结果相差无几。我还常常试着去联合九个暗帧来生成一个主暗帧，这是最少的了，虽然是暗帧越多越好（我曾经用过64个）。我的所有的暗帧都是以我能用到的最长的曝光时间（十分钟）拍摄的。这样它们就可以在较短的曝光时间内同比例缩小。
你还需要制作一个本底帧来生成一个可缩放的主暗帧。本底帧对于去除暗帧是很必要的。这些本底帧可以平均地叠加在一起然后再从主暗帧中去除掉，从而生成一个主热帧。这种热噪点是随着曝光时间的延长同步消除的，这样就可以在校准的过程中被准确无误地去除掉来与亮帧想匹配了。记录本底帧的方法与暗帧一样，只是用的是最短的曝光时间。与制作主暗帧的原理一样，我们要拍摄一系列本底帧来生成一个好的主本底帧。
运用校准帧
记录下所有的主暗帧和本底帧之后，接下来要做的就是把它们合成在一起来制作主校准帧了。这一步骤最好是用天文图像处理软件来完成。
第一步就是要将所有的暗帧合成为一个主暗帧。而这类软件会为图像的合成加工设计诸如sigma 剪辑、均值滤波、中值滤波处理等高级选项。这在里我就不细讲了，因为每个都有自己的优缺点，但都很好用。只是要确保你没有用总计这项功能来合成暗帧。并且要将结果以单独的帧来保存。
第二步是本底帧的合成，这与暗帧的处理方法一样。然后把主本底帧从主暗帧中去掉再以可扩展的主热帧保存结果。同样地，再将主暗帧从每个单独的亮帧中去掉。而在对它们进行校准之前，要将个别的亮帧同主热噪帧进行校准，这会用到软件中自动暗帧匹配的功能；这要先校准亮帧。
而进行校准后的亮帧很可能还是有很多热噪点。甚至是校准过的暗弱的深空天体的单次曝光也会被其他光源影响。但还是有办法避开这种噪点的。
更好的讯噪比
简单地说，你采集的讯号越多，照片质量就会越好。所以你应该把这一点放在天体摄影的第一位：就是尽可能多地采集讯号！
在数字天体摄影中有几种方法可以做到这一点：
以最长的时间对单帧进行曝光
将许多张个别曝光图像合成到一起 在同一焦距上用大一点的光圈 运用高量子效率传感器
因为我们只能用相机上的传感器、望远镜的口径和相机的镜头，所以收集更多讯号最简单的方法就是运用长效曝光。而越多的讯号就意味着更高的讯噪比。
通过合成许多张短时曝光的图片来记录下更多的讯号，要使它与长时间曝光的图片相同的话，我们就要计算出每个单帧所需的合适的曝光时间。如果曝光太多，图像中的加亮区就会曝光过度。而曝光太少的话，讯号又会被噪点盖过。
正确的曝光会因你的设备和拍摄位置而异。你不能在光污染的天空下拍摄长时间的曝光，但可以将许多短时曝光的图像合成到一起。
想要测定单反相机的正确曝光时间的话就要做个曝光试验然后查看柱状图。这个柱状图显示的是一张图像当中有多少象素有特殊亮度。阅读你的相机的说明手册可以找到如何检查LCD显示器的柱状图，这样你就可以检查视区里的柱状图了。这个柱状图是诊断相机的最好的依据，并且它也是所有图像处理软件中的标准工具。在图表的左侧可以显示图像中最暗像素的数目，右面显示的则是最亮的像素数目。噪点包括在较暗的区域里。你可以将图像做长时间曝光以使柱状图的顶点距左侧远一点。如果图像的细节曝光不够的话，它将会被噪点盖住。
大多数天体照片的像素都会记录下天空背景，并且这会在柱状图的“柱子”上体现出来，因此波峰会出现在曲线图左侧1/4或1/3宽度的地方。而只要你的设备处于这样的状态，你就可以以同样的曝光设置来拍摄除最亮的天体之外的所有天体了。对于深空天体照片的长时曝光拍摄，建议你用800或1600的高感光度设置。你可能知道高感光度会产生带有很多噪点的日间图像。这没错，但是对于需要长时曝光的深空天体摄影来说，高感光度在记录暗弱天体时更有优势。而一旦你测定了理想的曝光时间，那就要尽多地拍摄亮帧。例如，我用五英寸口径f/8焦比的折射镜拍摄一个适中的深空位置。通常我会用1600的感光度来拍摄，每帧曝光约5到10秒钟。我建议用原始模式来拍摄亮帧，因为JPEG格式的压缩会在每张图像上都丢掉一些数据，而这些数据是你用了整晚记录下来的。不过，正确的校准还是可以充分提高JPEG图像的质量的。
这些简单的步骤将会大大提高图像的质量。在拍摄亮帧时用到它们，会极大地降低各种不需要的讯号。为你的设备找到适当的曝光时间会使图像的细节从噪点中突出出来，而合成那些单独的图像会提高总的讯噪比。每一个步骤都会有小小的提高，这些加在一起就会产生巨大的变化，并且还会产生更易于加工的高讯噪比图像。
PS： 一、背景知识： 讯号与噪点 每一个用CCD或是数码单反相机拍摄的照片都会记录下各种各样的讯号——有些是有用的而有一些则没有用。大多数被我们叫作噪点的东西都是可以通过校准帧来移除的，当许多业余爱好者得知这一点时都十分惊讶。
通过相机传感器形成的深空天体的天体讯号都是来自光的量子，它们在宇宙中穿越了令人难以置信的距离。相机在亮帧中记录下这种讯号，即对深空天体的实际曝光。
本底帧是相机传感器中固有的以固定讯号的形式存在的低电荷，它在每一帧当中都会出现。
热电流，也被叫作暗电流，它是由传感器当中的热能产生的电子所产生的。即使没有光进入到传感中也会产生热电流，并且在数码单反相机图像中的像白色和彩色斑点的像素中也会出现热电流。
天空背景，尽管经常因为它那煞风景的颜色而让人不快，事实上却是在每个长时曝光的图像中真正被记录下的东西。光污染、散射和那即使是在地球上最暗的天空中都弥漫着的大气气辉，所有这一切都会对天空背景产生影响。但可以用图像处理软件移除掉。
数字图像的噪点部分在相机内部和外部有许多来源。在一张图像中，真正的噪点是指那些杂乱的，不重复的讯号。
光噪点来自于光量子的自然属性。它的噪点是由光子的统计差异产生的。在固定的时间间隔中会有多少图像通过传感器是很难确定的。同样地，暗电流也会有噪点成分。
由于当一张图像从相机的传感器中读取出来时含有统计不确定性，所以就会在这一过程中产生被称作读噪点的电子噪点。幸运的是，你可以通过校准单次曝光来将那些不需要的讯号减少到最少。但是天下没有免费的午餐，图像校准仍会增加噪点，不过通过留意细节再加上文章中说明的技巧，你可以大大改进你的天体照片。
二、简单的冷却方法：
在气温较暖（高于40华氏度）时，可以在曝光时用扇子在相机上方使空气流动来降低温度。这看起来很简单，却十分有效。
——原创翻译，转载请注明出处及译者
原文载于美国《Sky & Telescope》杂志 作者Jerry Lodriguss，是《CD-ROM book》一书的作者，这篇文章介绍的是如何用数码单反相机拍摄天体照片。
Improving your DSLR Photos
-- Proper exposure and careful calibration are keys to great astrophotos
TAKING GREAT images of faint galaxies and nebulae is all about signal and noise. The relationship between the two is called the signal-to-noise(S/N) ratio, and the higher the value, the better the images. While digital cameras can capture faint objects with short exposures, long exposures make the best pictures. You can do two very simple things to greatly improve the S/N ratio in your deep-sky astrophotographs: gather more signal and use calibration frames to remove unwanted signals.
Light, Dark, and Bias Frames
We gather signal by shooting pictures called light frames. But what are calibration frames? Simply put, these are pictures used to identify the unwanted information and its associated noise that’s recorded along with every light frame. Once identified, you can subtract this information from the light frame to improve your result. Bias and dark calibration frames are captured without the camera’s detector being exposed to light. The most important calibration image is known as a dark frame, which is often called a “dark” in astrophotographer lingo. A bias frame records the camera’s inherent electronic background signal, which is present in every image you shoot.
Darks should be recorded with the same exposure time, ISO, and temperature as the light exposures they will be used to calibrate. They record the signal generated by thermal current inherent in the camera’s detector.
Because the sensors in DSLR cameras are not cooled like those in most dedicated astronomical cameras, the thermal signal in a DSLR image can be quite noticeable in long exposures made at normal ambient temperatures.
But you can remove the thermal signal by subtracting dark frames from light frames. Indeed, this is what in-camera noise reduction is doing, and it’s why your camera stays busy and you can’t take another picture immediately after the shutter closes for your light frame. This busy time is an inefficient use of clear nights because it eats up time that can be better spent gathering more photons from your target. Read your camera manual to find out how to disable this function.
Shooting Darks
Because DSLRs are not cooled, heat from the camera’s electronics builds up during use and its internal temperature drifts upwards in each subsequent image until it stabilizes after a couple of hours. Since darks are supposed to match the light frames for optimum calibration, how do to handle this problem?
The best solution that I’ve found is to shoot a lot of darks and create what’s known as a scalable master dark, and then use specialized software such as Images Plus (mlunsold.com), Max DSLR (cyanogen.com), or Nebulosity (stark-labs.com) for image calibration.
Taking one dark to calibrate an entire series of images made during the course of the night won’t improve your images as much as calibrating with one made by averaging a series of darks. Averaging multiple darks will reduce the amount of noise in the master dark. While you can record dark frames at the end of the evening if the temperature hasn’t changed dramatically, I find it far more convenient to record my darks in a controlled environment on cloudy nights or even during daylight hours.
To do this, I set up my Canon 20Da camera in my garage at night and use the programmable TC80-N3 cable release to shoot many darks all night long with the body cap on the camera and the viewfinder eyepiece covered. I periodically monitor the temperature in the garage, but it usually stays fairly constant, I keep notes of how many exposures I take and at what temperature.
Since I let the camera shoot darks all night, I can combine many individual frames to create a master dark for each ISO setting and temperature. The temperature varies enough at my location that I’ve been able to amass a library of dark frames recorded at 40°, 50°, 60°, 70°,and 76°F.That way, no observing session has a difference of more than 5°F from one of my master darks. This is close enough that I can use a calibration program with automatic dark-frame matching and have my results be virtually indistinguishable from those using darks taken between each light frame. I usually try to combine a minimum of nine darks to create a master dark frame, though more would be better (I’ve used as many as 64). I shoot all of my darks at the longest exposure time I might use, 10 minutes. This way they can be scaled down for use with shorter exposures.
You’ll also need to record bias frames to create a scalable master dark. Bias frames are necessary to properly scale your darks. These bias frames can be averaged together then subtracted from your master dark to create a master thermal frame that contains only thermal signal. Since this signal scales linearly with exposure time, it can be scaled precisely to match your light images during image calibration. Bias frames are recorded like dark frames, but with the shortest exposure time possible. Shoot a lot of bias frames to create a good master bias frame for the same reason you create a master dark.
Applying Calibration Frames
With all the master darks and biases you recorded, it’s simply a matter of combining them to make master calibration images. This is best done with an astronomical image-processing program.
You first combine all the darks to make one master dark. The software will probably have advanced potions for the combining process, such as sigma clip, mean, median, or others. I won’t go into the details here, since each has advantages and disadvantages, but they all work. Just make sure you don’t use the sum function to combine the darks. Save the result as a separate frame.
Do the same for the bias frames. Then subtract the master bias from the master dark and save the result as a scalable master thermal frame. Subtract the master bias from each individual light frame also. Calibrate your individual light frames with the master thermal frame using the software’s function for automatic dark-frames before calibrating them; calibrate the individual light frames first.
The calibrated light frames will probably still appear to be quite noisy. Even calibrated single exposures of faint deep-sky objects will be dominated by noise from other sources. But there are ways to beat this noise.
Better Signal-to-Noise Ratio
Simply stated, the more signal you collect, the better your photos will be. You should consider this to be your number one priority in astrophotography: collect as much signal as possible!
I n digital astrophotography, there are several ways to do this:
◆ Expose individual frames as long as possible
◆ Combine many individual exposures
◆ Use a larger aperture at the same focal length
◆ Use a high-quantum-efficiency sensor
Since we have to work with the sensors in our cameras, as well as the apertures of our telescopes and camera lenses, the easiest way to gather more signal is to use long effective exposures. More signal means a high S/N ratio.
To record more signal by combining many short images to equal a longer one, we need to figure out the correct exposure for each individual frame. With too much exposure, the highlights in the image may be overexposed. With too little exposure, the signal will be drowned out by noise.
The correct exposure will vary depending on your equipment and shooting location. You won’t be able to shoot long exposures in light-polluted skies, but you can combine lots of shorter ones.
To determine the correct exposure with a DSLR, take a test exposure and examine the histogram. A histogram is a graph that displays how many pixels in an image have a particular brightness. Read your camera’s instruction manual and find out how to view the histogram on the LCD display so that you can check the histogram in the field. The histogram is the best diagnostic tool built into your camera, and it’s a standard tool in all image-processing programs. It shows tallies of the darkest pixels in the image on the left side of the graph, and the brightest pixels on the right. Noise resides within the dark region. Expose your image long enough so that the histogram’s peak is away from the left side. If your image detail is underexposed and dark, it will be lost in the noise.
Most of an astrophoto’s pixels record the sky background, and they are represented by the “mountain” in the histogram so that the peak is about 1/4 to 1/3 of the graph’s width away from the left side of the histogram. Once you achieve that condition with your equipment, you can use the same exposure settings for all but the brightest subjects in the sky. Use a high ISO setting, such as 800 or 1600, for your long-exposure deep-sky astrophotos. You’ve probably read that high ISO settings produce noisy daytime images. This is true, but for the long exposures required for deep-sky astrophotography, a high ISO will better record faint objects. Once you’ve determined the ideal exposure, shoot as many light frames as you can. For example, I shoot with a 5-inch f/8 refractor at a moderately dark-sky site. I usually shoot at ISO 1600 and expose individual frames for 5 to10 minutes. I recommend shooting light frames in RAW mode because JPEG compression will throw away some of the data in each image that you just spent all night to record. However, proper calibration can still substantially improve JPEG images.
These easy steps will greatly increase the quality of your images. By shooting them to your light frames, you’ll greatly reduce the various unwanted signals. Finding the proper exposure for your setup will raise image detail out of the noise, and combining many individual images will increase the overall S/N ratio. Each of these steps represents small improvements, but added together they will pay huge dividends and result in a high signal-to-noise image that can be processed more easily and effectively.
SIMPLE COOLING
In warm temperatures (above 40°F), use a fan to blow air over the camera during exposures. This seems very simple, but it works extremely well。
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