Astrometrica can also be used to stack a number of images, adding up the signal of a faint object. When stacking images, the images can be shifted to compensate for the motion of an object (asteroid or comet), which will enable the user to get reliable positions even for very faint objects (so that the object cannot be captured in a single, short exposure), or fast moving objects (that would trail in a single, longer exposure).
本软件也可用于叠加图片来增加暗弱目标的信号强度。叠加处理时，各图片会相应移动来自动补偿天体（小行星或彗星）的运动，这样可以使使用者即使碰到暗弱的目标（在一张短时间暴光的图片上无法成像）也可以较准确的定位，或者快速移动的目标（在一张长时间暴光的图片上是一条短线轨迹）。
1. Stacking Images 叠加图片
Select the 'Track and Stack' command from the 'Astrometry' menu. A dialog will
appear that allows you to select the images that will be stacked. Use the [Add]
button to choose '2000AC6_1.st6' and '2000AC6_2.st6', and close this dialog
with the [OK] button. The software will now load the images and ask the user
to verify the date and time for the images, but the individual images will not
be displayed. Instead, another dialog window will pop up.
在Astrometry菜单中选择'Track and Stack'命令，弹出一个对话框请你选择需要叠加的图片。使用[Add]按钮选择'2000AC6_1.st6'和'2000AC6_2.st6'两个文件，并点击[OK]按钮来关闭这个对话框。软件将载入图片并要求用户确认图片拍摄的时间和日期，单张图片，软件不会叠加。然后，弹出另一个对话框。
The software will ask you to enter the coordinates and the apparent motion of the object designation. Again, you may enter '2000 AC6' in the 'Object' field. After pressing the [Tab] key, the position, the speed and direction of the apparent motion of the asteroid will appear. For stacking the images, you can choose between adding, averaging or median combining the images. 'Add' sums up the pixel values from individual images. Before that, a pedestal is subtracted from the individual images to avoid saturation of pixel values. 'Average' will set the pixel value to the mean of the individual images. The range of pixel values is therefore preserved, and pixels will not saturate when stacking images. For faint targets and/or images using only a small fraction of the dynamic range, pixel values in the stacked image might appear quantized. 'Median' will almost completely remove stationary objects (like stars) when more than two images are combined, and the object has moved more than the diameter of the stars on the images, which might be useful when tracking an object in crowded fields. In most cases, adding will be used, so you want to choose this option here. Pressing the [OK] button will start the data reduction process.
软件将要求输入坐标及目标天体的视运动情况。同样的，你可以在目标区域输入“2000 AC6”，然后按[Tab]键，位置、速度及视运动的方位角将会显示出来。你可以选择相加、平均或中值的方式进行叠加。“相加”运算是将每幅图片的像素值都加起来。在此之前，为了不使像素值达到饱和，相应的相减的操作已经进行。“平均”运算是取各图片像素的平均值。各像素值的变化情况将被保留下来，像素值也不会饱和。对于使用非常小的动态范围而产生的比较暗弱的目标和/或图片，在叠加后的图片中像素值将可以得到量化。“中值”运算将几乎可以去除所有的固定天体（像恒星那样），目标将移动超过一个恒星直径的距离，这在叠加恒星密集区的时候比较有用。在很多场合，叠加是比较有用的，你可以选择合适的叠加算法。按下[OK]按钮开始软件叠加操作。
After a while, the stacked images, shifted to compensate for the motion of the asteroid, will appear. Also, the software has already identified the stars and reference stars on the images. Maybe you are already able to identify the asteroid as the only point of light among the star trails (or strings of individual star images). But we will stack another pair of images to identify the asteroid by blinking the images: Use the same commands as before to stack another pair of images, but choose the images '2000AC6_3.st6' and '2000AC6_4.st6' now.
过一会儿，经过适当位移用以补偿小行星视运动的叠加后的图片就会出现。并且软件已经在图片上识别了恒星和参考星。或许你在有恒星拖迹的图片中已经发现了一各小行星不动点（或者是单张图片中的一条线）。但我们还将叠加另一对图片来确定该小行星：像刚才那样使用相同的命令来叠加'2000AC6_3.st6'和'2000AC6_4.st6'两个文件。
Note: 2000 AC6 was bright enough to allow precise astrometry on individual exposure, as in Tutorial I and Tutorial II. We will, nevertheless, use the same images for this tutorial, which will make it easier for you to compare the different modes of astrometric data reduction provided by Astrometrica.
注：像在指南1和指南2中那样，2000 AC6是一个足够亮的小行星用来做单张图片的天文测量，然而，在这里我们还是使用相同的图片，目的是让用户能比较容易的体会本软件的不同数据分类处理方法。
2. Blinking Images 动画图片
Just like with the single images in Tutorial I
(Basic Astrometry) you may use the 'Blink Images' command from the 'Tools' menu
to blink the stacked images, in order to located the minor planet. Close the
Blink Window after you have identified the asteroid.
就像在指南1（基本天文测量）中使用单张图片一样，你可以使用 'Tools'菜单中的'Blink Images'命令来动画叠加后的图片来定位小行星。你确定完该小行星后就可以关闭动画窗口了。
3. Measuring the Object 测量目标
Like in Tutorial I, measure the position of the
asteroid by clicking on it. The verification dialog, also known from Tutorial
I, will be displayed.
像在指南1中那样，你可以点击鼠标来测量小行星的位置。弹出的对话框也和指南1中的一样。
Enter the packed designation of the object ('K00A06C' for 2000 AC6) in the Object Designation field, and press the [Accept] Button to store the measured position.
在目标名称区域输入该天体的名称('K00A06C'或2000 AC6)并点[Accept]按钮来储存刚才测量的位置。
Note that clicking on any other object than the asteroid (i.e., any object that is not moving at the speed and in the direction entered in the dialog before stacking the images) will give meaningless results!
注意：点击其他的非该小行星目标（例如：任何不移动的目标或移动速度和方向与先前叠加图片时对话框中的不一致的）将得到无意义的结果！
4. Viewing the Results 查看结果
To view the results, select the 'View MPC
Report' command or the 'View Log File'
command from the 'File' menu. Note that the results refer to the position of
the asteroid at the mean date and time of all the individual images that were
used to created the stacked image.
在File菜单中选择'View MPC Report'或'View Log File'命令来查看结果。注意：结果所对应的是所有曾用于叠加的图片的拍摄时间的小行星位置。
Tip: Before you start your own astrometric observing campaign, you may want to read the MPC's Guide to Minor Body Astrometry at http://cfa-www.harvard.edu/iau/info/Astrometry.html.
技巧：在开始你个人的天体测量工作之前，请最好先阅读MPC的小行星天体测量指导，地址在http://cfa-www.harvard.edu/iau/info/Astrometry.html。