我在三脚架上拍摄了一组很多用相机拍摄的astrophotos。使用气泡水平仪确保框架的长边与地平线平行，我知道每张照片中心的alt / az（和赤道）坐标。
现在我正在编写一些python代码来覆盖每个图像上的指示符以标记北方向。对于给定的alt / az坐标，我可以使用pyephem获取北天极方向和水平方向之间的角度吗？任何线索？
我会尝试为你的每个图像创建一个“世界坐标系”（WCS），这实际上是你的像素坐标和天空坐标（即RA和Dec）之间的映射。您可以使用http://astrometry.net提供的工具，根据图像中可见的星形图案自动解决图像。这将为图像生成WCS。
astrometry.net解算器（如果安装了相应的依赖项）可以生成图像的png版本，并标记已知的天体。这可能足以满足您的需要，但如果没有，您可以使用astropy.wcs包将图像WCS读入python并使用它来确定图像的方向，然后根据需要标记图像。
这里有一些快速而脏的代码，您可以尝试适应您的目的：
import math
import subprocess
import astropy.units as u
import astropy.fits as fits
## Solve the image using the astrometry.net solve-field tool.
## You'll want to look over the options for solve-field and adapt this call
## to your images.
output = subprocess.check_output(['solve-field', filename])
## Read Header of Image (assumes you are working off a fits file with a WCS)
## If not, you can probably read the text header output my astrometry.net in
## a similar fashion.
hdulist = fits.open(solvedFilename)
header = hdulist[0].header
hdulist.close()
CD11 = float(header['CD1_1'])
CD12 = float(header['CD1_2'])
CD21 = float(header['CD2_1'])
CD22 = float(header['CD2_2'])
## This is my code to interpet the CD matrix in the WCS and determine the
## image orientation (position angle) and flip status. I've used it and it
## seems to work, but there are some edge cases which are untested, so it
## might fail in those cases.
## Note: I'm using astropy units below, you can strip those out if you keep
## track of degrees and radians manually.
if (abs(CD21) > abs(CD22)) and (CD21 >= 0):
North = "Right"
positionAngle = 270.*u.deg + math.degrees(math.atan(CD22/CD21))*u.deg
elif (abs(CD21) > abs(CD22)) and (CD21 < 0):
North = "Left"
positionAngle = 90.*u.deg + math.degrees(math.atan(CD22/CD21))*u.deg
elif (abs(CD21) < abs(CD22)) and (CD22 >= 0):
North = "Up"
positionAngle = 0.*u.deg + math.degrees(math.atan(CD21/CD22))*u.deg
elif (abs(CD21) < abs(CD22)) and (CD22 < 0):
North = "Down"
positionAngle = 180.*u.deg + math.degrees(math.atan(CD21/CD22))*u.deg
if (abs(CD11) > abs(CD12)) and (CD11 > 0): East = "Right"
if (abs(CD11) > abs(CD12)) and (CD11 < 0): East = "Left"
if (abs(CD11) < abs(CD12)) and (CD12 > 0): East = "Up"
if (abs(CD11) < abs(CD12)) and (CD12 < 0): East = "Down"
if North == "Up" and East == "Left": imageFlipped = False
if North == "Up" and East == "Right": imageFlipped = True
if North == "Down" and East == "Left": imageFlipped = True
if North == "Down" and East == "Right": imageFlipped = False
if North == "Right" and East == "Up": imageFlipped = False
if North == "Right" and East == "Down": imageFlipped = True
if North == "Left" and East == "Up": imageFlipped = True
if North == "Left" and East == "Down": imageFlipped = False
print("Position angle of WCS is {0:.1f} degrees.".format(positionAngle.to(u.deg).value))
print("Image orientation is North {0}, East {1}.".format(North, East))
if imageFlipped:
print("Image is mirrored.")
## Now you have position angle and flip status and can mark up your image
无论你想用十字或点标记北天极，还是在地平线的北角放一个标记，你都会问一个关于你的相机镜头的问题：你的特定镜头是什么时候拍摄照片时使用的精确焦距，将弯曲的天空映射到相机传感器的平面上？
这不仅是天文学面临的挑战，也是任何拍摄照片然后想要将图像用于测量或空间计算的人所面临的挑战。
我听说专业天文学家使用的是名为FITS的图书馆。我的印象是，如果你向图书馆解释你的相机具有什么样的镜头以及它产生什么样的失真，它可以告诉你每个像素的坐标 - 这应该让你找到天体北点：
只是为了获得正确的方向的第一点，所有可以简化为：
positionAngle = np.degrees(np.arctan2(CD12, CD11))
因为有NumPy（np）帮助处理所有情况下的arctan函数（象限）。
米歇尔
编辑：如果您想检测图像是否已镜像：计算矩阵的行列式，如
mirrored = (CD11 * CD22 - CD12 * CD21) < 0
并检查它是否<0。比转换内部有镜像。