Question

我在三脚架上拍摄了一组用相机拍摄的许多天文照片。使用气泡水平仪确保框架的长边与地平线平行，并且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？

Answer 1

无论你是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真正问题的问题：你的特定镜头怎么样？，当你拍摄照片时使用的精确焦距时，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：

http://fits.gsfc.nasa.gov/fits_libraries.html

Answer 2

我会尝试为每个图像创建一个“世界坐标系”（WCS），这实际上是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下可用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，然后按照您喜欢的方式标记图像。

这里有一些快速而脏的代码，您可能会尝试适应您的目的：

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

Answer 3

仅仅为了获得正确的方向，所有这些都可以简化为：

    positionAngle = np.degrees(np.arctan2(CD12, CD11))

因为NumPy（np）可以帮助在所有情况下（象限）正确处理arctan函数。

米歇尔

编辑：如果您想检测图像是否被镜像：像这样计算矩阵的行列式

mirrored = (CD11 * CD22 - CD12 * CD21) < 0

并检查它是否为<0。转换内部具有镜像。

如何使用pyephem找到给定的altaz coords的北和地平线之间的角度？

3 个答案: