將赤經和赤緯翻譯成圖像

Question

我想讀一個星系的目錄的赤經（以小時爲單位），赤緯（以度爲單位）和大小（以圓弧爲單位），然後將它們全部放在指定的像素大小。

我試着將ra，dec和size轉換爲像素來爲每個星系創建一個Bounds對象，但是得到一個錯誤，「BoundsI必須用整數值初始化」。我知道像素必須是整數...

但是有沒有辦法將大圖像放在指定的ra和dec中，然後輸入每個星系的ra和dec作爲參數來繪製它？

預先感謝您！

Answer 1

GalSim使用CelestialCoord類來處理天空中的座標和任何一些WCS類來處理從像素到天體座標的轉換。

使用CelestialWCS（世界座標系統使用天體座標的WCS類的基類）的教程系列中的兩個演示demo11和demo13。所以你可能想看看他們。但是，沒有人會做與你正在做的事情非常接近的事情。

所以這是一個腳本，或多或少做你所描述的。

import galsim 
import numpy 

# Make some random input data so we can run this. 
# You would use values from your input catalog. 
ngal = 20 
numpy.random.seed(123) 
ra = 15 + 0.02*numpy.random.random((ngal)) # hours 
dec = -34 + 0.3*numpy.random.random((ngal)) # degrees 
size = 0.1 * numpy.random.random((ngal))  # arcmin 
e1 = 0.5 * numpy.random.random((ngal)) - 0.25 
e2 = 0.5 * numpy.random.random((ngal)) - 0.25 

# arcsec is usually the more natural units for sizes, so let's 
# convert to that here to make things simpler later. 
# There are options throughout GalSim to do things in different 
# units, such as arcmin, but arcsec is the default, so it will 
# be simpler if we don't have to worry about that. 
size *= 60 # size now in arcsec 

# Some plausible location at which to center the image. 
# Note that we are now attaching the right units to these 
# so GalSim knows what angle they correspond to. 
cen_ra = numpy.mean(ra) * galsim.hours 
cen_dec = numpy.mean(dec) * galsim.degrees 

# GalSim uses CelestialCoord to handle celestial coordinates. 
# It knows how to do all the correct spherical geometry calculations. 
cen_coord = galsim.CelestialCoord(cen_ra, cen_dec) 
print 'cen_coord = ',cen_coord.ra.hms(), cen_coord.dec.dms() 

# Define some reasonable pixel size. 
pixel_scale = 0.4 # arcsec/pixel 

# Make the full image of some size. 
# Powers of two are typical, but not required. 
image_size = 2048 
image = galsim.Image(image_size, image_size) 

# Define the WCS we'll use to connect pixels to celestial coords. 
# For real data, this would usually be read from the FITS header. 
# Here, we'll need to make our own. The simplest one that properly 
# handles celestial coordinates is TanWCS. It first goes from 
# pixels to a local tangent plane using a linear affine transformation. 
# Then it projects that tangent plane into the spherical sky coordinates. 
# In our case, we can just let the affine transformation be a uniform 
# square pixel grid with its origin at the center of the image. 
affine_wcs = galsim.PixelScale(pixel_scale).affine().withOrigin(image.center()) 
wcs = galsim.TanWCS(affine_wcs, world_origin=cen_coord) 
image.wcs = wcs # Tell the image to use this WCS 

for i in range(ngal): 
    # Get the celestial coord of the galaxy 
    coord = galsim.CelestialCoord(ra[i]*galsim.hours, dec[i]*galsim.degrees) 
    print 'gal coord = ',coord.ra.hms(), coord.dec.dms() 

    # Where is it in the image? 
    image_pos = wcs.toImage(coord) 
    print 'position in image = ',image_pos 

    # Make some model of the galaxy. 
    flux = size[i]**2 * 1000 # Make bigger things brighter... 
    gal = galsim.Exponential(half_light_radius=size[i], flux=flux) 
    gal = gal.shear(e1=e1[i],e2=e2[i]) 

    # Pull out a cutout around where we want the galaxy to be. 
    # The bounds needs to be in integers. 
    # The fractional part of the position will go into offset when we draw. 
    ix = int(image_pos.x) 
    iy = int(image_pos.y) 
    bounds = galsim.BoundsI(ix-64, ix+64, iy-64, iy+64) 

    # This might be (partially) off the full image, so get the overlap region. 
    bounds = bounds & image.bounds 
    if not bounds.isDefined(): 
     print ' This galaxy is completely off the image.' 
     continue 

    # This is the portion of the full image where we will draw. If you try to 
    # draw onto the full image, it will use a lot of memory, but if you go too 
    # small, you might see artifacts at the edges. You might need to 
    # experiment a bit with what is a good size cutout. 
    sub_image = image[bounds] 

    # Draw the galaxy. 
    # GalSim by default will center the object at the "true center" of the 
    # image. We actually want it centered at image_pos, so provide the 
    # difference as the offset parameter. 
    # Also, the default is to overwrite the image. But we want to add to 
    # the existing image in case galaxies overlap. Hence add_to_image=True 
    gal.drawImage(image=sub_image, offset=image_pos - sub_image.trueCenter(), 
        add_to_image=True) 

# Probably want to add a little noise... 
image.addNoise(galsim.GaussianNoise(sigma=0.5)) 

# Write to a file. 
image.write('output.fits') 

Answer 2

GalSim使用圖像座標處理圖像邊界和位置。將天空上的真實位置（RA，dec）連接到圖像座標的方法是使用GalSim中的世界座標系（WCS）功能。我從您的描述中收集到，有一個從RA/dec到像素座標的簡單映射（即沒有失真）。因此，基本上，您將設置一個簡單的WCS，用於定義大圖像的像素比例（RA，dec）中心。然後，對於給定的星系（RA，dec），可以使用WCS的「toImage」方法找出銀河系應該存在的大圖像的位置。任何子圖像邊界都可以使用該信息構建。

對於一個簡單的世界座標系統的示例，您可以在GalSim存儲庫中檢出demo10。