WCS 未出现在生成的 FITS 文件中

Question

我正在尝试生成带有随机星星的 FITS 文件。由于某种原因，当我在 SAO DS9 中打开文件时，鼠标悬停不显示天空坐标，而是仅显示我鼠标悬停的像素位置。下面我展示了我在 Python 中使用的脚本来生成 FITS 文件，以及我在 DS9 中看到的内容：

import numpy as np
from pathlib import Path
import matplotlib.pyplot as plt
import pandas as pd
import os
from astropy.wcs import WCS
from astropy import units as u
from astropy.io import fits
from astropy.visualization import AsinhStretch, LogStretch
from astropy.visualization.mpl_normalize import ImageNormalize
from convenience_functions import show_image
from photutils.datasets import make_gaussian_sources_image, make_random_gaussians_table
from astropy.nddata import Cutout2D



def add_wcs(cntrpix,cntrval,cdelt,ctype, radesys):
    #zhdrbb.remove('ZODIL')
    #CREATE WCS KEYWORDS
    w = WCS(naxis=2)
    w.wcs.crpix = cntrpix
    w.wcs.crval = cntrval
    w.wcs.cdelt = cdelt
    w.wcs.ctype = ctype
    w.wcs.radesys = radesys
    w.wcs.equinox = 2000
    w.wcs.set_pv([(2, 1, 45.0)])
    w.wcs.cunit = [u.deg, u.deg]

    header = w.to_header()
    return header

def stars(image, number, mag_min, mag_max):
    """
    Add some stars to the image.
    """

    # Most of the code below is a direct copy/paste from
    # https://photutils.readthedocs.io/en/stable/_modules/photutils/datasets/make.html#make_100gaussians_image

    flux_min = 10**mag_min
    flux_max = 10**mag_max
    flux_range = [flux_min, flux_max]

    y_max, x_max = image.shape
    xmean_range = [0.01 * x_max, 0.99 * x_max]
    ymean_range = [0.01 * y_max, 0.99 * y_max]
    xstddev_range = [4, 4]
    ystddev_range = [4, 4]
    params = dict([('flux', flux_range),
                  ('x_mean', xmean_range),
                  ('y_mean', ymean_range),
                  ('x_stddev', xstddev_range),
                  ('y_stddev', ystddev_range),
                  ('theta', [0, 2*np.pi])])
    
    sources = make_random_gaussians_table(number, params, seed=int(random.random()*1e4))

    star_im = make_gaussian_sources_image(image.shape, sources)

    return sources, star_im

RA_0, RA_F = 30, 70
DEC_0, DEC_F = -20,20
MAG_0, MAG_F = 3, 8
N_STARS = 250

D_RA = np.abs(RA_0 - RA_F)
D_DEC = np.abs(DEC_0 - DEC_F)


GRID_LENGTH = int(2e3) # N PIXELS PER SIDE

# WCS STUFF
CRPIX = (int(GRID_LENGTH / 2), int(GRID_LENGTH / 2))
CRVAL = (RA_0 + D_RA / 2., DEC_0 + D_DEC / 2.)

cdelt_x = D_RA / GRID_LENGTH
cdelt_y = D_DEC / GRID_LENGTH
CDELT = (cdelt_x, cdelt_y)
CTYPE = ["RA-TAN", "DEC-TAN"]
RADESYS = 'ICRS'
base_pix_full_image = np.zeros((GRID_LENGTH,GRID_LENGTH))
star_data, star_cat_img = stars(base_pix_full_image, N_STARS, MAG_0, MAG_F)
# WCS
wcs_header = add_wcs(CRPIX, CRVAL, CDELT, CTYPE, RADESYS)

hdr = fits.Header()
hdu_cat = fits.PrimaryHDU(header=wcs_header,data=star_cat_img)

# WRITE FITS PIXEL FILE
hdu_cat.writeto(fits_pixel_file, overwrite=True)

从图像中，鼠标悬停在图像上会显示 x 和 y 指向像素坐标，即使标题中包含 WCS 信息。

我觉得我错过了一些非常简单的东西，只需要一双新的眼睛来解决问题。

Answer 1

看来需要将

CTYPE

设置为

CTYPE = ["RA---TAN", "DEC--TAN"]

。

这里引用了FITS 标准。

CTYPEi – [字符串；索引;默认值：'␣'（即线性、未定义的轴）]。输入中间坐标轴 i。本标准或官方认可的 FITS 公约未涵盖的任何坐标类型均应视为线性。所有非线性坐标系名称必须以“4-3”形式表示：前四个字符指定坐标类型，第五个字符是连字符（“-”），其余三个字符指定用于计算的算法代码世界坐标值。名称少于四个字符的坐标类型在右侧用连字符填充，少于三个字符的算法代码在右侧用空格填充11。算法代码应该是三个字符。