python - Healpy map2alm 和 alm2map 不一致？

Question

我刚开始使用 Healpy 并注意到如果我使用 map 获取 alm's，然后使用这些 alm's 生成新 map ，我不会得到我开始使用的 map 。这是我正在查看的内容:

import numpy as np
import healpy as hp
nside = 2 # healpix nside parameter
m = np.arange(hp.nside2npix(nside)) # create a map to test
alm = hp.map2alm(m) # compute alm's
new_map = hp.alm2map(alm, nside) # create new map from computed alm's

# Let's look at two maps
print(m)
[ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47] # as expected

print(new_map)
[-23.30522233 -22.54434515 -21.50906755 -20.09203749 -19.48841773
-18.66392484 -16.99593867 -16.789984   -15.14587061 -14.57960049
-13.4403252  -13.35992138 -10.51368725 -10.49793946 -10.1262039
-8.6340571   -7.41789272  -6.87712224  -5.75765487  -3.75121764
-4.35825512  -1.6221964   -1.03902923  -0.41478954   0.52480646
 2.34629955   2.1511705    2.40325268   5.39576497   5.38390848
 5.78324832   7.24779083   8.4915595    9.0047257   10.15179735
 12.1306303   12.62672772  13.4512206   15.11920678  15.32516145
 16.96927483  17.53554496  18.67482024  18.75522407  20.42078855
 21.18166574  22.21694334  23.6339734 ] # not what I was expecting 

如您所见，new_map 与输入 map m 不匹配。我想我遗漏了这些功能的一些微妙之处。有什么想法吗？

Answer 1

我得到了不同的结果:

打印(新 map )
[ 0.15859344, 0.91947062, 1.95474822, 3.37177828,
4.01808325, 4.84257613, 6.51056231, 6.71651698,
8.36063036、8.92690049、10.06617577、10.1465796、
12.98620654、13.00668621、13.3736899、14.87056857、
16.08200108, 16.62750343, 17.74223892, 19.75340803,
19.13441288, 21.8704716 , 22.45363877, 23.07787846,
24.01747446, 25.83896755, 25.6438385, 25.89592068,
28.89565876, 28.88853415, 29.28314212, 30.7524165 ,
31.9914533, 32.50935137, 33.65169114, 35.63525597,
36.13322869, 36.95772158, 38.62570775, 38.83166242,
40.47577581, 41.04204594, 42.18132122, 42.26172504,
43.88460433, 44.64548151, 45.68075911, 47.09778917]


healpy 的旧版本会在转换前自动从 map 中删除常量偏移量，最好将 healpy 更新到最新版本。

残差与像素化引入误差有关，低nside误差较大。