Treat data as big-endian

Tests/test_file_fits.py

@@ -24,6 +24,8 @@ def test_open() -> None:
 
 def test_gzip1() -> None:
     with Image.open("Tests/images/m13_gzip.fits") as im:
+        assert im.getpixel((0, 0)) == 111
+
         assert_image_equal_tofile(im, "Tests/images/m13.fits")
 
 
@@ -36,6 +38,22 @@ def test_invalid_file() -> None:
         FitsImagePlugin.FitsImageFile(invalid_file)
 
 
+def test_unsupported_number_of_bits() -> None:
+    image_data = b"".join(
+        data.ljust(80, b" ")
+        for data in [
+            b"SIMPLE  = T",
+            b"BITPIX  = 128",
+            b"NAXIS   = 1",
+            b"NAXIS1  = 0",
+            b"END",
+        ]
+    )
+    with pytest.raises(OSError, match="Unsupported number of bits"):
+        with Image.open(BytesIO(image_data)):
+            pass
+
+
 def test_truncated_fits() -> None:
     # No END to headers
     image_data = b"SIMPLE  =                    T" + b" " * 50 + b"TRUNCATE"

src/PIL/FitsImagePlugin.py

@@ -107,17 +107,23 @@ class FitsImageFile(ImageFile.ImageFile):
 
         number_of_bits = int(headers[prefix + b"BITPIX"])
         if number_of_bits == 8:
-            self._mode = "L"
+            self._mode = rawmode = "L"
         elif number_of_bits == 16:
             self._mode = "I;16"
+            rawmode = "I;16B"
         elif number_of_bits == 32:
             self._mode = "I"
-        elif number_of_bits in (-32, -64):
+            rawmode = "I;32B"
+        elif number_of_bits == -32 or (decoder_name == "raw" and number_of_bits == -64):
             self._mode = "F"
+            rawmode = f"F;{number_of_bits * -1}BF"
+        else:
+            msg = "Unsupported number of bits"
+            raise OSError(msg)
 
         args: tuple[str | int, ...]
         if decoder_name == "raw":
-            args = (self.mode, 0, -1)
+            args = (rawmode, 0, -1)
         else:
             args = (number_of_bits,)
         return decoder_name, offset, args
@@ -133,11 +139,13 @@ class FitsGzipDecoder(ImageFile.PyDecoder):
 
             rows = []
             offset = 0
-            number_of_bits = min(self.args[0] // 8, 4)
+            number_of_bytes = abs(self.args[0]) // 8
             for y in range(self.state.ysize):
                 row = bytearray()
                 for x in range(self.state.xsize):
-                    row += value[offset + (4 - number_of_bits) : offset + 4]
+                    row += value[
+                        offset + 4 - 1 : offset + (4 - number_of_bytes) - 1 : -1
+                    ]
                     offset += 4
                 rows.append(row)
         self.set_as_raw(bytes([pixel for row in rows[::-1] for pixel in row]))