Assigning new data to an existing HDU won't update scaling consistently #5559
Description
When creating a brand new ImageHDU, this:
im = ImageHDU(data=data, header=existing_header)
will deal properly with the header scaling, adding or subtracting BZERO/BSCALE to fit the data type, no matter what the data type is, or whether the original header has scaling information on it or not. If we set the data after creation, the behavior is not consistent. Let's show this.
>>> uint8 = np.array([[1,2,3,4],[4,3,2,1]], dtype='uint8')
>>> uint64 = np.array([[1,2,3,4],[4,3,2,1]], dtype='uint64')
Now, let's create a new image with a data type that doesn't require scaling, and replace the data with the unsigned 64 bit array:
>>> im = ImageHDU(uint8)
>>> im.header
XTENSION= 'IMAGE ' / Image extension
BITPIX = 8 / array data type
NAXIS = 2 / number of array dimensions
NAXIS1 = 4
NAXIS2 = 2
PCOUNT = 0 / number of parameters
GCOUNT = 1 / number of groups
>>> im.data = uint64
>>> im.header
XTENSION= 'IMAGE ' / Image extension
BITPIX = 64 / array data type
NAXIS = 2 / number of array dimensions
NAXIS1 = 4
NAXIS2 = 2
PCOUNT = 0 / number of parameters
GCOUNT = 1 / number of groups
BSCALE = 1
BZERO = 9223372036854775808
It behaves as expected. Now:
>>> im = ImageHDU(uint64)
>>> im.header
XTENSION= 'IMAGE ' / Image extension
BITPIX = 64 / array data type
NAXIS = 2 / number of array dimensions
NAXIS1 = 4
NAXIS2 = 2
PCOUNT = 0 / number of parameters
GCOUNT = 1 / number of groups
BSCALE = 1
BZERO = 9223372036854775808
>>> im.data = uint8
>>> im.header
XTENSION= 'IMAGE ' / Image extension
BITPIX = 8 / array data type
NAXIS = 2 / number of array dimensions
NAXIS1 = 4
NAXIS2 = 2
PCOUNT = 0 / number of parameters
GCOUNT = 1 / number of groups
BSCALE = 1
BZERO = 9223372036854775808
As we can see, scaling info is preserved, when it shouldn't. This works properly if the new array is of 16/32/64 integer (signed or unsigned), but fails with 8 bit and floating point data.