【Tips】How to change column format of FITS file?

# Problem and Demand
- In a standard SPECTRA file of FITS format, there is a column of source `COUNTS` in extension. 
- Default data type of `COUNTS` is `1I`, meaning *interger*.
- **manually** change the data type of `COUNTS` to *float*.

# Try
- directly assign a float array
    - **failed**
    - the array will be reformatted to *interger*
- change keyword `TFORM2` in header
    - **failed**
    - *TFORM2* cannot be manually changed
    
# Solution
- with `astropy`, using `fits.Columns`
- `filename_raw` is the raw spectra fits file
- `counts_new` is the new counts array with *float* type 
```python
from astropy.io import fits
from astrioy.table import Table

hdu_raw = fits.open(filename_raw)
hd_pri = hdu_raw[0].header # extract primary header
hd_spec = hdu_raw[1].header # extract spectra header
da_spec = hdu_raw[1].data

# extract data columns
col_chan = fits.Column(name='CHANNEL',format='1I',array=da_spec['CHANNEL']) # unchanged
col_cts_new = fits.Column(name='COUNTS',format='1D',unit='count',array=test_counts) # '1D' for double precision, instead of originial '1I' for interger
col_bin_lo = fits.Column(name='BIN_LO',format='1D',unit='angstrom',array=da_spec['BIN_LO'])
col_bin_hi = fits.Column(name='BIN_HI',format='1D',unit='angstrom',array=da_spec['BIN_HI'])

#generate new extension
new_hdu0 = fits.PrimaryHDU(header = hd_pri)
new_hdu1 = fits.BinTableHDU.from_columns(columns=[col_chan,col_cts_new,col_bin_lo,col_bin_hi], header = hd_spec)
new_hdu_tot = fits.HDUList([new_hdu0, new_hdu1])
new_hdu_tot.writeto(filename_new, overwrite = True)
```

# Notes
- Then you will get a fits file with `COUNTS` in type of *float*, and can also be accepted by *Xspec*

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