Re: Using astscript-psf-select-stars on HST flc images

[Mohammad Akhlaghi]

Hi Alex,

Your welcome, I am happy it was useful ;-).

> I am wondering if the astcrop trick always cuts the secondary HST WCS 
> or if other files will not behave correctly

For limiting the part of the header that is used for reading the WCS, 
Crop has two options (which you can get from the command below: both 
options have the format of '--h*wcs=INT', the 'h' is for header):

$ astcrop --help | grep wcs=
      --hendwcs=INT          Header keyword number to stop reading WCS.
      --hstartwcs=INT        Header keyword number to start reading WCS.

With these two, you can tell Crop to only use the given keywords for the 
WCS of its output.

About usage on other files, usually low-level datasets produced from a 
certain version of a pipeline write their keywords on the same place for 
all the files they produce. So in theory, this should work on all files 
that you get from the same place.

> Do you think that there is a way to automatically detect which 
> keywords should stay and which ones should go?

This will need some manual trial and error (for example to increase the 
'--hendwcs' until you find the problematic keyword. You can then use 
Gnuastro's FITS program to remove/edit that keyword (until bug 63644 in 
[1] is fixed).

> I would need to find and rotate the spikes, which is a nightmare 
> by itself.

Indeed, that will be a nightmare! Just have in mind that the shape of 
the PSF and the spikes are also be affected by the titling. So for 
constructing the PSF use bright stars that are in similar parts of the 
CCD, not just all stars. It would later be interesting to see how much 
they differ by position.

Cheers,
Mohammad

[1] https://savannah.gnu.org/bugs/?63644