Couple of API questions

[obser...@stratis.co.uk]

Starting to use the API from Python for the first time, using the Client script.  I can't find much documentation but the script is fairly easy to understand. However I have a couple of basic questions which maybe someone can help with:

1. If I solve by uploading a FITS file (using Client.upload) how do I then download the solved file, with the FITS header updated with the WCS info?

2. Alternatively, if I solve by uploading an x,y text file of extracted star pixel coords (also using Client.upload) how do I get the WCS info, including distortion coefficients, into the FITS header of my original image file? Which send_request method should I use to retrieve this data and is there a preferred way of updating the FITS headers with it? (I can roll my own but I'm hoping there's an existing function that does this).

Many thanks

Nigel Brooks

Stratis Observatory

[Stratis Observatory]

OK I've solved the downloading - the link to the api documentation on the astrometry.net site at http://nova.astrometry.net/api_help doesn't allow access but I finally found the docs via the readme on github.

I'm still looking for advice on how best to update the wcs in a fits file with what I download from astrometry.net.

I can download the wcs.fits file after solving but I need a Python function that will put that wcs data into a FITS image file (overwriting any wcs that's already there).

Any pointers very gratefully received.

Nigel

[Dustin Lang]

In the Astrometry.net code there is a "new-wcs" program for plugging a WCS header file into an image.

cheers,

--dustin

[Stratis Observatory]

Many thanks Dustin. However as I don't really want to get into C I'll probably roll my own using astropy.WCS class methods which I've been checking out today.

Nigel

[Dustin Lang]

Fair enough; with astropy, you could also work at the FITS header level -- I think you should be able to read in the image (in read/write mode), read in the WCS header, and plug most of the WCS headers into the image header, and write the image to a new out file, and that'll do it.  Omit SIMPLE, BITPIX, NAXIS, NAXIS1, NAXIS2, but keep the rest.

cheers,

--dustin

[Stratis Observatory]

Great minds think alike.  Just what I did earlier.  Seems to work.

def update_wcs(input_image, astrometry_data, output_image=None):
    # Adds or updates the astrometry data in input_image from
    # astrometry_data. If output_image is not specified then
    # input_image is updated in-situ and resaved,
    # otherwise the updated file is copied to output_image
    # and input_image is unchanged.
    #
    # astrometry_data is assumed to be a minimal FITS file
    # (e.g. as returned by astrometry.net after plate-solving).
    #
    # Mandatory FITS cards are ignored (don't want to change them).
    # Where other keys match the existing card is updated.
    # Where astrometry_data contains new keys those cards are added.
    # All other existing cards in input_image are retained unchanged.
    #
    # Input:    input_image         String. File path/name of
    #                               existing FITS file.
    #
    #           astrometry_data     String. File path/name of  
    #                               FITS file containing astrometry
    #                               cards in hdu0 header.
    #          
    #           output_image        Optional string. File path/name
    #                               where updated file is to be copied.
    #
    # Output:   Updated input_image file (if output_image not
    #           specified) or updated copy of input_image file
    #           saved to output_image (overwrites any existing).


    from astropy.io import fits
    image_file = fits.open(input_image)
    astrometry_file = fits.open(astrometry_data)
    new_cards = list([])
    for card in astrometry_file[0].header.cards:
        if (not(card[0] in ['SIMPLE','BITPIX','END']) and
            not(card[0][:5] == 'NAXIS')):
            new_cards.append(tuple(card))
    image_file[0].header.update(new_cards)
    if output_image is None:
        image_file.writeto(input_image)
    else:
        image_file.writeto(output_image)
    return