Swarp

From http://www.astromatic.net/software/swarp:

SWarp is a program that resamples and co-adds together FITS images
using any arbitrary astrometric projection defined in the WCS
standard.

Create default a config file called default.swarp:

swarp -d >  default.swarp

edit default.swarp to change the following parameters. Use any text editor
like gedit, nedit or pico.

 PIXELSCALE_TYPE        MEDIAN          # MANUAL,FIT,MIN,MAX or MEDIAN
 PIXEL_SCALE            0.0             # Pixel scale

to

 PIXELSCALE_TYPE        MANUAL          # MANUAL,FIT,MIN,MAX or MEDIAN
 PIXEL_SCALE            5             # Pixel scale

then run swarp on your files, eg

swarp file1.fits file2.fits file3.fits
 
or use wildcards:

swarp IPHASXJ06041*+13*Hb*.fits    

to select all Hb observations of the same object, output file (also
set in default.swarp) is coadd.fits

Also works to resample one file, eg

swarp IPHASXJ060413.8+133319_Ha_300s.fits    

again output file is coadd.fits

Tips

SUBTRACT_BACK          n               # Subtraction sky background (Y/N)?
 
chang y to n