I have found ASIfitsview to be a very well done usefull app. However I have fouund a problem with the latest two versions of the app. Prior to these two versions I could double click a fits file and the ASIfits view file would open and display the fits file without having the ASIStudio open. The latest two versions now do not open the ASIfitsview module of ASIStudio by clicking on a .fits document.
I use the Mac OS with M series processors. and the latest version of the MacOS. I do not know if he change is caused by changes by Apple or Zwo. I now request ZWO also provide a stand alone version of the ASIfitsview app which would allow clicking on a fits document taken with my ZWO cameras to open the app and display the image for use.
I do not know if .fits files from other sources can be opened with ASIfitsview, But since all the Astro Cameras I own are from ZWO/asi there would be no problem if ASIfitsview were limited to only open images created by ASI cameras.
Allen Ruckle