In principle, every image from the CCD astronomical camera is unique. No repeating of heaven's mysteries is possible. Therefore, the utilities of nightview package creates every saved image as read-only by default. (Technically the unmask mode is set to 444 on Unix systems.) There is no switch for overwriting of this feature except patching and re-compilation of sources. If you can rewrite or delete your data (NOT RECOMMENDED), use chmod command for this purpose.
I was added this feature after unfortunately removing all of my images of the 1998WT24 asteroid, BL Lac observations and time series of SAX xxxx+xxx. Please, be careful.