I have long been intrigued with this globular. I first observed it in 1981 with my 8" F7 reflector and made the note: "It's like a dull, torchlight pointed at me, but shining through a mist."
I had the same impression when I observed it many years later with my 12" F5 Dob. I observed it 3 nights ago with my new Tak 100D (F7.4) and still saw that effect with the lessor aperture.
In the Stephen James O'Meara Calwell Objects (P 291), Stephen says that ; "The overall impression is that of the great Hercules globular M13 seen through a fog." NGC 1851 is also known at Caldwell 73.
I have seen this 'fog/mist' affect only on this globular. I am interested if others see it? Strangely, Hershel saw it, but only with his right eye; not his left eye! (Ibid P288).
On researching, this globular has a very colourful past with indications that it may be a merger of two previous globulars and that it may have lost from a nearby Dwarf Galaxy. Also there are studies indicating a halo extending around the globular. There are a surprising number of studies on the Web concerning this cluster.
More information:
https://britastro.org/node/6857
https://books.google.com.au/books?id...201851&f=false
https://arizona.pure.elsevier.com/en...bular-ngc-1851
https://www.aanda.org/articles/aa/pd...aa17269-11.pdf