The episode Black Sun, was written by British writer David Weir in November 1973, before filming on the series began. The original title was "The Black Sun", but it lost the definite article for a second version of the script on 5th December 1973 (the third day of filming the series).
The final shooting script was issued on 21 January 1974, mostly written by series story consultant Christopher Penfold but still solely credited to Weir. It was revised on 24th January.
Filming began on 31st January, and was scheduled to film for ten days. In fact, it shot for 16 days, finally completing on 21st February. Filming would then resume for four days on Breakaway; additional scenes would also be shot for Black Sun later.
David Weir's original script is pretty close to the final episode. As it was completed before filming started, there are some old names (Eagles are called MTUs, the commlock is an IDX, Sandra's surname is Sabatini, the chief pilot is Catani). Koenig's title is "Alcom" (Alpha commander). The computer expert is Ouma; Kano's name first appears in the 21st January script (in Matter of Life and Death it was still Ouma).
Weir's script has a page of notes at the end. He describes the visualisation of the black sun and forcefield effects, and signs off with "With apologies to H.G.Wells". Rather than the suggested "ghosting effects", Brian Johnson's effects has an airbrushed halo of dense stars. This is actually scientifically accurate (the black hole will bend light around it, creating a characteristic ring).
The imagery of the forcefield is also different. The description of this scene is as follows: "High angle SHOT (not view from space) of an area of Alpha's surface just above the overlapping, circular forcefields, which also shimmer different colours and look, perhaps, like nothing so much as translucent chain-mail. (Do it with discs of perspex, and light?) Ahead the BG of the Black Sun..."
The notes below include a hand-drawn diagram with overlapping circles, and hand-written notes. Rather than a simple ring of towers, there would be a field of Bergman generators, interacting to form a larger mesh, with different iridescent colours. This is the effect that Sandra calls "like fish-scales".
Sandra still describes it like fish-scales, but in the episode it a simpler green glow with white spokes. The different circles and colours would have been much more challenging to produce.
NOTES:
I'm afraid I've spent a small fortune on SFX.
But what's the point of hiring a genius designer,
and under-using him?
I won't presume to tell him how - but a couple of
notes:
Each circle is one antigrav screen -
its nucleus a Bergman generator.
A shielded area drawn around.
---- Line of safely shielded space
Which would look very pretty in different
colours, but might be too complex. Then, and
it's allowed for in the script, you could
assume the antigrav fields transform into
forcescreens merely by touching edges; and
do not require to overlap, to double, over
the whole shielded area.
With apologies to H.G. Wells.
Luck.
D.W.