I found something on youtube, not sure if it will work on that large of a scale, but I still have a few days of work before I worry about the roof. I am new here so not sure I can post the link here. If I am allowed to let me know and I will post it for anyone who is interested.
Without adding a massive footprint of probably around 10 blocks thick to the roof, the only way I can think of doing it is by using 'glass' instead air blocks for when the roof is open, and some other block when its closed, this of course has a limit of probably 12 or 13 wide. If you didn't mind taking the roof down yourself you could go with 2 to 4 cobble/smoothstone generators which would 'fill' in the roof and could be as wide as 26, but as I already said there's no way to 'retract' it. In the most recent PC snapshots there are slimeblocks that might enable you to do this but it will likely be awhile before we have these to play with.
In respects to posting links to tutorials and other videos, its usually permitted so long as its not 'promoting' so to speak.
Sorry it took me so long to back on here. been busy with work but I will post the link. the guy in the video is doing on a small scale so I do not know how it will translate. Also it does have the glass blocks I am pretty sure it will not work without some kind of block.
In respects to posting links to tutorials and other videos, its usually permitted so long as its not 'promoting' so to speak.
not actually open but switch to glass. Would it be possible to do more than one set of the 13 blocks?