I don't know how it is meant to be written actually.  I tend to write it as if I were saying the letters (therefore a you-eff-oh). I don't know if that is correct.

I suppose in the most literal way if it is a vowel then it should be an, not a.

I always make the mistake with 'historical' too.

Ad astra per aspera

DoDo,
I like the original also.
Trying to work a whole image sometimes works less positively on one element of the image while ameliorating another.
In the example, I took the liberty of altering,(only because I saw you wanted Mr.Sven, to have a go at it) by separating the sky & the pasture, & working on them separately.
With the selection "lasso" set at 10, I just followed the tree line roughly & around the bottom.
You can work the bottom, then invert the selection to modify the sky.
By doing it this way you have control over the elements you want to work on.

The difference between theists and atheists is that the atheists don't set the theists on fire for refusing to agree with them.