updated to be supplementary answer only. There was a question about the two cameras on either side of the symmetry line and if they pointed in "nearly the same" direction or pointed closer to orthogonal directions.
The question asks:
...why do the two lower ones point nearly in the same direction?
If I understand the linked document correctly, it turns out that the angle between the two is less than 45 degrees and while not "nearly the same" closer to parallel than to orthoginal.
Table 3. in the linked paper Hubble Space Telescope Reduced-Gyro Control Law Design, Implementation and On-Orbit Performance; AAS 08-278 in @OrganicMarble's answer appears to give just that, unit vectors for the directions of the three cameras. Arccosines of the dot products give us the angle between pairs, which is
pair angle (deg)
1 - 2 49.1,
2 - 3 44.4
3 - 1 49.1
Number 1 [0, 0, -1] is the top camera in the question since it is the only one pointing perpendicular to the telescope's axis. I'm getting the angle between the other two cameras as 44.4 degrees.
import numpy as np
degs = 180/np.pi
vecs = np.array([[0, 0, -1],
[-0.6547, -0.3779, -0.6546],
[-0.6547, +0.3779, -0.6546]])
print([degs*np.arccos(np.dot(vecs[i], vecs[(i+1)%3])) for i in range(3)]) # they're fairly well normalized
These are borrowed from @OM's linked answer, click for full size or better yet enjoy them while reading that answer!