I thought I had a 1.062" diagonal until I measured it. It's actually a 1.25" diagonal. Purchased about 4 years ago from Edmund Scientific. Here is the writeup from their website:
"Our precision 45° Elliptical Flat Mirrors are ideal for both technical and astronomical applications. Because of their elongated major axis, they are perfect for bending/folding light at precise angles with minimum wave distortion. All are ground, polished, and tested to our exacting standards. Each size is available with protected aluminum.
Material: Pyrex® – Fine annealed Brand No. 7740
Axis Dimension Tolerance: ±1%
Surface Quality: 60/40 Scratch/Dig
Surface Accuracy: 1/8 Wave (l) [l = 632.8nm]
Approx. 3/8" thick."
Asking 20.00 shipped.