DIY Spherometer

So...you are crazy enough to build your own telescope mirror or other optics? Well, you'll definitely need a spherometer!
This design has 2 configurations, based on the central hole - 8mm and 8.3mm. When I print a hole, my printer has an accuracy of about 0.3mm so in my case, 8.3mm is actually close to 8.1mm. The body of the dial indicator is 8mm so it fits like a glove. If you don't know your printer accuracy - use the 8mm version and drill it after the print.
You'll also need the following parts:
1. Dial indicator (0.01mm accuracy) with 0-10mm travel. If you can find 0.001mm one for less money - use it! I bought mine on eBay for about 10-12$.
2. 3 x M5x20 DIN912 bolts with 3-6 nuts and 6 washers.
3. 1 x M3x12 DIN912 bolt with a nut for the nut trap.
4. Stainless steel bearing balls - preferably 1/4 inch (6.35mm) version. You can use watever balls you want but keep in mind that it's important that you know the exact diameter! If you have old bearings - you'll need a micrometer