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$.
2. 3 x M5x20 DIN912 bolts with 3-6 nuts and 6 washers.
3. 1 x M3x12 DIN912 bolt with a nut for the nut trap.
4. Stainless steel bearing balls - preferably 1/4 inch (6.35mm) version. You can use watever balls you want but keep in mind that it's important that you know the exact diameter! If you have old bearings - you'll need a micrometer to measure the diameter of the balls. I bought mine on eBay for less than 3$.

Put the M5 bolts, nuts and washers as shown in the pictures. Epoxy the balls on top of the heads of the bolts and try to not put any glue on the working surface of the balls!!! Put a M3 nut in the nut trap and screw the M3x12 bolt into the central hole. Put the dial indicator and screw the small bolt untill tight (Don't be crazy here - just a gentle twist).
Keep in mind that this is pretty precise measuring tool! If you need your dial indicator to sit higher - use more nuts and put the head of the bolts lower. Be carefull and level everything!
If you make everything correctly - you'll have a nice tool for measuring the sagita of the mirror that you're grinding! The biggest diameter (far most position of the M5 bolts) is 125mm diameter (or almost 5 inches). If you're measuring smaller diameters - put the bolts in other holes.
How to calculate and use this tool:
Part 1 - https://www.youtube.com/watch?v=htBx5SRFeMI
Part 2 - https://www.youtube.com/watch?v=fLiodHOESYM
Happy measuring!

P.S. Sorry about the stl files. Some of them are not correctly aligned so use the proper ones! The system don't want to give me the choice to remove the old files. :)