[QUOTE=multiweb;1519244]You still have a sharp angled obstruction in your circular scope aperture so that statement is incorrect.
I did not think of that and you are right. I found some more information on this site
https://skyandtelescope.org/wp-conte...-Secondary.pdf.
To make a flat and to test it for flatness. It is similar to scraping a cast iron table flat. It needs 3 pieces and altenative scraping with prussian blue as marker. Testing for the glass flat is with an optical flat and monochrome light.