Quote:
Originally Posted by Top_oz
This is very impressive.
Can I ask what tolerances you are working to with the machining?
When you say the backplate was a challenge to get 'flat', what are you aiming for?
After dabbling with some machine work I have developed a much greater appreciation for tolerancing and metrology.
Flat for me is a multi faceted challenge of getting two sides flat, and parallel, and a certain distance apart. And then being able to actually measure the result with any degree of accuracy.
I've not found any specific advice for astro so very interested to find out how tight this work needs to be.
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Hi Tim,
My OTAs are made to tight tolerances in order to eliminate the need for collimation of the primary mirror.
The back plate was held, on the lathe, against a sturdy steel ring, with three screws. The steel ring was about half the diameter of the aluminium plate.I used washers as spacers between the steel ring and the aluminium plate so that the plate was free to change flatness as internal stresses were released during machining. Flatness was tested with a dial indicator that has 2 micron divisions.
The finished disc was reading deviations of no more than 10 microns near the periphery on both sides.
If the primary mirror is adjustable for collimation then such accuracy is not needed.