Saturday, April 10, 2021

made another diffraction mask

Made a diffraction mask for the C8.

Wasn't keen to haul out the Celestron Optical Tube Assembly to do measurements. But then I remembered the Bahtinov mask project. So tracked down the PDF. I recalled the grating was at the exposed glass edges so I could use that as a reference... Hold the phone! I grabbed the actual focusing mask I made. I had modified it to fit inside the OTA! That would help me easily and quickly get all the measures, including the secondary mirror!

Measured the diameters.

  • OTA inner diameter (ID): 22.7 cm
  • opening: 20.5 cm (that seems good, an 8-inch mirror is 203 mm)
  • central obstruction (CO): 70 mm

OK. Fired up Visio and located the apodisation mask I had made for the little Meade ETX.

Considered scaling up the ETX but didn't feel completely comfortable. So I started from scratch.

vector diagram for C8 apodisation mask

Like with the focusing mask, it ran off the page, too big for a single "letter" size sheet. So I cut the mask at a logical place. Printed on the laser in greyscale and cut out the pieces. Couldn't find the white glue so I used wood. It went fine.

Ready to try.

But now I'll probably have to wait 10 months... Hrrm.

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