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HELP with optical testing for DIY 3.5" secondary m

Started by BABOafrica, 11/01/2012 08:34AM
Posted 11/01/2012 08:34AM Opening Post
I did my first test (ever in my life) looking at diffraction patterns. I'm hoping someone can help me sort out some ideas here.

I have three secondary mirrors (still in square shape) pretty close to final polish. Let's call them A, B & C. All three are about 5" x 7", aiming at making one usable 3.5" secondary.

I set up a red laser light shining thru a barlow and after the barlow an ample piece of ground glass. The red light is positioned to strike the target at about a 45 degree angle as the target lies on the table. I'm looking at the target from the other side, my line of sight also at about 45 degrees relative to the target -- just like in the book!

I laid A on C and saw about 40 lines, each line stretching from one edge of the glass to the other. All the lines where curved.

I laid B on C and got the same result.

I laid A on B and saw about 40 lines but all the lines were almost perfectly straight, stretching from one edge of the glass to the other.

Does this mean that both A and B are flat?

Clears,
Joe

In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 11/01/2012 09:31AM #1
In case this makes a difference, I also noticed the following:

With A on C or B on C, the curved lines diverge as I move my head up and start squishing together as I lower my below the 45 degree angle.

With A on B, the straight lines do not diverge or squish together no matter where I move my head.

Clears,
Joe

In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm