CHEAP INTERFEROMETER FOR OPTICAL TESTING OF TELESCOPE MIRRORS.

THINGS REQUIRED

· XY-Axis stage/ plywood board of one square feet.

· Laser pointer

· Camera lens filters coated one side

· Flat mirror

· TEST MIRROR. Primary or secondary mirrors.

· Planoconvex lens.

· Water level/ right angle triangle scale.

Step 1: SETTING UP THE ASSEMBLY.

· If XY axis stage not available, take a square plywood stand.

· Set up water level on stand. Will require 2 such stands of same height.

· Take wooden stand for laser source.

· Measure the height of laser beam with right angle near the source and away at one feet. Now secure laser beam with glue on to the stand..

· Take flat mirror with adjustable tilt and keep in front of laser beam. Redirect beam in laser pointer.

· Keep beamsplitter in between path of laser beam. Now check two beams from beam splitter. One will pass/transmit in straight line and other will reflect at right angle.

· Instead of using beamsplitter you can use plain camera lens which is coated on one side. Also you can get it from old scrap cd drive lens.

· One beam directed at right angle, should focus the test mirror. if your test mirror is flat mirror you can keep it close to the splitter beam. If you want to test primary mirror it has to be kept at radius of curvature(ROC).

NOTE: ROC IS CALCULATED AS TWICE THE FOCAL LENGTH. ROC CAN BE MEASURED BY FOCUSING A TORCH IN FRONT OF THE MIRROR AND TRY TO GET THE SHARP FOCUS IN FRONT OF THE TORCH.

· IN OUR CASE WE ARE TESTING a flat mirror which is placed in front of the right angle beam. Adjust the beam at the same point of focus as that of the beam splitter

· You will see a laser beam opposite the beam splitter.

· Your assembly is ready for the test.

Step 2: RESULTS OF INTERFEROMETRY.

Place a Plano convex mirror in front of the laser source at the same height as that of the beam. You will see the fringes dark and light bands on the screen. If you see alternating circles of dark& light, give a small tilt carefully to the test mirror and you will see the bands of interference pattern.

Download the photographs and check the result on fringes software available on the internet.

Divert back the laser beam to the point of origination of laser from beam splitter.