4.1 Alignment

1. Mount the camera and the point-mass lens in their holders (if they’re not there already). Use the blue-fingered holder for the lens and make sure the lens is secure.
2. Slide the lens so that it is relatively close to the camera and adjust the position of the lens so that it is well-centered with the camera’s field of view. Then, slide the lens so that it is far away from the camera, and adjust the camera angle so that the lens is well-centered with the camera. You may need to do a few iterations of these steps to get the camera and lens well aligned.
3. Slide the lens to about halfway between the camera and the source object. Tape the image of the black dot on the backdrop so that you see a nice, full Einstein ring. Slide the lens closer and further away from the camera. Hopefully the observer sees an Einstein ring over the whole distance. But you can always make small adjustments as you do the experiment; they won’t affect your results.

4.2 Strong Gravitational Lensing

1. Place the lens from 25% to 75% of the distance between the lens and the backdrop (or a wider range if you are feeling brave).  The idea here is that you will test to see if the position of the lens affects the mass measurement (ideally, it wouldn’t).
2. You will need at least 5 data points. Be sure that you can see the entire lens in each image. The diameter of the lens is used to calibrate the image from pixels to real units. Take careful note of the component positions; you will need them to compare data across experiments.

4.3 Weak Gravitational Lensing

1. Remove the paper with the single dot and replace it with the sheet with many dots.
2. Use a light isothermal lens or one of the lightest point-mass lenses. Adjust the distances until you can see the dots being distorted into ellipses, particularly near the center of the lens.  Try to fill up as much of the field of view of the camera as possible.  Anything outside of the lens isn’t useful.
3. Save a frame of the image for analysis. The contrast of the image will be important, so please use the desk lamp or distance adjustment of the components to get the best contrast between the lines and the paper as possible.
4. Move the lens and take another picture. You may want several pictures at various distances, then you can choose the one with the clearest and most ellipses for your analysis.
5. Use ImageJ and go to Image→Adjust Color Threshold. Set the threshold colour to black and adjust the brightness so that the ellipses are as clearly visible as possible. See the Appendix for images of this and the following steps.
6. Go to Analyze→Set Measurements… and make the same choices as shown in the Appendix. Then go to Analyze→Analyze Particles…and make the same choices as in the Appendix. You should end up with a table of measurements similar to the one shown in the Appendix.
7. In your analysis/report you will plot the ellipticity as a function of radius to determine the Einstein angle, and thereby the mass emulated by the lens.

4.4 Gravitational Micro-Lensing

1. Remove the paper from the backdrop.
2. Make sure you are using the same lens as in the weak gravitational lensing experiment.
3. Set up the gear box, string and bead so that the bead hangs over the backdrop and in the field of view of the camera. When the gearbox is activated, the bead should be pulled up through the field of view of the camera. Remember that you are trying to mimic microlensing, which is a very weak lensing action.
4. Watch the video as the bead is pulled behind the lens. Be sure that Einstein rings or other strong gravitational lensing phenomenon are not taking place. Look at the text display of the Python program so you know when the bead is passing through the lens so you know what section of data to look at.
5. Plot your data and see that it looks something like the light curve in Figure 5. You will likely see some sharp peaks in the intensity as the bead enters and exits the area of the lens, which is due to refraction at the edges of the lens. Fit your data to extract the value t_E, and use it to determine the Einstein radius and ‘mass’ of the lens.
6. Try to save frames at the minimum and maximum magnifications for your lab report