Reproducing the Velador Experiment

I.                    Calibration

1.      This is optimally performed as a series of exercises, intended to examine the performance of the instrument and to gather information necessary for its operation.  As a minimum, it should include the following:

a.       A full field photo of a still life target with the lens in the camera, for scale and to check camera function.

b.      Photographs of the beam interior using the camera lens in the camera’s approximate installed position, showing the relative position of the vertical and horizontal axes in the frame.  The beam top or laser cell should be removed to admit sufficient light.

c.       A photo of the beam interior using the camera lens with the laser installed but not activated, in order to check for light leakage.

d.      A lensless photo of the beam interior with the laser installed and activated but off-center, in order to show the system response when not aligned.

e.       A set of photos taken without the lens while the beam is in coarse alignment, to assess the response of the laser cell during fine alignment.  This set should be obtained with the goal of verifying which adjustments of the laser cell will move the laser image in which direction relative to the CCD.

f.        A photo of the centered laser beam image taken without the lens, for assessment of the central peak and surrounding interference pattern.

g.       A set of photos of the centered image without lens with the apparatus turned at north, south, east and west orientations for a fixed elevation, to gauge response.  Preferably, at least three images should be taken at each orientation without changing the location of the apparatus, to assess beam deformation before and after 360 degrees of rotation.

h.       If the camera is capable of video recording at any resolution, a video clip of the centered laser beam image while the apparatus is stationary, to assess thermal deformation of the image.

i.         If the camera is capable of video recording at any resolution, a video clip of the centered laser beam image while the apparatus is in motion, to assess beam deformation and jarring during motion.

j.        During these calibration measurements, it is useful to decide a nomenclature and file system for image files, and begin renaming downloaded images according to that system.  Most digital cameras repeatedly apply the same file name suffixed with a sequential number, and will overwrite file images if the files are not renamed during copying.

II.                 Collimation and Preparation

1.      Assemble all components, including computer. 

2.      Move the apparatus to the approximate position where it will be used. 

3.      Turn on computer.  Disconnect camera USB cable from computer until calibration. 

4.      Prepare laser cell.

a.       Remove laser from holder.

b.      Install laser batteries.

c.       Secure laser switch in “on” position.  Verify laser is on, then reinstall it in the laser cell.

d.      Remove the camera baffle.

e.       Using the adjusting screws on the laser mounting cell, center the laser on the camera CCD position.  If necessary, loosen or remove the camera cell from the support beam for aiming purposes.  A marked sheet of paper may be substituted for the removed camera.  Alternately, a sight may be installed on the camera cell which is visible with the baffle removed.

5.      Prepare camera cell.

a.       Once coarse alignment of the laser cell is complete, loosen or remove the camera cell to access the camera memory card.  After downloading and clearing the memory card, install it in the camera for use.

b.      Connect the camera USB cable to the camera.

c.       Reinstall the camera cell and camera baffle. 

d.      Using the adjusting screws, align the camera so that the CCD is perpendicular to the laser beam path.

e.       Install camera batteries.

6.      Fine adjustment of laser cell.

a.       Using camera mode switch, turn camera on.  Wait at least five to ten seconds for beam motion to damp out.

b.      Using camera shutter switch, record the current image.

c.       Connect the camera USB cable to the computer USB port, placing the camera in PC mode.

d.      Download the most recent image to the computer hard drive.

e.       Disconnect the camera USB cable, deactivating the camera.

f.        Open the downloaded image and evaluate it.

g.       If the center of the beam image or any portion of the surrounding diffraction pattern are not visible, compare to initial calibration photos to verify that laser and camera are functioning, then troubleshoot the coarse alignment of the laser cell. 

h.       If the image center or surrounding diffraction pattern are visible, use the laser cell adjusting screws to center the laser beam image in the frame.  (Relative motion of the image is dependent on the camera and its position, and should be assessed during an initial calibration trial before measurements are taken.)

i.         Advance one laser cell adjusting screw by 1/12 turn (30 degrees) or less.  Do not turn more than one adjusting screw at a time between calibration images for fine adjustment, and never advance the screws more than 30 degrees at a time.

j.        After one turn of one adjusting screw, reactivate the camera using the camera mode switch, then record the current image using the camera shutter button.

k.      Repeat steps a through j until the laser is aligned at the approximate center of the frame.

7.      Once aligned, the laser and camera cells should remain approximately aligned for short term storage between trials without disassembly of the aparatus.  Optimally, the coarse adjustment should only need to be performed once.  Unless indicated during fine alignment, you may skip all disassembly steps for any future measurements.  (The steel parts in the laser and camera cells can hold the same coarse alignment over time.)  To begin future measurements, install batteries and power cables, then proceed directly to fine alignment.

III.               Collection of Data

1.      After fine alignment is complete, connect the camera USB cable, download any remaining images and clear the camera memory, then disconnect the USB cable to deactivate the camera.

2.      Set the apparatus to a fixed elevation.  (Do not attempt to swing the apparatus vertically during an ongoing experimental trial.  The mount cannot correct for this motion, and the beam will deform.)

3.      Select the pattern of rotational motion and timing for each rotation step.

4.      I intend to use three different patterns during separate trials:

·        360 degree circular rotation in 45 degree increments, repeated three times during the same trial (25 photos, including a final photo in the starting position).

·        360 degree circular rotation in 15 degree increments, repeated three times during the same trial (73 photos). 

·        180 degree circular rotation in 10 degree increments, repeated three times during the same trial (55 photos).

5.      Measure and mark a horizontal motion pattern for the apparatus, according to the selected pattern for the current trial.  Useful marks include electrical tape, sidewalk chalk, etc.  Marks should be placed so that the apparatus can be visually lined up with each to within 5 degrees.  I will be using numbered strips of tape positioned so that the wheels of the apparatus can be lined up with them at each rotational position.  The markers should be positioned so that the apparatus wheels are at the same position on the floor (and experiencing the same variation in floor elevation) every time its rotation returns it to the marked orientation.

6.      Multiple repetitions of the pattern during each trial is important, as it allows comparison of the beam state at different times at approximately the same orientation, to check for pendular motion effects and other causes of deflection. 

7.      Care should be taken to accomplish all repetitions within 20 minutes (the time increment corresponding to 5 degrees of rotation for the earth – the same angle as the tolerance for rotation position).  This allows between 15 and 48 seconds for each photo, including beam damping after rotation.  An additional 5 minutes leeway is acceptable, raising the rotational error to 6 degrees, but should be avoided where possible.  This inherent error can be compensated somewhat by using more accurate measurement of the rotational position of the beam.

8.      Care should be taken to follow the same approximate schedule for all measurements made during a given trial, with a fixed number of seconds between each measurement.  At least 10 seconds must be allowed for damping of the beam’s pendular motion and after every time the beam is moved, jarred or touched.  With an expected error of up to 5 seconds in measurement spacing, this implies a minimum duration of 15 seconds between photos, and a maximum duration of 53 seconds.

9.      Position the apparatus on Mark 1.

10.  Verify that the USB cable is disconnected and activate the camera using the mode switch.

11.  Double check the orientation of the apparatus.

12.  Using the remote shutter switch, photograph the position of the laser image central peak.

13.  Rotate the apparatus and position it on Mark 2. 

14.  Wait a minimum of 10 seconds, but not more than 43 seconds.

15.  Using the remote shutter switch, photograph the laser image central peak at Mark 2.

16.  Rotate the apparatus and position it on the next mark in sequence.

17.  Wait a minimum of 10 seconds, but not more than 43 seconds.

18.  Using the remote shutter switch, photograph the laser image central peak at this mark.

19.  Repeat steps 16 through 18 until the apparatus has been through three complete rotations.

20.  With these tolerances for timing and rotational orientation, the expected mean error for rotational orientation will be less than 10 degrees (position measurement error plus earth’s rotation over the course of the experimental trial).

21.  Upon completion of each trial, connect the USB cable to the computer.  Download, file and/or rename all images, and clear the camera memory.

22.  Unplug the USB cable to deactivate the camera.

23.  Deactivate the laser by removing it from the cell and removing its batteries.

IV.              Preliminary Analysis of Data