- Achieving reference spot measurement -
Achieving reference spots set
The goal of this step, is to get a reference spot pattern, that is required to perform wavefront measurment later on.
Install the 5 or 10 µm pinhole in the following way :
The pinhole diameter mark shall be visible, the pinhole shall not be installed upside down !
Warning : pinholes are fragile parts !
Then screw the cap on top of it.
Insert the collimator inside the holder
Push smoothly the collimator inside the holder.
Push it till it touches the top of the holder.
Clamp the collimator to the holder.
Collimator should not have dust on it, it will "discard" optically some microlenses.
The Shack Hartmann can be cleaned also, a dust ca affect some microlenses.
Assemble the collimator with the SH body.
To achieve a reference spot array, a classic office lamp ca be used to illuminate the small pinhole.
At that stage you are ready to perform a reference spot array, the incoming wavefront will be your reference wavefront and
should be as perfect as possible. Nevertheless, relative measurements with respect
to a given wavefront can be achieved, provided a given spot don't get
mixed with its neighboring spots. Enough space (say some pixels) must
be left between spots. In case of pure flat wavefront or achieved by a small pinhole, the distance between
spots should be arround 27 pixels.
In the next section the incoming wavefront is fairly flat.
Now turn on the "Combine incoming frames (SNR improvement). By doing this the image gets much less noisy, and stacking 100 frames leads to an improvement of square root of 100 of the SNR (so by a factor of 10). This improves also the accuracy of the measurements, because centroiding of the spots gets much easier.
When the required amount of combined frames is reached, check the menu "Camera/Pause". The acquisition will get paused.
Then pay attention to the green box, "Signal to background noise ratio". This is currently set to 5 (SNBR). It means when it detects a spot, as valid provided the spot maximum is larger than SNBR time the background noise plus the Mean. For instance, here the background noise (blue box) has been computed as 7.96 ADU and the Mean is equal to 13.5, any spot that has a maximum larger than 5*7.96+13.5 = 53.3 ADU is regarded as a valid spot.
Then click to the menu "Measurements/3.Spot position calibration"
The next window pops up. Fill the approximate spot distance with the figure that seems to you the most appropriate (say 27 for a flat wavefront), the detection threshold can be figured out from the MAX value from the background box definition (here it is 40.4 ADU) , let's add 10 ADU to avoid noise spike detection, so yielding to 50 ADU.
The first stages of centroid detection is running.. All the valid spots are
surrounded by a red thick square. A selection window appears : it gives to the
user the position (X,Y) and also the Full Width at Half Maximum of the spot.
By clicking to the position in the selector, the spot can be : traced, removed
(deselected).
Spots to close to the image's edges are ignored.
For instance the Yellow arrow indicates a user deselected spot and the green arrow the currently selected spot.
Once the selection, checking phase is done, click "OK". A file save dialog box pops up. It will generate the text file that contains all the spot positions.
Then all the positions are validated according to your choices. Note that the unselected spot has been discarded.
If one opens the "demo.txt" file, with wordpad it looks to this :
18569789
335
13
1 205.000 31.812 1.718 1.772
2 260.012 31.461 0.368 0.332
3 287.853 30.925 0.111 0.017
4 315.348 31.089 0.787 0.592
5 343.429 30.987 0.513 0.625
6 371.105 30.528 0.601 0.574
7 398.981 30.523 0.342 0.392
8 426.841 30.560 0.190 0.125
9 454.472 30.579 0.833 0.844
10 482.012 30.702 0.179 0.287
11 509.491 31.126 0.482 0.481
.....
First line 1856789 : is a magic number, to help the software to recognize this
txt file as a valid spot file position.
2nd line : 335 is the amount of valid spots (not valid spots are not written
in this file)
3rd line : 13 is the radius of the box in which the spot is allowed to move
further wise. This is a box of 26x26 in which the spot will be allowed to move
to measure local wavefront distortion.
4 th lines and the remainder : this describes the spot reference positions where
:
1st column is : spot rank (arbitrary)
2nd column : X position
3rd column : Y position
4th and 5th : for debugging purposes, not documented.
You can edit the file and modify, delete, add the position of a spot. In case of manual adding deleting operations don't forget to update the 2nd line (amount of spots).
Now you're set to carry out measurements