OVERVIEW
RATDATA is an Excel Workbook with several spreadsheets that help
you analyze Foucault tests using a five zone Couder Mask. The
worksheets display the data graphically and in table format. It
is modeled after the work of Jean Texereau in How to Make a Telescope.
The user easily enters the data and quickly sees what needs to
be done to figure the mirror. The user may also print a single
sheet report to keep in the figuring notebook. The current version,
11, allows either a fixed or moving light source.
Because it uses the standard Windows and Excel interface, the
program is easily adapted to individual tastes and needs.
The workbook itself contains a welcome screen, a "Readme"
worksheet of instructions and two worksheets. "Working Data"
is for day to day figuring. "Two Axes" follows the same
program logic, but allows checking for astigmatism on more than
one axis.
DIRECTIONS
1) Enter requested information in appropriate fields. Entry fields
have red fonts. Information in cells with a blue background needs
be entered only once. Information in cells with a gray background
must be entered with every set of readings. In "Working Data"
you may enter three sets of readings. The computer will average
them.
2) Determine the appropriate
"Constant" by clicking the yellow control box to make
the deviation equal "0.".
3) Analyze the Data.
HOW IT WORKS
A full analysis of the parabola and its uses is contained in Texereau's
How to Make a Telescope.
The spreadsheet roughly follows Texereau's methodology through
line 5 of the Texereau data table (Row 48 of the worksheet). Lines
6, 7, and 8 of the Texereau sheet are replaced by the "averaged
readings" (of worksheet row 9). Texereau then suggests one
determine (for his line 9) a constant to be subtracted from all
the readings so that when one calculates line 10 and 11, one finds
the two extreme zones are equal in absolute value, but different
in sign. The spreadsheet does this by performing the line 10 and
11 calculations, and identifying the minimum and maximum deviations
(in I54 and J54). The "Average" of these two values
is put into Cell I6. The Maximum and Minimum must then be made
to average 0 (because they must be equal in absolute value but
opposite in sign). To make this happen, the user calls a subroutine
(by clicking the control box) that runs Excel's "Goal Seek"
tool. The subroutine changes the constant and re-runs the figures
until the average is 0. When the "Average Deviation"
in Cell I6 is 0, the proper constant has been selected.
The worksheet then returns to Texereau's methodology to the end
of his line 13. At this point, Texereau asks the reader to use
graph paper, a ruler, and a pencil. The spreadsheet takes over
and actually calculates the "Y" values by using the
rise and the run of the line segments to determine where each
"Y" point will be.
Texereau then asks the reader to fit a parabola to these points.
The worksheet first calculates parabolas for all possible points
of intercept (rows 65 to 78). In the next set of cells (Row 81
to 94), the worksheet calculates the differences between the wavefront
and the "best fit" parabola. The best fit requires that
the differences are all either positive (or 0) or negative (or
0). So, the worksheet selects those parabolas that meet that criteria
(Column I would indicate a 1). It then finds the maximum absolute
value of the differences in that row (Column J), and looks for
the smallest of the maximum differences of those parabola that
qualify (Column B shows the ranking). The information about the
selected parabola is then transferred to row 95, and subsequently
to row 59.
The worksheet then calculates the remaining datapoints, the difference
between the best fit and the actual parabola, and the percentage
of difference between the ideal wavefront reading and the actual.
This information is transferred into the various report and graph
sections.
The workbook also contains a sheet called "Two Axes"
that allows a more sophisticated approach, with two sets of data,
entered on different axes so that the mirror maker can check astigmatism.
It uses the same logic as the Working Data sheet. By watching
the differences between the graphs on various axes, the mirror
maker may see astigmatism in the mirror.
If you have suggestions, or bugs,
please contact Alex at alexmcconahay@roadrunner.com.
