19 inch Plate Glass Mirror

Testing

image1.jpg (8257 bytes)

The five zone Couder mask used in conjunction with the Foucault test to assess the mirror.

 One has to be realistic about what is achievable using the tests available to an amateur with a mirror of this size, focal length and with a plate glass substrate. That is not to say that a good mirror cannot be produced with these resources and techniques. My  goal was to achieve a mirror with a wavefront error of no less  than 1/4 lambda, corresponding to a maximum surface error of 1/8 lambda.

To find out more about rating the surface accuracy of a mirror click here.

In order to test the mirror I used  the standard combination of the Foucault test with a Couder mask. The readings were reduced using the software, TEX . In addition to the Foucault test  I used the matching ronchi test using Mel Bartels software which can be found here, along with details of the test. Although I have made many mirrors in the past, this was only the second occasion I had used the matching ronchi test. I recommend the test particularly to beginners who might have found the foucault test a little trying with large fast mirrors. The test basically involves matching observed ronchi patterns to those predicted at particular offsets inside and outside of the centre of curvature.  The results for the 19 inch f5.3 mirror are shown below.

 

19 inch F5.3 Mirror

Matching Ronchi Test
Predicted Observed

mrt-3.jpg (3394 bytes)

act-3.jpg (1046 bytes)

 Offset = -0.3
mrt-1.jpg (3212 bytes)

act-1.jpg (1100 bytes)

 Offset = -0.1
mrt+1.jpg (3568 bytes) act+1.jpg (1040 bytes) Offset = +0.1
mrt+5.jpg (3326 bytes) act+5.jpg (1055 bytes) Offset = +0.5

mrt+7.jpg (3309 bytes)

act+7.jpg (1210 bytes)

 Offset = +0.7

 

Mirror Diameter = 19 inches   Focal Length = 100.5inches 

Line Frequency = 85  Parabola = 1 (full)

Predicted patterns were generated using Mel Bartels Software.

Actual images photographed with a hand held digital camera.

The table below shows the results generated using TEX software.

19 inch f5.3 Mirror 19/04/00

RoC= 201 inches

Focal Length= 100.5

1 ZONE                        1                            2                          3                        4                                5

2 h(x)                      4.3140                    6.0190               7.3380                  8.4530                              9.4380

3 h(m)                     2.1570                    5.1665               6.6785                  7.8955                              8.9455

4 hm**2/R              0.0231                    0.1328                0.2219                  0.3101                             0.3981

5 hm/4f                   0.0054                    0.0129                0.0166                  0.0196                             0.0223

6 D1                      1.0560                    1.1770                1.2530                   1.3510                              1.4480

7 D2                      1.0560                    1.1770                1.2530                   1.3510                              1.4480

8 D12                     1.0560                   1.1770                 1.2530                  1.3510                              1.4480

9 D12 - 1.0419        0.0141                   0.1351                 0.2111                   0.3091                              0.4061

10 Lamda c            -0.0090                     0.0023              -0.0108                  -0.0010                             0.0080

11 Lamda f * 1e5    -4.83                        3.02                   -17.87                     -1.96                               17.87

12 Lamda f / rho     -0.343                      0.214                  -1.268                      -0.139                            1.268

13 u * 1E6               0.48                      -0.30                      1.78                        0.19                              -1.78

14 Wavefront          1.58                         0.60                      2.47                        2.22                               0.00

Reference parabola: y = 0.0266199 * x**2 + 0

passing through ( 0.00, 0.00) and ( 9.44, 0.00)

Maximum wavefront error = 1 / 8.7 wave at zone 3

Results for individual sets of readings:

Reading set 1: 1 / 5.8 lamda, Max Trans Abberation=0.76

Reading set 2: 1 / 4.5 lamda, Max Trans Abberation=2.55

Results data above is in file mirror.txt

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