Comparison of Old and New Cells

I have claimed that cells designed with PLOP are better than older cell designs. In order to support that claim I present here some images of PLOP evaluations of different cells.

Image showing mirror deformation This image shows the residual deformations of an 18-point cell for a 20-inch diameter, 7/8-inch thick, f/5 mirror with 3.1-inch diagonal. The cell dimensions were copied from The Dobsonian Telescope, Kriege and Berry, 1st. English Edition, ©1997, Table 5.5, page 132. The color range from deep blue to bright red covers a range of 1/20 wave. Blue is high, that is deformations above the mean. Red is low. Green is in the middle of the range. This image is actually a little "overexposed" in that the peak-to-valley deformation of the mirror in this cell is actually 1/17 wave.

Remember that these deformations are surface deformation. Light arriving at your eye traverses this distance twice. 1/17 wave surface deformation becomes 1/(8.5) wave path difference at the image.

Image showing mirror deformation This image shows the residual deformations of the same 20-inch mirror in an 18-point cell as optimized with PLOP. The peak to valley deformation with this cell is 1/43 wave. The color range is still 1/20 wave.

It doesn't require much thought to see that you would rather have your mirror in the second cell than the first. This is especially so when you realize that the central low zone (red) is obscured by the diagonal mirror so that the light reaching your eye comes only from the green and blue areas.

The next seven images were also produced by PLOP. The colors encode, from bright blue to bright red, a range of 5 x 10-5mm or1/10 wave. All deformations are listed as surface deformations, not wavefront errors, which would be twice as large. The best fit paraboloid has been subtracted from each plot, so that these plots show not the total deflection, but the deviation from a parbolic curve.

All seven mirrors represented have 1:6 thickness ratios. All are f/6 and each was calculated assuming an appropriately sized secondary.

Image showing mirror deformation This plot shows an 8-inch diameter mirror supported by three points located at 0.393 r. This is the optimum spacing as calculated by PLOP. The P-V deformation is 1/75 wave. The RMS deformation is 1/365 wave.

Image showing mirror deformation This plot shows an 8-inch diameter mirror supported by three points located at 0.7071 r. This is the spacing recommended by Hindle. The P-V deformation is 1/30 wave. The RMS deformation is 1/135 wave.

Image showing mirror deformation This plot shows an 8-inch diameter mirror supported by three points located at 0.95 r. This is the spacing recommended by Texerau. The P-V deformation is 1/13 wave. The RMS deformation is 1/74 wave.

Image showing mirror deformation This plot shows a 10-inch diameter mirror supported by three points located at 0.396 r. This is the optimum spacing as calculated by PLOP. The P-V deformation is 1/48 wave. The RMS deformation is 1/233 wave.

Image showing mirror deformation This plot shows a 10-inch diameter mirror supported by three points located at 0.7071 r. This is the spacing recommended by Hindle. The P-V deformation is 1/19 wave. The RMS deformation is 1/86 wave.

Image showing mirror deformation This plot shows a 12.5-inch diameter mirror supported by three points located at 0.401 r. This is the optimum spacing as calculated by PLOP. The P-V deformation is 1/30 wave. The RMS deformation is 1/149 wave.

Image showing mirror deformation This plot shows a 12.5-inch diameter mirror supported by three points located at 0.7071 r. This is the spacing recommended by Hindle. The P-V deformation is 1/12 wave. The RMS deformation is 1/56 wave.


© 2002 Mark D. Holm