This will be the fourth of a series about how to optimize telescope designs using the OSLO software program. This is a free download available from Sinclair Optics at: http://www.sinopt.com
Please email me, Steve Fejes with questions or comments at: email@example.com
I will show how to use the OSLO slider wheels to optimize a lens design. This is not an automatic method, you will make each step manually. This method is useful for finding a starting design, or for fine tuning a good design.
I will start with a 6 inch f/4 Maksutov Newtonian. Enter this
design, then save it. I will show you step by step how to use the
Click Optimize, then click Slider-Wheel Design.
Below is the new window you will see. I have entered several items here. Starting with the top I have clicked on Spot diagram, then below this I have clicked on All points. Next I entered Surf 1 then 2, these are the two surfaces of the corrector lens. Then under Item, I entered cv and pressed enter, which I did for Surface 1 and 2. This accepted the values as Curvature (cv).
You can add more sliders, I have left it at the default of 2 for this demostration. I find more than 3 or 4 sliders confusing because it is hard to keep track of so many changes in my mind.
Now click the green check mark on the upper left corner to accept the values you have entered.
Below are the new windows which will open. The analysis on these new windows will change as you modify the design. The original analysis window will not change until you click an icon on it. So, you can leave the starting spot diagram analysis on the original window in the top right for comparison. I often do this.
GW31 is a new window which shows the spot diagram. Your window may not say GW31, it may have different letters and numbers. The center spot is on axis, the top spot is off axis. I don't pay attention to the bottom spot image. You can click any of the icons on the top of this window for different kinds of analysis, just like you use the main analysis window.
Behind the spot diagram window (GW31) is a lens drawing window.
On the right is the Slider Window. You will notice there are two rows here. One is called CV1, which is curve 1. The other is CV 2, which is the second surface we will modify.
On the right of CV1 is a value of -0.004454, this is the inverse of the radius of curvature. 1 / -224.5 = -0.0044543, the slider wheel does not show the last digit.
On the right of this value we see two arrows, one pointing left the other pointing right. Clicking on one of these will change the radius of curvature. You might want to write down the starting point for reference.
Next it says Step 0.001, this is the amount of change for each click of the arrow. To the right of this we see two more arrows, the top one faces up, the bottom one faces down. A click on one of these will change the size of the Step.
I have clicked the down arrows twice each to make the Steps 1e-05. This will be a good starting point for my demonstration. The spot diagrams still look the same because I have not clicked on the left or right arrow yet.
Below is what happens when I click the right arrow on CV 1. The new value is -0.004444. If you have the Surface Data window open you can see the new radius for surface 1, which is now -225.005137. This is a change of about 0.5mm from the starting radius of curvature.
Below on the left we see the spot diagram is changed a lot. Most of the light is scattered outside the window, far away from the Airy disk in the center. The main reason for this is because the focus has changed when I entered a new radius of curvature. We could refocus the design now, but this would change the focal length. So I will adjust CV2 to bring the light back to the original focus.
If the Step would have been bigger the image would be so spread out that you would not see it in the window at all. You would have to click CV2 to find where the light comes closer to a focus. This would be trial and error because at first you don't know which direction to change CV 2.
Below shows what happens when I click the right arrow on CV2. The light is closer to the original focus, but it is not properly focused yet.
I have changed the Step on CV2 by clicking twice on the down arrow. Now each step is 1e-07. I have clicked the left arrow about 5 times to get the focus as the image below shows. You will notice that the value for CV 2 is still -0.004279, which is the same as the value above. My focusing at 1e-07 has changed the radius of curvature very little. The Surface Data window shows the change in radius of curvature.
So, I have shown you one step in how to change the radius of curvature for surface 1, then how to adjust the radius of curvature for surface 2 to find a good focus. At this point you can save the modified design if you want. Or you can modify surface 1 more and see if the images improve. We have changed the radius of curvature of surface 1 only 0.5mm, so the images are similar. A change of 2 millimeters would affect the off axis images more.
You can use this same procedure for other types of telescope designs. You can use slider windows for different items - curvature, thickness, and conic constant. Below is a drop down dialog box which shows all the possibilities. Tilts and decentration are not very useful for most optical designing. But can be used for checking tolerances, or for off axis telescope designs.
Please email me if you have comments or questions at: firstname.lastname@example.org