Refractor Data Entry

  This page will show how to enter data into the OSLO program for a refractor objective lens.  This page has many images which should make this lesson easy to learn.  These images also make the page slow to load because of their combined size.  I have tried to keep each image small while showing good details.

  OSLO LT is a free software program which is available from

  There are three things I want to cover in this section.  The first is how to enter and save a refractor design.  This will be a two element design with an oiled surface between the two glass lenses.  This design is a 6" diameter f/20 lens.  The next thing I will do is show how to change a lens surface to an aspherical shape.  The third thing I will show is how to add an air space between the lenses.

  I plan to make a couple more pages about how to create a reflecting telescope design, and then modify this design.  Next I want to show how to enter a Maksutove design, then an eyepiece.  I plan to make other pages also which will cover evaluation and optimization also.

  Let's start by creating a new file.  Click File, then New Lens.

You should see this text box open.

  Enter 3 in the Number of surfaces box, then click OK.

Now we see the Surface Data box (image below).  This is where we will enter the information to make our design.  You can enter data into most of the boxes here, and many of the grey boxes can be right clicked to show a list of options for each.  I will cover this in some detail later.

I want to go over some of the information on the Surface Data window.  There is a row of grey boxes near the center of this window which lists:
SRF                This is the surface number.

RADIUS        This is the radius of curvature for a given surface.

THICKNESS     This is the thickness of the glass, or the amount of air space.

APERTURE RADIUS        This is the semi-diameter of the lens or other surface.

GLASS        This is the type of glass or other material

SPECIAL        This is where special data is entered, when needed.

I have entered a name or description for this design.  This is just for my use so I can quickly see what this design is.  You are limited to the amount of letters and numbers which will fit on this line.  Just click in the space after Lens: and type in your information.  Click enter (on your keyboard) to accept the information.

I am going to select a type of glass for the first lens element.  Click on the grey box to the right of AIR.  This is in the Glass column, under GLASS.  And it is in the row of the first surface.  In this case the first surface does not have a number 1 under SRF because it is the default aperture stop (AST).

When you click on the grey box to the right of AIR you will see a new window open which has several options.  I have moved my mouse cursor over Catalog, which then opens a second window.  This second window lists the glass catalogs available.  I will now click on Schott.  This is one of the glass catalogs.

Now we have the Schott glass catalog opened.  I have clicked on BAK1.  This is the glass I want to select for this lens design.  You will see that two rows of information are displayed near the top of this window.  This shows the refractive index (n), dispersion (V), and other information.  At the end of the second row you will see the relative cost (1.80 in this case), and the availability (V which means very available).

BAK1 is a fairly common glass which is moderate in cost and very available.

Now click on the green check mark in the upper left corner.  I have moved my mouse cursor over this green check mark, and you can see that it says "Accept pending entry / Close spreadsheet.  Clicking on the green check mark will accept your changes, in this case we will be changing from the material AIR to the glass BAK1.

Now the Surface Data window is open and we can see that the GLASS for surface 1 is now BAK1.

Next I will change the glass for surface 2 to from AIR to F2.  This is a common flint glass.  You can do this by repeating the above steps.  Or you can enter the name directly by clicking on AIR in surface 2, then typing in F2, then enter.

Now I will enter the radius of curvature for surface 1 (AST).  My design is for a radius of curvature of 1996mm.  So, I enter this value (1996) into the RADIUS box for surface number 1, then click enter on my keyboard.

Now I will enter the rest of the data for my design.  You can see the other two radii of curvature for surface 2 and 3.  Next I enter data into the boxes for thickness.  The BAK1 lens will be 16mm thick, and the F2 lens will be 12mm thick.  The third thickness I will leave as 0.  The final thickness I will enter a value almost equal to the focal length.  This will be where the image is formed.  I will show you how to focus the lens a little later.

The highlighted box shows the semi-diameter of the lens.  In this case I have chosen 76mm, which is about 1/2 of the 6" diameter.

On the upper right side of this window you can find the effective focal length (Efl).  In this case it is 2.9990e+03.  This is 2999mm.  So, the focal ratio of this design is f/19.73.

To the right of the entrance beam radius is the field angle.  Again this is the semi-diameter of the field of view.  I have chosen 0.5 degrees, which will give a 1 degree field diameter.

Now I will focus the lens.  Click on the grey box under the thickness column of surface number 4 (IMS).  A window will open, move your cursor over Autofocus - minimum RMS spot size, and another window will open.  Now click on ON-Axis (monochromatic).  This will give you the best focus for the primary wavelength used for this design.  The default primary wavelength is 0.58756, which is green light.

As you can see there are other options for  which focus to select.  You can try these and examine the image quality.  You can also enter in any value in the IMS thickness box and examine the image at this focus.  I will make a web page to explain how to evaluate optical designs, it would take too much time to do now.

Now that we have made a design, let's save it.  Click on File, then on Save Lens As...      I don't like to click on Save Lens because this will overwrite the existing design which I am working on.  I suggest getting into the habbit of using Save Lens As... instead.

I save my lenses in the Private Library Directory.  You can see that I have made three folders here, atmfree, Reflectors, and Refractors.  This will help me keep my design organized.  You need to type the name of your lens in the File name box, then click Save.



I want to modify this design by changing the first lens surface to a non-spherical shape.  This will correct the slight amount of spherical aberration in this design.  The conic constant of this surface will be about 1/5 as much as a parabola.  The conic constant of my design is 0.2, while a parabola is -1.0

We will do this by clicking on the grey box under SPECIAL on the surface 1 row (AST).  A new window will open, move your mouse cursor over Polynomial Asphere (A) and then click on Conic/Toric.

Enter the value 0.2 in the conic constant box.  Enter the value then click the green check mark twice.  This will accept the conic value.  You can see that a conic constant of 0.2 is an oblate spheroid.

The green check mark will accept pending entry, and close this spreadsheet.

Now the Surface Data window shows the letter A in the grey box under the SPECIAL column for surface number 1.  This is an Aspherical surface.

We will need to refocus now.

And save the updated file.  I have put the letter "b" at the end of the origina file.  I ofen name my changes as b, c, d, etc.   This helps me see the progression as I work to improve a lens design.



I want to add an air space between the two lenses.  So, I will need to add another row because our file uses one row to describe two surfaces.  I did not actually put a thin layer of oil between the two pieces of glass.  I simplified the design by having the two glass surfaces touch.  The image evaluation is very similar with this type of lens set up, so I normally do it this way.

Click on the number 2 under the SRF column.  This will highlight the second surface row.  Now click on the blue arrow on the upper right of the Surface Data window.  I have moved my cursor over this icon, and it displays Insert row above selected row(s).  This will add another row between surface 1 and surface 2.  This is where I want the air space to be - between the lenses.

Now the Surface Data window shows 4 lens surfaces.  I have added the radius of curvature for the new surface number 2, and added the air space of 2mm.

Click the green check mark to accept these changes.

Now re-open the Surface Data window by clicking on the blue lens icon on the upper left corner.  I have moved my cursor over the blue lens icon, which displays Open the lens spreadsheet.  Left click on this.

Now the Surface Data window shows all 4 lens surface and the image surface IMS.  Let's refocus the lens because our design has changed.

Now, save this lens.  I have called it Bak1F2air.


I hope this has been clear.  Please email me with question or comments at:

Steve Fejes