This paper includes the design and evaluation of the current Wynne corrector system for a 2.3-meter telescope. It

also summarizes the specification for, design, and evaluation of the new Wynne corrector that I worked on this

summer.

The University of Wyoming maintains a telescope observatory near Laramie, Wyoming. At this observatory

stands an f / 2 telescope that is 2.3 meters in diameter. Professor Edwin Loh of Michigan State University

designed the original Wynne corrector lenses for that telescope. The old corrector system was used to obtain a

field of 15 arcmin. My project was to design a series of corrector lenses for a 1.2-degree field, which is 5 times as

large.

Wynne (1987) Correctors are a grouping of either three or four lenses in a telescope that correct for the

aberrations of the primary mirror.  

My REU project was to help design a new Wynne corrector for the telescope in Wyoming by modifying the

design of the original corrector. The following information was obtained using an optics program called Zemax and

notes from Dr. Edwin Loh at Michigan State University.

Some specifics for the Wynne corrector and telescope currently in place:

It is an f / 2 telescope.

The entrance pupil diameter of the telescope is 2,300 mm.

The focal length from Zemax is 4,780.77 mm. (This should theoretically be 4,600 mm but the corrector increases the focal length slightly.)

BK7 glass was used for all surfaces except the mirror. 

Using the above setup produced very good results. I used values of 0.8, 0.55, and 0.44 microns for the

wavelengths to test the spot diagrams of the system. I also sent bundles of rays into the system at angles of 0, 0.1,

and 0.12 degrees. The value of 0.12 degrees is very near the outer range for the system. By using that outer value,

I could see how the system reacted to degree values that were very near its limitations. I received spot diagrams

for the three different degree values with root-mean-square sizes of 2.5, 5.2, and 6.4 microns respectively. This

was much improved over the values that were taken with just the mirror, which has coma. The values obtained

with just the mirror were 0.3, 190.1, and 228.2 microns.

Following is the lens data for the original four-lens corrector system.

I checked the tolerance of the system to errors in positioning the lenses with respect to the mirror. I studied the

changes in the RMS radius of the spot diagrams while I varied the decentering of the lenses in the x and y

direction. I also evaluated how it would effect the system if the lenses were tilted in the x or y direction. I found

that the lenses could only be off center approximately 0.15 mm for there to still be a reasonable sized spot

diagram. I found that decentering by a value in the x direction makes the spot diagram larger than by decentering

by that same value in the y direction. I found that the lenses could be tilted about the respective axis by an angle of

approximately 0.07 degrees before the spot diagrams became too large by our standards.

I evaluated what happens to the images at the corners of the pictures when the system is tilted or decentered. I

changed the field location to 0.175 degrees. At this angle, the fields are outside of the circle of good images, but

still in the square of the detector. I also checked to see what happens to the corners when the length between the

mirror and the other lenses is varied. The changes in the corner images for decentering and tilting were hard to

distinguish in the range of change that I was working with. The moving back and forth of the mirror with respect to

the other lenses made a large difference though. When I made the distance between the mirror and the lenses

smaller, the image took on an almost "fan-like" pattern. One end was narrow and the other feathered out like a

fan. When I increased the distance between the mirror and the lenses from 4407 mm, the image spread out into

almost a triangle or diamond shape. It was very easy to tell from the test images if the distance between the

corrector and the mirror is too long, too short, or the correct length.

I designed the new Wynne corrector. I started by making the corrector lenses 4 times bigger. Almost every value

from the original system was multiplied by four and put into Zemax for evaluation of the new system. The focal

length of the telescope was kept the same because the new lens system must replace the old one directly in the

same telescope when it is completed. I also changed the weight function that acts on the different degree fields so

that it put more emphasis on the spot diagrams toward the outer range of approximately 0.6 degrees. I did that

because the program was trying to really focus in on the center regardless of how large the outer-range spot

diagrams were becoming.

There were many different variables I had to take into consideration while designing this new system. I allowed the

Zemax program to change the radii of curvature for the four corrector lenses in order to optimize the system. I

also let the program change the distances between the corrector lenses. Zemax has an "optimization" function that

finds the placement of the system that gives the smallest RMS values possible. I soon realized that Zemax was

trying to allow the edge thickness of the lenses to be larger than we wished them to be. It would be in our best

interests to have the lenses as thin and narrow as physically possible so as to minimize the cost of producing the

new lenses.

The original Wynne corrector lenses were made of BK7 glass. There were four different glass types that I

considered while designing the new system. The glass types were BK7, F2, F6, and LaSFN9. I chose to analyze

the other three glass types (besides BK7) because their indexes of refraction are very close to that of BK7.

Therefore, all four glass types are somewhat similar in optical nature.

After my evaluation of the system with each of the four types I found that BK7 was still the best glass to use. The

RMS values produced by the other three types of glass were greater than that of BK7 in all of my computations.

Following is the lens data for the four-lens corrector system that produced the smallest RMS values.

I designed two different three-lens corrector systems. Making a system that requires only three lenses would

greatly diminish the costs of the finished product. Is a three-lens system's effectiveness comparable to a four-lens

system? That was the next question that I had to answer.

Wynne (1974) described a three-lens corrector system.. His specifications were for an f / 3.25 telescope. I

modified this corrector lens package for an f / 2 telescope. I gradually changed the length of the system until it was

f / 2. After that, I slowly modified the thickness of the lenses until they were very near the values that I had been

testing on the four-lens system. The Wynne design that I modified from the journal article did not work better than

the four-lens system. In fact, it was much worse than the four-lens system that I had designed. The RMS values of

the three lenses were 10.2, 13.6, 21.3, 28.2, and 36.2 microns. These range from approximately a factor of two

to four times worse than the values of 5.3, 6.5, 8.5, 9.1, and 9.5 that I received from the four-lens system.

I experimented with a three-lens system of my own. I took the four-lens system that I already had and just deleted

out the smallest lens in the system. I then optimized the system and randomly changed a few of the distances and

semi-diameters just to see what would happen. Somehow, I came up with a very efficient system that gave me

RMS values of 7.1, 7.3, 8.4, 10.6, and 13.4 microns. Those values are very good when compared to the values

that I had been receiving from the system that I modified from the Wynne (1974) design. Also, the numbers are

fairly close to the values obtained from the four-lens system.

References: