by Chris Krauskopf
The January 2006 edition of S&T had an article by Carl Anderson that gave me the last piece of information I felt necessary to go ahead with the project. Subsequently, Carl has provided me with valuable insight, information and even parts for the telescope. His enthusiasm for this project has been most helpful.
Changing the size of the scope was more of a practical matter rather than a technical one. I simply did not have the space or funds available to make a 24 incher. Also, the use of the program Winspot, allowed me to test and adjust parameters to see how they would work with various mirror sizes. Therefore, after changing a few parameters, an 18" f/15 scope seemed the most practical.
My daughters helped me with the name once it was set up. Yo = Yolo, Max = Maximus
Similiar to Dob construction, Ebony Star laminate is used for the smooth operating bearings.
The Two mirrors required for this scope had some unusual aspects to them.
* Each mirror is quite shallow. (F/24.4 for the 18" primary and F/35.2 for the 12.5" secondary mirror)
* Each mirror was ground and polished to a sphere with the same radius of curvature. The difference between a more common parabolic mirror and a sphere at these very long focal lengths is so very small that it can be completely ignored.
* In my Yolo design one of the mirrors had to have a compound curve generated into it in order to compensate for astigmatism that is introduced when both mirrors are tilted. I chose to polish the compound curve (torroid shape) into the 12.5" secondary mirror. The use of the Winspot program indicated that one axis of the secondary mirror needed to be 8 and 3/4 inches shorter than the other axis.
* Foucault testing at the radius of curvature for both mirrors involved a separation distance of approximately 73 feet. Air turbulence and the extreme shallowness of the curves made using the foucault tester quite challenging.
Building the actual telescope is similar to building a large Newtonian with a few differences. The secondary mirror is much larger than a typical Newtonian secondary. It is also shifted to the front of the scope and away from the incoming light path.
Also, the telescope focus point is located very near the primary mirror. This location requires that the back end of the scope be elevated somewhat for comfortable viewing.
The weight of a full thickness 12.5 inch secondary mirror is about 22 lbs. This weight must be compensated for at the back end of the scope. The weight also requires a substantially rigid support structure. I chose a typical truss tube design with 8 hollow fiberglass tubes. This system has proven to be quite satisfactory. A motorized collimation system was built to correct for structural sagging, however it appears that this system is not needed.
The back primary mirror cell and the front secondary mirror cell contain fully adjustable collimation bolts. In addition, the front mirror cell is able to move toward and away from the primary optical axis in order to gain the right separation distances of the axes.
The 2 inch focuser likewise has the capacity to move along the optical axis as well as toward and away from the primary optical axis. These adjustments became crucial during the optical alignment phase of testing.
I was able to wipe out stray light by using a pvc tube in front of the focuser. I lined the inside with sawdust gathered from my workbench! First thing is to spray the inside of the tube with contact cement, then I poured sawdust inside plugged the ends and shook it like mad for a few seconds. Then I dumped the excess out, and used flat black spray paint inside the tube. The results are really amazing! The rough, black texture is stray lights worst nightmare.
Several weeks worth of testing were needed in order to align the optics properly. The telescope naturally sags a slight amount due to the considerable weight at each end. This sagging causes the optics to become misaligned and introduces astigmatism (in my case) when the scope is moved around the sky. My solution was to "fix" the telescope into a permanent sagged position. I do this by tightening all the components of the tube assembly once the scope has settled down with the mirrors in place. First, I very gently tapped things to make sure they were seated properly. Then I carefully marked each component as to where it was in relation to other parts so that I could repeat the assembly process for perfect alignment every time. This has virtually eliminated astigmatism caused by flexure of the scope body. And once the telescope optics are aligned, very slight astigmatism is sometimes seen at very high powers. Most often, the sky conditions limit optical clarity, not misalignment.
The out of focus diffraction rings of a bright star are quite beautiful. I have never seen anything like them. I counted 7 rings before they became too small to see clearly with the rather poor sky conditions. Probably there are about 10-12 or so of them. (405x mag.) It really does look like a cross section of a log.
Star tests revealed some slight edge defects in this system. That is understandable because the primary and secondary mirrors have several clips holding them in place. A simple solution would be to mask off the outer 1/4 inch of the primary mirror, another might be to redesign one of the mirror mounts. I however don't find the defects to be much of a nuisance.
The mounting box under the scope is only temporary.
Primary mirror is 18 inches diameter, secondary mirror is 12.5 inches diameter. Both mirrors are full thickness and are coated with high efficiency coatings (98% to 99%) from Spectrum Coatings, Florida.
I have been using and building telescopes for around 30 years. When I got my first view of the moon through this telescope the word WOW! kept on coming out of my mouth. When it is adjusted properly, the views of the lunar surface are absolutely astounding. The craters take on new crispness and contrast that I have never seen in any telescope or photograph before. Also the subtle coloration and shading of surface textures makes the view seem truly 3-D. I have often described a "floating above the moon looking down" sensation to other people. The field of view of this telescope is narrow, about 2/3 of a full moon with a 56mm 2 inch eyepiece. At high powers (400+), the views are still nice and crisp.
Planets:
The views of planets through this telescope can be outstanding also, however the seeing conditions greatly limit this pleasure. The views of Jupiter are negatively influenced by the large column of turbulent air that the scope must see through. When conditions are right, Jovian detail is a real pleasure. In fact, I believe that I have seen a small amount of detail on one of the moons.
Deep sky:
Under average conditions, a great deal of detail can be seen in star clusters and nebulas. I have not had the opportunity to really "open up" this scope yet on many deep sky objects because I can't see them from my light polluted home sky. However the ones I have seen are quite impressive. M-13 showed bright, sharp, blue, gold and white stars across the whole face. And M-57 showed structure and hinted at the central star with averted vision.
Overall, this telescope has been a real pleasure to make and use. I look forward to really pushing it to it's limits.
I realize that there may be others out there who may not have the skill, time, or desire required to make one of these very special telescopes, so I offer my services for hire. The 6-12 inch range would be the most practical to make for a variety of reasons. However I would not exclude other sizes. Cost for this type of scope? That will depend on several variables. We can discuss ideas and talk over the phone or internet and come up with an appropriate price after that.
So, if there is a real interest in obtaining the telescope makers dream, an unobstructed two mirror reflecting telescope, please e-mail your request.
Thank you,
Chris Krauskopf
e-mail Chris Krauskopf