Our ATM eyepieces started as a father daughter summer Astronomy project before she left for college. The parts for the eyepieces were acquired from Surplus Shed, but they were not any type of kit. Our primary design guide was the book .All About Telescopes., 2nd printing, by Sam Brown. This book has been my bible for telescopes since I acquired the book in grade school. Edmond Scientific has a new printing with a few update available on their web site.
The original eyepiece design intent was to replicate two of our TeleVue Plossl eyepieces. Part of our design process was to carefully disassemble our 32 mm and 20 mm eyepieces and take notes. Our two ATM eyepieces are not exactly TeleVue quality, but they are quite good, and we use the 34mm as primary eyepieces (we have several now). The 20mm has some optical distortion issues caused primarily because of the lens design.
We hope to make a 25mm to use as a public stargaze eyepiece. The 34mm would be fine except that it has a longer eye relief than the public expects. The long eye relief causes difficulty for the public stargazers. Over the years we have gone to a 28mm RKE that seems to be a good compromise for public stargazes. To be fair to the Plossl design, our 32mm TeleVue also has a long eye relief that is also difficult for the public to use.
Because you can really get tied up in optical design, I am going to present some generalities and basically give you construction details. Further reading and design information is available in the list of books at the end of this article.
The selection of lenses started with focal length ranges on Surplus Shack.s lens finder (see their web page). After getting a list of achromatic lenses in the focal length range, we selected the specific lenses based on focal length and diameter. The lenses needed to fit inside the PVC pipe housing I describe later. The general rule for symmetrical achromatic eyepieces (aka Plossls) is that your final focal length is ½ that of your single lens. focal length. My measurements would indicate it closer to 52-55%. So for other eyepieces, select lenses with a focal length slightly shorter than 2x the desired eyepiece focal length.
Material used for our 34mm eyepieces:
-200 grid sand paper
-card stock from a discarded package in the recycle bin
-fine point and wide point black permanent marker
-blue painters masking tape (easier to work with vs regular masking tape)
-1 ½ inch PVC tail pipe (found at Home Depot). Do not use the waxy poly pipe as it does not glue
-1 ¼ inch metal tail pipe to use as a .stop fixture.. PVC glue does not stick to metal
-threaded barrel for 1-1/4" eyepieces- Surplus Shack part number M2104
-2 achromatic 34mm diameter by 65mm fl ctd . Surplus Shack part number L10560
-razor blade carton knife
-6 in metal machinist scale. Has a very easy to use 1/64th of an inch scale
-metal straight edge
-model makers backsaw (Zona 35-251)
|Marking the lines on the PVC Tailpipe|
Use the backsaw to carefully cut the PVC pipe. I find that if I first score the line around the pipe with the box knife it is easier to keep the backsaw aligned. The best way to cut the pipe is to slowly rotate the pipe while you saw. Don.t get in a hurry. Your goal is to cut slowly and produce as smooth a cut as possible.
|Your first finished cut|
When finished with the cut, set the saw aside and sand the end of the large portion. Put the pipe on end and sand in a circular motion. That is hold the pipe with your hand, and sand it in a large circular motion on the paper. If you cut well, there is not a lot of sanding that needs done. The idea is to have a nice edge on the pipe so that it looks and feels good when completed.
Saw again for the second line. Sand the large section of pipe, but don't worry about the 1 ½ inch section. You will use this end with the retained ring. This end will be sanded after the retainer is glued. Set the remainder of the long PVC pipe aside for future projects.
Take the retainer ring and cut a slit into it. Next sand any burrs, but don.t sand too much as you do not want the edges rounded. Take the ring and put it inside the larger PVC piece. You will have to overlap it. Mark the overlap and remove the ring. Cut the ring a second time close to the mark. Be careful and cut so that you are long. You can always cut or sand a little more, but if you cut too short, you will need to cut another ring and try again.
|Section cut out to create the retainer ring|
To glue the ring inside the tube, use the metal pipe as a stop. Use masking tape to shim up the pipe to fit very snugly into the PVC pipe.
Push it into the pipe until it is close to the other end.
Now install the retainer ring and push it down to the metal pipe.
Use the metal pipe to push the retainer ring back toward the opening and into place.
You can put the larger PVC pipe on the table retainer ring down. Then push the metal pipe until things stop. You need to get the retainer ring flat with the front of the outside pipe like the picture. Now pop the retainer ring out, but do not move the metal stop pipe.
Use PVC glue and carefully coat the outside of the retainer ring. Put the ring into the tube. You should be able to get things to fit snugly. Be fairly quick about this, the PVC welding glue set up rather quickly. After about 2 minutes, you can carefully pull the metal stop tube out of the pipe. Set everything aside to dry overnight.
Once the PVC weld has cured, sand the retainer ring end of the pipe. Use the circular motion as described earlier. You can sand this until you have a smooth top. You can also continue sanding until you have a thinner retainer ring. Be careful to leave enough to guarantee a sold hold. I usually do not go smaller then about 1/16 inch retainer ring.
Get some card stock thickness cardboard from your recycle bin. Using a straight edge and box knife, cut the card stock about 4 ½ inch long and two strips. One strip should be as close to ¼ inch wide as possible, and the second should be about 9/16 inch. See exploded eyepiece image at end of this article.
Carefully roll these pieces, trying not to kink them, into the inside of the PVC pipe. The idea is to get the card stock accustom to being curled. You need to determine exactly how long to cut them. By putting them inside, you can take the box knife and .nip. the edge where it needs cut. Cut a square edge and see if the card stock fits snuggly inside the pipe. The snugger the fit the better.
Now assemble your eyepiece to see how things are fitting. For the 1¼ inch barrel, use blue making tape to shim up to fit snugly into the PVC. Run the tape so that it hangs over the edge. Then several times during the taping, use the box knife to shave off the top edge.
|Be sure to use smooth card stock that is not creased (poorly staged pic)|
It is now time to attempt to reduce glare inside the eyepiece. Take the wide point permanent marker and blacken the edges of the lenses and the card stock. Blacken the inside of the retainer ring area.
When you assemble, you want the flint glass to be away each other. When you look at the edge of the lens, there will be an indented glue line. One side is thin and one thick. Assemble the lenses so that the thin sides face one another.
After you have done all the hard work, set back and enjoy some great views. We primarily use these eyepieces in Schmidt type scopes (aka long f-ratio). It really works well there, but also holds its own with faster scopes. You should know that like all long focal length, low power, eyepieces, it can have center obstruction "issues". Nothing abnormal, just something to be aware of. It is not a TeleVue, but it does quite well given the price. Most of all, it sure is fun looking through an eyepiece you made yourself.
Good luck, and clear skies,
Stillwater Stargazers, Troy Ohio
Applied Optics and Optical Design, Part One and Two, by A. E. Conrady
Telescope Optics, a Comprehensive Manual for Amateur Astronomers, by Harrie Rutten and Martin van Venrooij